JBoss.org Community Documentation

# Drools Planner User Guide

by The JBoss Drools team

Version 5.4.0.Beta2

## Chapter 1. Planner introduction

## 1.1. What is Drools Planner?

Drools Planner optimizes planning problems. It solves use cases, such as:

- Employee shift rostering: timetabling nurses, repairmen, ...

- Agenda scheduling: scheduling meetings, appointments, maintenance jobs, advertisements, ...

- Educational timetabling: scheduling lessons, courses, exams, conference presentations, ...

- Vehicle routing: planning vehicles (trucks, trains, boats, airplanes, ...) with freight and/or people

- Bin packing: filling containers, trucks, ships and storage warehouses, but also cloud computers nodes, ...

- Job shop scheduling: planning car assembly lines, machine queue planning, workforce task planning, ...

- Cutting stock: minimizing waste while cutting paper, steel, carpet, ...

- Sport scheduling: planning football leagues, baseball leagues, ...

- Financial optimization: investment portfolio optimization, risk spreading, ...

Every organization faces planning problems: provide products and services with a limited set of constrained resources (employees, assets, time and money).

Drools Planner enables normal JavaTM programmers to solve planning problems efficiently. Under the hood, it combines optimization algorithms (including Metaheuristics such as Tabu Search and Simulated Annealing) with the power of score calculation by a rule engine.

Drools Planner, like the rest of Drools, is business-friendly open source software under the Apache Software License 2.0 (layman's explanation).

## 1.2. What is a planning problem?

### 1.2.1. A planning problem is NP-complete

All the use cases above are probably NP-complete. In layman's terms, this means:

- It's easy to verify a given solution to a problem in reasonable time.

- There is no silver bullet to find the optimal solution of a problem in reasonable time (*).

## Note

(*) At least, none of the smartest computer scientists in the world have found such a silver bullet yet. But if they find one for 1 NP-complete problem, it will work for every NP-complete problem.

In fact, there's a $ 1,000,000 reward for anyone that proves if such a silver bullet actually exists or not.

The implication of this is pretty dire: solving your problem is probably harder than you anticipated, because the 2 common techniques won't suffice:

- A brute force algorithm (even a smarter variant) will take too long.

- A quick algorithm, for example in bin packing, putting in the largest items first, will return a solution that is usually far from optimal.

Drools Planner does find a good solution in reasonable time for such planning problems.

### 1.2.2. A planning problem has (hard and soft) constraints

Usually, a planning problem has at least 2 levels of constraints:

- A (negative) hard constraint must not be broken. For example: 1 teacher can not teach 2 different lessons at the same time.

- A (negative) soft constraint should not be broken if it can be avoided. For example: Teacher A does not like to teach on Friday afternoon.

Some problems have positive constraints too:

- A positive soft constraint (or reward) should be fulfilled if possible. For example: Teacher B likes to teach on Monday morning.

  In practice, these are just like negative soft constraints, but with a positive weight.

Some toy problems (such as N Queens) only have hard constraints. Some problems have 3 or more levels of constraints, for example hard, medium and soft constraints.

These constraints define the score function (AKA fitness function) of a planning problem. Each solution of a planning problem can be graded with a score. Because we 'll define these constraints as rules in the Drools Expert rule engine, adding constraints in Drools Planner is easy and scalable.

### 1.2.3. A planning problem has a huge search space

A planning problem has a number of solutions. There are several categories of solutions:

- A possible solution is a solution that does or does not break any number of constraints. Planning problems tend to have a incredibly large number of possible solutions. Most of those solutions are worthless.

- A feasible solution is a solution that does not break any (negative) hard constraints. The number of feasible solutions tends to be relative to the number of possible solutions. Sometimes there are no feasible solutions. Every feasible solution is a possible solution.

- An optimal solution is a solution with the highest score. Planning problems tend to have 1 or a few optimal solutions. There is always at least 1 optimal solution, even in the case that there are no feasible solutions and the optimal solution isn't feasible.

- The best solution found is the solution with the highest score found by an implementation in a given amount of time. The best solution found is likely to be feasible.

Counterintuitively, the number of possible solutions is huge (if calculated correctly), even with a small dataset. As you'll see in the examples, most instances have a lot more possible solutions than the minimal number of atoms in the known universe (10^80). Because there is no silver bullet to find the optimal solution, any implementation is forced to evaluate at least a subset of all those possible solutions.

Drools Planner supports several optimization algorithms to efficiently wade through that incredibly large number of possible solutions. Depending on the use case, some optimization algorithms perform better than others. In Drools Planner it is easy to switch the optimization algorithm, by changing the solver configuration in a few XML lines or by API.

## 1.3. Status of Drools Planner

Drools Planner is production ready. The API is almost stable but backward incompatible changes can occur. With the recipe called `UpgradeFromPreviousVersionRecipe.txt` you can easily upgrade and deal with any backwards incompatible changes between versions. That recipe file is included in every release.

## 1.4. Get Drools Planner and run the examples

### 1.4.1. Get the release zip and run the examples

You can download a release zip of Drools Planner from the Drools download site. Unzip it. To run an example, just open the directory `examples` and run the script (`runExamples.sh` on Linux and Mac or `runExamples.bat` on Windows) and pick an example in the GUI:

    $ cd examples
    $ ./runExamples.sh

    $ cd examples
    $ runExamples.bat

### 1.4.2. Run the examples in an IDE (IntelliJ, Eclipse, NetBeans)

To run the examples in your favorite IDE, first configure your IDE:

- In IntelliJ and NetBeans, just open the file `examples/sources/pom.xml` as a new project, the maven integration will take care of the rest.

- In Eclipse, open a new project for the directory `examples/sources`.

  - Add all the jars to the classpath from the directory `binaries` and the directory `examples/binaries`, except for the file `examples/binaries/drools-planner-examples-*.jar`.

  - Add the java source directory `src/main/java` and the java resources directory `src/main/resources`.

Next, create a run configuration:

- Main class: `org.drools.planner.examples.app.DroolsPlannerExamplesApp`

- VM parameters (optional): `-Xmx512M -server`

- Working directory: `examples` (this is the directory that contains the directory `data`)

### 1.4.3. Use it with maven, gradle, ivy, buildr or ANT

The Drools Planner jars are available on the central maven repository (and the JBoss maven repository).

If you use maven, just add a dependency to `drools-planner-core` in your project's `pom.xml`:

        <dependency>

            <groupId>org.drools.planner</groupId>

            <artifactId>drools-planner-core</artifactId>

            <version>5.x</version>

        </dependency>

This is similar for gradle, ivy and buildr.

If you're still using ant (without ivy), copy all the jars from the download zip's `binaries` directory and manually verify that your classpath doesn't contain duplicate jars.

### 1.4.4. Build it from source

You can also easily build it from source yourself.

Set up Git and clone `drools-planner` from GitHub (or alternatively, download the zipball):

    $ git clone git@github.com:droolsjbpm/drools-planner.git drools-planner
    ...

Then do a Maven 3 build:

    $ cd drools-planner
    $ mvn -DskipTests clean install
    ...

After that, you can run any example directly from the command line, just run this command and pick an example:

    $ cd drools-planner-examples
    $ mvn exec:exec
    ...

## 1.5. Questions, issues and blog

Your questions and comments are welcome on the user mailing list. Start the subject of your mail with `[planner]`. You can read/write to the user mailing list without littering your mailbox through this web forum or this newsgroup.

Feel free to report an issue (such as a bug, improvement or a new feature request) for the Drools Planner code or for this manual to the drools issue tracker. Select the component `drools-planner`.

Pull requests (and patches) are very welcome and get priority treatment! Include the pull request link to a JIRA issue and optionally send a mail to the dev mailing list to get the issue fixed fast. By open sourcing your improvements, you 'll benefit from our peer review and from our improvements made upon your improvements.

Check our blog, Google+(Drools Planner, Geoffrey De Smet) and twitter (Geoffrey De Smet) for news and articles. If Drools Planner helps you solve your problem, don't forget to blog or tweet about it!

## Chapter 2. Use cases and examples

## 2.1. Introduction

Drools Planner has several examples. In this manual we explain Drools Planner mainly using the n queens example. So it's advisable to read at least the section about that example. For advanced users, the following examples are recommended: curriculum course and nurse rostering.

You can find the source code of all these examples in the distribution zip under `examples/sources` and also in git under `drools-planner/drools-planner-examples`.

## 2.2. N queens example

### 2.2.1. Problem statement

The n queens puzzle is a puzzle with the following constraints:

- Use a chessboard of n columns and n rows.

- Place n queens on the chessboard.

- No 2 queens can attack each other. Note that a queen can attack any other queen on the same horizontal, vertical or diagonal line.

The most common n queens puzzle is the 8 queens puzzle, with n = 8. We 'll explain Drools Planner using the 4 queens puzzle as the primary example.

A proposed solution could be:

Figure 2.1. A wrong solution for the 4 queens puzzle

The above solution is wrong because queens `A1` and `B0` can attack each other (as can queens `B0` and `D0`). Removing queen `B0` would respect the "no 2 queens can attack each other" constraint, but would break the "place n queens" constraint.

### 2.2.2. Solution(s)

Below is a correct solution:

Figure 2.2. A correct solution for the 4 queens puzzle

All the constraints have been met, so the solution is correct. Note that most n queens puzzles have multiple correct solutions. We 'll focus on finding a single correct solution for a given n, not on finding the number of possible correct solutions for a given n.

### 2.2.3. Screenshot

Here is a screenshot of the example:

Figure 2.3. Screenshot of the n queens example

### 2.2.4. Problem size

These numbers might give you some insight on the size of this problem.

Table 2.1. NQueens problem size

# queens (n) | # possible solutions (each queen its own column) | # feasible solutions (= optimal in this use case) | # optimal solutions | # optimal out of # possible
--- | --- | --- | --- | ---
4 | 256 | 2 | 2 | 1 out of 128
8 | 16777216 | 64 | 64 | 1 out of 262144
16 | 18446744073709551616 | 14772512 | 14772512 | 1 out of 1248720872503
32 | 1.46150163733090291820368483e+48 | ? | ? | ?
64 | 3.94020061963944792122790401e+115 | ? | ? | ?
n | n ^ n | ? | # feasible solutions | ?

The Drools Planner implementation has not been optimized because it functions as a beginner example. Nevertheless, it can easily handle 64 queens.

### 2.2.5. Domain model

Use a good domain model: it will be easier to understand and solve your planning problem with Drools Planner. This is the domain model for the n queens example:

    public class Column {

        private int index;

        // ... getters and setters
    }

    public class Row {

        private int index;

        // ... getters and setters
    }

    public class Queen {

        private Column column;
        private Row row;

        public int getAscendingDiagonalIndex() {...}
        public int getDescendingDiagonalIndex() {...}

        // ... getters and setters
    }

    public class NQueens implements Solution<SimpleScore> {

        private int n;
        private List<Column> columnList;
        private List<Row> rowList;

        private List<Queen> queenList;

        private SimpleScore score;

        // ... getters and setters
    }

A `Queen` instance has a `Column` (for example: 0 is column A, 1 is column B, ...) and a `Row` (its row, for example: 0 is row 0, 1 is row 1, ...). Based on the column and the row, the ascending diagonal line as well as the descending diagonal line can be calculated. The column and row indexes start from the upper left corner of the chessboard.

Table 2.2. A solution for the 4 queens puzzle shown in the domain model

A solution | Queen | columnIndex | rowIndex | ascendingDiagonalIndex (columnIndex + rowIndex) | descendingDiagonalIndex (columnIndex - rowIndex)
--- | --- | --- | --- | --- | ---
| A1 | 0 | 1 | 1 (**) | -1
B0 | 1 | 0 (*) | 1 (**) | 1
C2 | 2 | 2 | 4 | 0
D0 | 3 | 0 (*) | 3 | 3

When 2 queens share the same column, row or diagonal line, such as (*) and (**), they can attack each other.

A single `NQueens` instance contains a list of all `Queen` instances. It is the `Solution` implementation which will be supplied to, solved by and retrieved from the Solver. Notice that in the 4 queens example, NQueens's `getN()` method will always return 4.

## 2.3. Cloud balancing example

### 2.3.1. Problem statement

Assign each process to a server.

Hard constraints:

- Every server should be able to handle the sum of each of the minimal hardware requirements (CPR, RAM, network bandwidth) of all its processes.

Soft constraints:

- Each server that has one or more processes assigned, has a fixed maintenance cost. Minimize the total cost.

This is a form of bin packing.

### 2.3.2. Domain model

## 2.4. Machine reassignment example (ROADEF 2012)

### 2.4.1. Problem statement

Assign each process to a machine. All processes already have an original (unoptimized) assignment. Each process requires an amount of each resource (such as CPU, RAM, ...). This is more complex version of the Cloud balancing example.

The problem is defined by the Google ROADEF/EURO Challenge 2012.

Hard constraints:

- Maximum capacity: The maximum capacity for each resource for each machine must not be exceeded.

- Conflict: Processes of the same service must run on distinct machines.

- Spread: Processes of the same service must be spread across locations.

- Dependency: The processes of a service depending on another service must run in the neighborhood of a process of the other service.

- Transient usage: Some resources are transient and count towards the maximum capacity of both the original machine as the newly assigned machine.

Soft constraints:

- Load: The safety capacity for each resource for each machine should not be exceeded.

- Balance: Leave room for future assignments by balancing the available resources on each machine.

- Process move cost: A process has a move cost.

- Service move cost: A service has a move cost.

- Machine move cost: Moving a process from machine A to machine B has another A-B specific move cost.

### 2.4.2. Problem size

    model_a1_1: 2 resources, 1 neighborhoods, 4 locations, 4 machines, 79 services, 100 processes and 1 balancePenalties with flooredPossibleSolutionSize (10^60).
    model_a1_2: 4 resources, 2 neighborhoods, 4 locations, 100 machines, 980 services, 1000 processes and 0 balancePenalties with flooredPossibleSolutionSize (10^2000).
    model_a1_3: 3 resources, 5 neighborhoods, 25 locations, 100 machines, 216 services, 1000 processes and 0 balancePenalties with flooredPossibleSolutionSize (10^2000).
    model_a1_4: 3 resources, 50 neighborhoods, 50 locations, 50 machines, 142 services, 1000 processes and 1 balancePenalties with flooredPossibleSolutionSize (10^1698).
    model_a1_5: 4 resources, 2 neighborhoods, 4 locations, 12 machines, 981 services, 1000 processes and 1 balancePenalties with flooredPossibleSolutionSize (10^1079).
    model_a2_1: 3 resources, 1 neighborhoods, 1 locations, 100 machines, 1000 services, 1000 processes and 0 balancePenalties with flooredPossibleSolutionSize (10^2000).
    model_a2_2: 12 resources, 5 neighborhoods, 25 locations, 100 machines, 170 services, 1000 processes and 0 balancePenalties with flooredPossibleSolutionSize (10^2000).
    model_a2_3: 12 resources, 5 neighborhoods, 25 locations, 100 machines, 129 services, 1000 processes and 0 balancePenalties with flooredPossibleSolutionSize (10^2000).
    model_a2_4: 12 resources, 5 neighborhoods, 25 locations, 50 machines, 180 services, 1000 processes and 1 balancePenalties with flooredPossibleSolutionSize (10^1698).
    model_a2_5: 12 resources, 5 neighborhoods, 25 locations, 50 machines, 153 services, 1000 processes and 0 balancePenalties with flooredPossibleSolutionSize (10^1698).

## 2.5. Manners 2009 example

### 2.5.1. Problem statement

In Manners 2009, miss Manners is throwing a party again.

- This time she invited 144 guests and prepared 12 round tables with 12 seats each.

- Every guest should sit next to someone (left and right) of the opposite gender.

- And that neighbour should have at least one hobby in common with the guest.

- Also, this time there should be 2 politicians, 2 doctors, 2 socialites, 2 sports stars, 2 teachers and 2 programmers at each table.

- And the 2 politicians, 2 doctors, 2 sports stars and 2 programmers shouldn't be the same kind.

Drools Expert also has the normal miss Manners examples (which is much smaller) and employs a brute force heuristic to solve it. Drools Planner's implementation employs far more scalable heuristics while still using Drools Expert to calculate the score..

## 2.6. Traveling Salesman Problem example (TSP)

### 2.6.1. Problem statement

Given a list of cities, find the shortest tour for a salesman that visits each city exactly once. See the wikipedia definition of the traveling Salesman Problem.

It is one of the most intensively studied problems in computational mathematics. Yet, in the real world, it's often only part of a planning problem, along with other constraints, such as employee shift time constraints.

## 2.7. Traveling Tournament Problem example (TTP)

### 2.7.1. Problem statement

Schedule matches between n teams with the following hard constraints:

- Each team plays twice against every other team: once home and once away.

- Each team has exactly 1 match on each timeslot.

- No team must have more than 3 consecutive home or 3 consecutive away matches.

- No repeaters: no 2 consecutive matches of the same 2 opposing teams.

and the following soft constraint:

- Minimize the total distance traveled by all teams.

The problem is defined on Michael Trick's website (which contains several world records too).

### 2.7.2. Simple and smart implementation

There are 2 implementations (simple and smart) to demonstrate the importance of some performance tips. The `DroolsPlannerExamplesApp` always runs the smart implementation, but with these commands you can compare the 2 implementations yourself:

    $ mvn exec:exec -Dexec.mainClass="org.drools.planner.examples.travelingtournament.app.simple.SimpleTravelingTournamentApp"
    ...
    $ mvn exec:exec -Dexec.mainClass="org.drools.planner.examples.travelingtournament.app.smart.SmartTravelingTournamentApp"
    ...

The smart implementation performs and scales exponentially better than the simple implementation.

### 2.7.3. Problem size

These numbers might give you some insight on the size of this problem.

Table 2.3. Traveling tournament problem size

# teams | # days | # matches | # possible solutions (simple) | # possible solutions (smart) | # feasible solutions | # optimal solutions
--- | --- | --- | --- | --- | --- | ---
4 | 6 | 12 | 2176782336 | <= 518400 | ? | 1?
6 | 10 | 30 | 1000000000000000000000000000000 | <= 47784725839872000000 | ? | 1?
8 | 14 | 56 | 1.52464943788290465606136043e+64 | <= 5.77608277425558771434498864e+43 | ? | 1?
10 | 18 | 90 | 9.43029892325559280477052413e+112 | <= 1.07573451027871200629339068e+79 | ? | 1?
12 | 22 | 132 | 1.58414112478195320415135060e+177 | <= 2.01650616733413376416949843e+126 | ? | 1?
14 | 26 | 182 | 3.35080635695103223315189511e+257 | <= 1.73513467024013808570420241e+186 | ? | 1?
16 | 30 | 240 | 3.22924601799855400751522483e+354 | <= 2.45064610271441678267620602e+259 | ? | 1?
n | 2 * (n - 1) | n * (n - 1) | (2 * (n - 1)) ^ (n * (n - 1)) | <= (((2 * (n - 1))!) ^ (n / 2)) | ? | 1?

## 2.8. Curriculum course scheduling example (ITC 2007 track 3)

### 2.8.1. Problem statement

Schedule each lecture into a timeslot and into a room.

The problem is defined by the International Timetabling Competition 2007 track 3.

## 2.9. Examination timetabling example (ITC 2007 track 1)

### 2.9.1. Problem statement

Schedule each exam into a period and into a room. Multiple exams can share the same room during the same period.

There are a number of hard constraints that cannot be broken:

- Exam conflict: 2 exams that share students should not occur in the same period.

- Room capacity: A room's seating capacity should suffice at all times.

- Period duration: A period's duration should suffice for all of its exams.

- Period related hard constraints should be fulfilled:

  - Coincidence: 2 exams should use the same period (but possibly another room).

  - Exclusion: 2 exams should not use the same period.

  - After: 1 exam should occur in a period after another exam's period.

- Room related hard constraints should be fulfilled:

  - Exclusive: 1 exam should not have to share its room with any other exam.

There are also a number of soft constraints that should be minimized (each of which has parameterized penalty's):

- 2 exams in a row.

- 2 exams in a day.

- Period spread: 2 exams that share students should be a number of periods apart.

- Mixed durations: 2 exams that share a room should not have different durations.

- Front load: Large exams should be scheduled earlier in the schedule.

- Period penalty: Some periods have a penalty when used.

- Room penalty: Some rooms have a penalty when used.

It uses large test data sets of real-life universities.

The problem is defined by the International Timetabling Competition 2007 track 1.

### 2.9.2. Problem size

These numbers might give you some insight on the size of this problem.

Table 2.4. Examination problem size

Set | # students | # exams/topics | # periods | # rooms | # possible solutions | # feasible solutions | # optimal solutions
--- | --- | --- | --- | --- | --- | --- | ---
exam_comp_set1 | 7883 | 607 | 54 | 7 | 10^1564 | ? | 1?
exam_comp_set2 | 12484 | 870 | 40 | 49 | 10^2864 | ? | 1?
exam_comp_set3 | 16365 | 934 | 36 | 48 | 10^3023 | ? | 1?
exam_comp_set4 | 4421 | 273 | 21 | 1 | 10^360 | ? | 1?
exam_comp_set5 | 8719 | 1018 | 42 | 3 | 10^2138 | ? | 1?
exam_comp_set6 | 7909 | 242 | 16 | 8 | 10^509 | ? | 1?
exam_comp_set7 | 13795 | 1096 | 80 | 28 | 10^3671 | ? | 1?
exam_comp_set8 | 7718 | 598 | 80 | 8 | 10^1678 | ? | 1?
? | s | t | p | r | (p * r) ^ e | ? | 1?

Geoffrey De Smet (the Drools Planner lead) finished 4th in the International Timetabling Competition 2007's examination track with a very early version of Drools Planner. Many improvements have been made since then.

### 2.9.3. Domain model

Below you can see the main examination domain classes:

Figure 2.4. Examination domain class diagram

Notice that we've split up the exam concept into an `Exam` class and a `Topic` class. The `Exam` instances change during solving (this is the planning entity class), when they get another period or room property. The `Topic`, `Period` and `Room` instances never change during solving (these are problem facts, just like some other classes).

## 2.10. Patient admission scheduling (hospital bed planning) example (PAS)

### 2.10.1. Problem statement

Assign each patient (that will come to the hospital) into a bed for each night that the patient will stay in the hospital. Each bed belongs to a room and each room belongs to a department. The arrival and departure dates of the patients is fixed: only a bed needs to be assigned for each night.

There are a couple of hard constraints:

- 2 patients shouldn't be assigned to the same bed in the same night.

- A room can have a gender limitation: only females, only males, the same gender in the same night or no gender limitation at all.

- A department can have a minimum or maximum age.

- A patient can require a room with specific equipment(s).

And of course, there are also some soft constraints:

- A patient can prefer a maximum room size, for example if he/she want a single room.

- A patient is best assigned to a department that specializes in his/her problem.

- A patient is best assigned to a room that specializes in his/her problem.

- A patient can prefer a room with specific equipment(s).

The problem is defined on this webpage and the test data comes from real world hospitals.

## 2.11. Nurse rostering example (INRC 2010)

### 2.11.1. Problem statement

For each shift, assign a nurse to work that shift.

The problem is defined by the International Nurse Rostering Competition 2010.

## Chapter 3. Planner configuration

## 3.1. Overview

Solving a planning problem with Drools Planner consists out of 5 steps:

1. Model your planning problem as a class that implements the interface `Solution`, for example the class `NQueens`.

2. Configure a `Solver`, for example a first fit and tabu search solver for any `NQueens` instance.

3. Load a problem data set from your data layer, for example a 4 Queens instance. Set it as the planning problem on the `Solver` with `Solver.setPlanningProblem(...)`.

4. Solve it with `Solver.solve()`.

5. Get the best solution found by the `Solver` with `Solver.getBestSolution()`.

## 3.2. Solver configuration

### 3.2.1. Solver configuration by XML file

You can build a `Solver` instance with the `XmlSolverFactory`. Configure it with a solver configuration XML file:

            XmlSolverFactory solverFactory = new XmlSolverFactory();

            solverFactory.configure("/org/drools/planner/examples/nqueens/solver/nqueensSolverConfig.xml");

            Solver solver = solverFactory.buildSolver();

A solver configuration file looks something like this:

    <?xml version="1.0" encoding="UTF-8"?>

    <solver>

      <!-- Define the model -->

      <solutionClass>org.drools.planner.examples.nqueens.domain.NQueens</solutionClass>

      <planningEntityClass>org.drools.planner.examples.nqueens.domain.Queen</planningEntityClass>

      <!-- Define the score function -->

      <scoreDrl>/org/drools/planner/examples/nqueens/solver/nQueensScoreRules.drl</scoreDrl>

      <scoreDefinition>

        <scoreDefinitionType>SIMPLE</scoreDefinitionType>

      </scoreDefinition>

      <!-- Configure the optimization algorithm(s) -->

      <termination>

        ...

      </termination>

      <constructionHeuristic>

        ...

      </constructionHeuristic>

      <localSearch>

        ...

      </localSearch>

    </solver>

Notice the 3 parts in it:

- Define the model

- Define the score function

- Configure the optimization algorithm(s)

We 'll explain these various parts of a configuration later in this manual.

Drools Planner makes it relatively easy to switch optimization algorithm(s) just by changing the configuration. There's even a `Benchmark` utility which allows you to play out different configurations against each other and report the most appropriate configuration for your problem. You could for example play out tabu search versus simulated annealing, on 4 queens and 64 queens.

### 3.2.2. Solver configuration by Java API

As an alternative to the XML file, a solver configuration can also be configured with the `SolverConfig` API:

            SolverConfig solverConfig = new SolverConfig();

            solverConfig.setSolutionClass(NQueens.class);
            Set<Class<?>> planningEntityClassSet = new HashSet<Class<?>>();
            planningEntityClassSet.add(Queen.class);
            solverConfig.setPlanningEntityClassSet(planningEntityClassSet);

            solverConfig.setScoreDrlList(
                    Arrays.asList("/org/drools/planner/examples/nqueens/solver/nQueensScoreRules.drl"));
            ScoreDefinitionConfig scoreDefinitionConfig = solverConfig.getScoreDefinitionConfig();
            scoreDefinitionConfig.setScoreDefinitionType(
                    ScoreDefinitionConfig.ScoreDefinitionType.SIMPLE);

            TerminationConfig terminationConfig = solverConfig.getTerminationConfig();
            // ...
            List<SolverPhaseConfig> solverPhaseConfigList = new ArrayList<SolverPhaseConfig>();
            ConstructionHeuristicSolverPhaseConfig constructionHeuristicSolverPhaseConfig
                    = new ConstructionHeuristicSolverPhaseConfig();
            // ...
            solverPhaseConfigList.add(constructionHeuristicSolverPhaseConfig);
            LocalSearchSolverPhaseConfig localSearchSolverPhaseConfig = new LocalSearchSolverPhaseConfig();
            // ...
            solverPhaseConfigList.add(localSearchSolverPhaseConfig);
            solverConfig.setSolverPhaseConfigList(solverPhaseConfigList);
            Solver solver = solverConfig.buildSolver();

It is highly recommended to configure by XML file instead of this API. To dynamically configure a value at runtime, use the XML file as a template and extract the `SolverConfig` class with `getSolverConfig()` to configure the dynamic value at runtime:

            XmlSolverFactory solverFactory = new XmlSolverFactory();
            solverFactory.configure("/org/drools/planner/examples/nqueens/solver/nqueensSolverConfig.xml");

            SolverConfig solverConfig = solverFactory.getSolverConfig();
            solverConfig.getTerminationConfig().setMaximumMinutesSpend(userInput);
            Solver solver = solverConfig.buildSolver();

## 3.3. Model your planning problem

### 3.3.1. Is this class a problem fact or planning entity?

Look at a dataset of your planning problem. You 'll recognize domain classes in there, each of which is one of these:

- A unrelated class: not used by any of the score constraints. From a planning standpoint, this data is obsolete.

- A problem fact class: used by the score constraints, but does NOT change during planning (as long as the problem stays the same). For example: `Bed`, `Room`, `Shift`, `Employee`, `Topic`, `Period`, ...

- A planning entity class: used by the score constraints and changes during planning. For example: `BedDesignation`, `ShiftAssignment`, `Exam`, ...

Ask yourself: What class changes during planning? Which class has variables that I want the `Solver` to choose for me? That class is a planning entity. Most use cases have only 1 planning entity class.

## Note

In real-time planning, problem facts can change during planning, because the problem itself changes. However, that doesn't make them planning entities.

In Drools Planner all problems facts and planning entities are plain old JavaBeans (POJO's). You can load them from a database (JDBC/JPA/JDO), an XML file, a data repository, a noSQL cloud, ...: Drools Planner doesn't care.

### 3.3.2. Problem fact

A problem fact is any JavaBean (POJO) with getters that does not change during planning. Implementing the interface `Serializable` is recommended (but not required). For example in n queens, the columns and rows are problem facts:

    public class Column implements Serializable {

        private int index;

        // ... getters
    }

    public class Row implements Serializable {

        private int index;

        // ... getters
    }

A problem fact can reference other problem facts of course:

    public class Course implements Serializable {

        private String code;

        private Teacher teacher; // Other problem fact
        private int lectureSize;
        private int minWorkingDaySize;

        private List<Curriculum> curriculumList; // Other problem facts
        private int studentSize;

        // ... getters
    }

A problem fact class does not require any Planner specific code. For example, you can reuse your domain classes, which might have JPA annotations.

## Note

Generally, better designed domain classes lead to simpler and more efficient score constraints. Therefore, when dealing with a messy legacy system, it can sometimes be worth it to convert the messy domain set into a planner specific POJO set first. For example: if your domain model has 2 `Teacher` instances for the same teacher that teaches at 2 different departments, it's hard to write a correct score constraint that constrains a teacher's spare time.

Alternatively, you can sometimes also introduce a cached problem fact to enrich the domain model for planning only.

### 3.3.3. Planning entity and planning variables

#### 3.3.3.1. Planning entity

A planning entity is a JavaBean (POJO) that changes during solving, for example a `Queen` that changes to another row. A planning problem has multiple planning entities, for example for a single n queens problem, each `Queen` is a planning entity. But there's usually only 1 planning entity class, for example the `Queen` class.

A planning entity class needs to be annotated with the `@PlanningEntity` annotation.

Each planning entity class has 1 or more planning variables. It usually also has 1 or more defining properties. For example in n queens, a `Queen` is defined by it's `Column` and has a planning variable `Row`. This means that a Queen's column never changes during solving, while it's row does change.

    @PlanningEntity
    public class Queen {

        private Column column;

        // Planning variables: changes during planning, between score calculations.
        private Row row;

        // ... getters and setters
    }

A planning entity class can have multiple planning variables. For example, a `Lecture` is defined by it's `Course` and it's index in that course (because 1 course has multiple lectures). Each `Lecture` needs to be scheduled into a `Period` and a `Room` so it has 2 planning variables (period and room). For example: the course Mathematics has 8 lectures per week, of which the first lecture is Monday morning at 08:00 in room 212.

    @PlanningEntity
    public class Lecture {

        private Course course;
        private int lectureIndexInCourse;

        // Planning variables: changes during planning, between score calculations.
        private Period period;
        private Room room;

        // ...
    }

The solver configuration also needs to be made aware of each planning entity class:

    <solver>
      ...
      <planningEntityClass>org.drools.planner.examples.nqueens.domain.Queen</planningEntityClass>
      ...
    </solver>

Some uses cases have multiple planning entity classes. For example: route freight and trains into railway network arcs, where each freight can use multiple trains over it's journey and each train can carry multiple freights per arc. Having multiple planning entity classes directly raises the implementation complexity of your use case.

## Note

Do not create unnecessary planning entity classes. This leads to difficult `Move` implementations and slower score calculation.

For example, do not create a planning entity class to hold the total free time of a teacher, which needs to be kept up to date as the `Lecture` planning entities change. Instead, calculate the free time in the score constraints and put the result per teacher into a logically inserted score object.

If historic data needs to be considered too, then create problem fact to hold the historic data up to, but not including, the planning window (so it doesn't change when a planning entity changes) and let the score constraints take it into account.

#### 3.3.3.2. Planning entity difficulty

Some optimization algorithms work more efficiently if they have an estimation of which planning entities are more difficult to plan. For example: in bin packing bigger items are harder to fit, in course scheduling lectures with more students are more difficult to schedule and in n queens the middle queens are more difficult.

Therefore, you can set a `difficultyComparatorClass` to the `@PlanningEntity` annotation:

    @PlanningEntity(difficultyComparatorClass = CloudProcessAssignmentDifficultyComparator.class)
    public class CloudProcessAssignment {
        // ...
    }

    public class CloudProcessAssignmentDifficultyComparator implements Comparator<CloudProcessAssignment> {

        public int compare(CloudProcessAssignment a, CloudProcessAssignment b) {
            return new CompareToBuilder()
                    .append(a.getCloudProcess().getRequiredMultiplicand(), b.getCloudProcess().getRequiredMultiplicand())
                    .append(a.getCloudProcess().getId(), b.getCloudProcess().getId())
                    .toComparison();
        }

    }

## Note

If you have multiple planning entity classes, the `difficultyComparatorClass` needs to implement a `Comparator` of a common superclass (for example `Comparator<Object>`) and be able to handle comparing instances of those different classes.

Alternatively, you can also set a `difficultyWeightFactoryClass` to the `@PlanningEntity` annotation, so you have access to the rest of the problem facts from the solution too:

    @PlanningEntity(difficultyWeightFactoryClass = QueenDifficultyWeightFactory.class)
    public class Queen {
        // ...
    }

    public interface PlanningEntityDifficultyWeightFactory {

        Comparable createDifficultyWeight(Solution solution, Object planningEntity);

    }

    public class QueenDifficultyWeightFactory implements PlanningEntityDifficultyWeightFactory {

        public Comparable createDifficultyWeight(Solution solution, Object planningEntity) {
            NQueens nQueens = (NQueens) solution;
            Queen queen = (Queen) planningEntity;
            int distanceFromMiddle = calculateDistanceFromMiddle(nQueens.getN(), queen.getColumnIndex());
            return new QueenDifficultyWeight(queen, distanceFromMiddle);
        }

        // ...

        public static class QueenDifficultyWeight implements Comparable<QueenDifficultyWeight> {

            private final Queen queen;
            private final int distanceFromMiddle;

            public QueenDifficultyWeight(Queen queen, int distanceFromMiddle) {
                this.queen = queen;
                this.distanceFromMiddle = distanceFromMiddle;
            }

            public int compareTo(QueenDifficultyWeight other) {
                return new CompareToBuilder()
                        // The more difficult queens have a lower distance to the middle
                        .append(other.distanceFromMiddle, distanceFromMiddle) // Decreasing
                        .append(queen.getColumnIndex(), other.queen.getColumnIndex())
                        .toComparison();
            }

        }

    }

None of the current planning variable state may be used to compare planning entities. They are likely to be `null` anyway. For example, a `Queen`'s `row` variable may not be used.

#### 3.3.3.3. Planning variable

A planning variable is a property (including getter and setter) on a planning entity. It changes during planning. For example, a `Queen`'s `row` property is a planning variable. Note that even though a `Queen`'s `row` property changes to another `Row` during planning, no `Row` instance itself is changed. A planning variable points to a planning value.

A planning variable getter needs to be annotated with the `@PlanningVariable` annotation. Furthermore, it needs a `@ValueRange*` annotation too.

    @PlanningEntity
    public class Queen {

        private Row row;

        // ...

        @PlanningVariable
        @ValueRangeFromSolutionProperty(propertyName = "rowList")
        public Row getRow() {
            return row;
        }

        public void setRow(Row row) {
            this.row = row;
        }

    }

#### 3.3.3.4. When is a planning entity initialized?

A planning entity is considered initialized if all it's planning variables are initialized.

By default, a planning variable is considered initialized if it's value is not `null`.

### 3.3.4. Planning value and planning value ranges

#### 3.3.4.1. Planning value

A planning value is a possible value for a planning variable. Usually, a planning value is problem fact, but it can also be any object, for example a double. Sometimes it can even be another planning entity.

A planning value range is the set of possible planning values for a planning variable. This set can be a discrete (for example row `1`, `2`, `3` or `4`) or continuous (for example any `double` between `0.0` and `1.0`). There are several ways to define the value range of a planning variable, each with it's own `@ValueRange*` annotation.

If `null` is a valid planning value, it should be included in the value range and the default way to detect uninitialized planning variables must be changed.

#### 3.3.4.2. Planning value range

##### 3.3.4.2.1. `ValueRangeFromSolutionProperty`

All instances of the same planning entity class share the same set of possible planning values for that planning variable. This is the most common way to configure a value range.

The `Solution` implementation has property which returns a `Collection`. Any value from that `Collection` is a possible planning value for this planning variable.

        @PlanningVariable
        @ValueRangeFromSolutionProperty(propertyName = "rowList")
        public Row getRow() {
            return row;
        }

    public class NQueens implements Solution<SimpleScore> {

        // ...

        public List<Row> getRowList() {
            return rowList;
        }

    }

##### 3.3.4.2.2. `ValueRangeFromPlanningEntityProperty`

Each planning entity has it's own set of possible planning values for a planning variable. For example, if a teacher can never teach in a room that does not belong to his department, lectures of that teacher can limit their room value range to the rooms of his department.

        @PlanningVariable
        @ValueRangeFromPlanningEntityProperty(propertyName = "possibleRoomList")
        public Room getRoom() {
            return room;
        }

        public List<Room> getPossibleRoomList() {
            return getCourse().getTeacher().getPossibleRoomList();
        }

Never use this to enforce a soft constraint (or even a hard constraint when the problem might not have a feasible solution). For example, when a teacher can not teach in a room that does not belong to his department unless there is no other way, the teacher should not be limited in his room value range.

## Note

By limiting the value range specifically of 1 planning entity, you are effectively making a build-in hard constraint. This can be a very good thing, as the number of possible solutions is severely lowered. But this can also be a bad thing because it takes away the freedom of the optimization algorithms to temporarily break such a hard constraint.

A planning entity should not use other planning entities to determinate it's value range. It would only try to solve the planning problem itself and interfere with the optimization algorithms.

##### 3.3.4.2.3. `ValueRangeUndefined`

Leaves the value range undefined. Some optimization algorithms do not support this value range.

        @PlanningVariable
        @ValueRangeUndefined
        public Row getRow() {
            return row;
        }

#### 3.3.4.3. Planning value strength

Some optimization algorithms work more efficiently if they have an estimation of which planning values are stronger, which means they are more likely to satisfy a planning entity. For example: in bin packing bigger containers are more likely to fit an item and in course scheduling bigger rooms are less likely to break the student capacity constraint.

Therefore, you can set a `strengthComparatorClass` to the `@PlanningVariable` annotation:

        @PlanningVariable(strengthComparatorClass = CloudComputerStrengthComparator.class)
        // ...
        public CloudComputer getCloudComputer() {
            // ...
        }

    public class CloudComputerStrengthComparator implements Comparator<CloudComputer> {

        public int compare(CloudComputer a, CloudComputer b) {
            return new CompareToBuilder()
                    .append(a.getMultiplicand(), b.getMultiplicand())
                    .append(b.getCost(), a.getCost()) // Descending (but this is debatable)
                    .append(a.getId(), b.getId())
                    .toComparison();
        }

    }

## Note

If you have multiple planning value classes in the same value range, the `strengthComparatorClass` needs to implement a `Comparator` of a common superclass (for example `Comparator<Object>`) and be able to handle comparing instances of those different classes.

Alternatively, you can also set a `strengthWeightFactoryClass` to the `@PlanningVariable` annotation, so you have access to the rest of the problem facts from the solution too:

        @PlanningVariable(strengthWeightFactoryClass = RowStrengthWeightFactory.class)
        // ...
        public Row getRow() {
            // ...
        }

    public interface PlanningValueStrengthWeightFactory {

        Comparable createStrengthWeight(Solution solution, Object planningValue);

    }

    public class RowStrengthWeightFactory implements PlanningValueStrengthWeightFactory {

        public Comparable createStrengthWeight(Solution solution, Object planningValue) {
            NQueens nQueens = (NQueens) solution;
            Row row = (Row) planningValue;
            int distanceFromMiddle = calculateDistanceFromMiddle(nQueens.getN(), row.getIndex());
            return new RowStrengthWeight(row, distanceFromMiddle);
        }

        // ...

        public static class RowStrengthWeight implements Comparable<RowStrengthWeight> {

            private final Row row;
            private final int distanceFromMiddle;

            public RowStrengthWeight(Row row, int distanceFromMiddle) {
                this.row = row;
                this.distanceFromMiddle = distanceFromMiddle;
            }

            public int compareTo(RowStrengthWeight other) {
                return new CompareToBuilder()
                        // The stronger rows have a lower distance to the middle
                        .append(other.distanceFromMiddle, distanceFromMiddle) // Decreasing (but this is debatable)
                        .append(row.getIndex(), other.row.getIndex())
                        .toComparison();
            }

        }

    }

None of the current planning variable state in any of the planning entities may be used to compare planning values. They are likely to be `null` anyway. For example, None of the `row` variables of any `Queen` may be used to determine the strength of a `Row`.

### 3.3.5. Planning problem and planning solution

#### 3.3.5.1. Planning problem instance

A dataset for a planning problem needs to be wrapped in a class for the `Solver` to solve. You must implement this class. For example in n queens, this in the `NQueens` class which contains a `Column` list, a `Row` list and a `Queen` list.

A planning problem is actually a unsolved planning solution or - stated differently - an uninitialized `Solution`. Therefor, that wrapping class must implement the `Solution` interface. For example in n queens, that `NQueens` class implements `Solution`, yet every `Queen` in a fresh `NQueens` class is assigned to a `Row` yet. So it's not a feasible solution. It's not even a possible solution. It's an uninitialized solution.

#### 3.3.5.2. The `Solution` interface

You need to present the problem as a `Solution` instance to the `Solver`. So you need to have a class that implements the `Solution` interface:

    public interface Solution<S extends Score> {

        S getScore();

        void setScore(S score);

        Collection<? extends Object> getProblemFacts();

        Solution<S> cloneSolution();

    }

For example, an `NQueens` instance holds a list of all columns, all rows and all `Queen` instances:

    public class NQueens implements Solution<SimpleScore> {

        private int n;

        // Problem facts

        private List<Column> columnList;

        private List<Row> rowList;

        // Planning entities

        private List<Queen> queenList;

        // ...

    }

#### 3.3.5.3. The getScore and setScore methods

A `Solution` requires a score property. The score property is `null` if the `Solution` is uninitialized or if the score has not yet been (re)calculated. The score property is usually typed to the specific `Score` implementation you use. For example, `NQueens` uses a `SimpleScore`:

    public class NQueens implements Solution<SimpleScore> {

        private SimpleScore score;

        public SimpleScore getScore() {

            return score;

        }

        public void setScore(SimpleScore score) {

            this.score = score;

        }

        // ...

    }

Most use cases use a `HardAndSoftScore` instead:

    public class CurriculumCourseSchedule implements Solution<HardAndSoftScore> {

        private HardAndSoftScore score;

        public HardAndSoftScore getScore() {
            return score;
        }

        public void setScore(HardAndSoftScore score) {
            this.score = score;
        }

        // ...
    }

See the Score calculation section for more information on the `Score` implementations.

#### 3.3.5.4. The getProblemFacts method

All objects returned by the `getProblemFacts()` method will be asserted into the drools working memory, so the score rules can access them. For example, `NQueens` just returns all `Column` and `Row` instances.

        public Collection<? extends Object> getProblemFacts() {

            List<Object> facts = new ArrayList<Object>();

            facts.addAll(columnList);

            facts.addAll(rowList);

            // Do not add the planning entity's (queenList) because that will be done automatically

            return facts;

        }

All planning entities are automatically inserted into the drools working memory. Do not add them in the method `getProblemFacts()`.

The method `getProblemFacts()` is not called much: at most only once per solver phase per solver thread.

#### 3.3.5.5. Cached problem fact

A cached problem fact is a problem fact that doesn't exist in the real domain model, but is calculated before the `Solver` really starts solving. The method `getProblemFacts()` has the chance to enrich the domain model with such cached problem facts, which can lead to simpler and faster score constraints.

For example in examination, a cache problem fact `TopicConflict` is created for every 2 `Topic`'s which share at least 1 `Student`.

        public Collection<? extends Object> getProblemFacts() {
            List<Object> facts = new ArrayList<Object>();
            // ...
            facts.addAll(calculateTopicConflictList());
            // ...
            return facts;
        }

        private List<TopicConflict> calculateTopicConflictList() {
            List<TopicConflict> topicConflictList = new ArrayList<TopicConflict>();
            for (Topic leftTopic : topicList) {
                for (Topic rightTopic : topicList) {
                    if (leftTopic.getId() < rightTopic.getId()) {
                        int studentSize = 0;
                        for (Student student : leftTopic.getStudentList()) {
                            if (rightTopic.getStudentList().contains(student)) {
                                studentSize++;
                            }
                        }
                        if (studentSize > 0) {
                            topicConflictList.add(new TopicConflict(leftTopic, rightTopic, studentSize));
                        }
                    }
                }
            }
            return topicConflictList;
        }

Any score constraint that needs to check if no 2 exams have a topic which share a student are being scheduled close together (depending on the constraint: at the same time, in a row or in the same day), can simply use the `TopicConflict` instance as a problem fact, instead of having to combine every 2 `Student` instances.

#### 3.3.5.6. The cloneSolution method

Most optimization algorithms use the `cloneSolution()` method to clone the solution each time they encounter a new best solution (so they can recall it later) or to work with multiple solutions in parallel.

The `NQueens` implementation only deep clones all `Queen` instances. When the original solution is changed during planning, by changing a `Queen`, the clone stays the same.

        /**

         * Clone will only deep copy the {@link #queenList}.

         */

        public NQueens cloneSolution() {

            NQueens clone = new NQueens();

            clone.id = id;

            clone.n = n;

            clone.columnList = columnList;

            clone.rowList = rowList;

            List<Queen> clonedQueenList = new ArrayList<Queen>(queenList.size());

            for (Queen queen : queenList) {

                clonedQueenList.add(queen.clone());

            }

            clone.queenList = clonedQueenList;

            clone.score = score;

            return clone;

        }

The `cloneSolution()` method should only deep clone the planning entities. Notice that the problem facts, such as `Column` and `Row` are normally not cloned: even their `List` instances are not cloned.

## Note

If you were to clone the problem facts too, then you'd have to make sure that the new planning entity clones also refer to the new problem facts clones used by the solution. For example, if you 'd clone all `Row` instances, then each `Queen` clone and the `NQueens` clone itself should refer to the same set of new `Row` clones.

#### 3.3.5.7. Build an uninitialized solution

Build a `Solution` instance to represent your planning problem, so you can set it on the `Solver` as the planning problem to solve. For example in n queens, an `NQueens` instance is created with the required `Column` and `Row` instances and every `Queen` set to a different `column` and every `row` set to `null`.

        private NQueens createNQueens(int n) {

            NQueens nQueens = new NQueens();

            nQueens.setId(0L);

            nQueens.setN(n);

            List<Column> columnList = new ArrayList<Column>(n);

            for (int i = 0; i < n; i++) {

                Column column = new Column();

                column.setId((long) i);

                column.setIndex(i);

                columnList.add(column);

            }

            nQueens.setColumnList(columnList);

            List<Row> rowList = new ArrayList<Row>(n);

            for (int i = 0; i < n; i++) {

                Row row = new Row();

                row.setId((long) i);

                row.setIndex(i);

                rowList.add(row);

            }

            nQueens.setRowList(rowList);

            List<Queen> queenList = new ArrayList<Queen>(n);

            long id = 0;

            for (Column column : columnList) {

                Queen queen = new Queen();

                queen.setId(id);

                id++;

                queen.setColumn(column);

                // Notice that we leave the PlanningVariable properties (row) on null

                queenList.add(queen);

            }

            nQueens.setQueenList(queenList);

            return nQueens;

        }

Figure 3.1. Uninitialized solution for the 4 queens puzzle

Usually, most of this data comes from your data layer, and your `Solution` implementation just aggregates that data and creates the uninitialized planning entity instances to plan:

            private void createLectureList(CurriculumCourseSchedule schedule) {
                List<Course> courseList = schedule.getCourseList();
                List<Lecture> lectureList = new ArrayList<Lecture>(courseList.size());
                for (Course course : courseList) {
                    for (int i = 0; i < course.getLectureSize(); i++) {
                        Lecture lecture = new Lecture();
                        lecture.setCourse(course);
                        lecture.setLectureIndexInCourse(i);
                        // Notice that we leave the PlanningVariable properties (period and room) on null
                        lectureList.add(lecture);
                    }
                }
                schedule.setLectureList(lectureList);
            }

## 3.4. Solver

### 3.4.1. The Solver interface

The `Solver` implementation will solve your planning problem. It's build based from a solver configuration, do not implement it yourself:

    public interface Solver {

        void setPlanningProblem(Solution planningProblem);

        void solve();

        Solution getBestSolution();

        // ...

    }

A Solver can only solve 1 problem instance at a time. A `Solver` should only be accessed from a single thread, except for the methods that are specifically javadocced as being thread-safe.

### 3.4.2. Solving a problem

Solving a problem is quite easy once you have:

- A `Solver` build from a solver configuration

- A `Solution` that represents the planning problem instance

Just set the planning problem, solve it and extract the best solution:

        solver.setPlanningProblem(planningProblem);

        solver.solve();

        Solution bestSolution = solver.getBestSolution();

For example in n queens, the method `getBestSolution()` will return an `NQueens` instance with every `Queen` assigned to a `Row`.

Figure 3.2. Best solution for the 4 queens puzzle in 8 ms (also an optimal solution)

The `solve()` method can take a long time (depending on the problem size and the solver configuration). The `Solver` will remember (actually clone) the best solution it encounters during its solving. Depending on a number factors (including problem size, how time the `Solver` has, the solver configuration, ...), that best solution will be a feasible or even an optimal solution.

## Note

The `Solution` instance given to the method `setPlanningProblem()` will be changed by the `Solver`, but it do not mistake it for the best solution.

The `Solution` instance returned by the method `getBestSolution()` will most likely be a clone of the instance given to the method `setPlanningProblem()`, which means it's a different instance.

## Note

The `Solution` instance given to the method `setPlanningProblem()` does not need to be uninitialized. It can be partially or fully initialized, which is likely in repeated planning.

### 3.4.3. Environment mode: Are there bugs in my code?

The environment mode allows you to detect common bugs in your implementation. It does not affect the logging level.

You can set the environment mode in the solver configuration XML file:

    <solver>

      <environmentMode>DEBUG</environmentMode>

      ...

    </solver>

A solver has a single `Random` instance. Some solver configurations use the `Random` instance a lot more than others. For example simulated annealing depends highly on random numbers, while tabu search only depends on it to deal with score ties. The environment mode influences the seed of that `Random` instance.

There are 4 environment modes:

#### 3.4.3.1. TRACE

The trace mode is reproducible (see the reproducible mode) and also turns on all assertions (such as assert that the delta based score is uncorrupted) to fail-fast on rule engine bugs.

The trace mode is very slow (because it doesn't rely on delta based score calculation).

#### 3.4.3.2. DEBUG

The debug mode is reproducible (see the reproducible mode) and also turns on most assertions (such as assert that the undo Move is uncorrupted) to fail-fast on a bug in your Move implementation, your score rule, ...

The debug mode is slow.

It's recommended to write a test case which does a short run of your planning problem with debug mode on.

#### 3.4.3.3. REPRODUCIBLE (default)

The reproducible mode is the default mode because it is recommended during development. In this mode, 2 runs in the same Planner version will execute the same code in the same order. Those 2 runs will have the same result, except if they rely on time based termination and they have a sufficiently large difference in allocated CPU time. This allows you to consistently reproduce bugs. It also allows you to benchmark certain refactorings (such as a score constraint optimization) fairly across runs.

The reproducible mode is not much slower than the production mode. If your production environment requires reproducability, use it in production too.

In practice, this mode uses the default random seed, and it also disables certain concurrency optimizations (such as work stealing).

#### 3.4.3.4. PRODUCTION

The production mode is the fastest and the most robust, but not reproducible. It is recommended for a production environment.

The random seed is different on every run, which makes it more robust against an unlucky random seed. An unlucky random seed gives a bad result on a certain data set with a certain solver configuration. Note that in most use cases the impact of the random seed is relatively low on the result (even with simulated annealing). An occasional bad result is far more likely caused by another issue (such as a score trap).

### 3.4.4. Logging level: What is the `Solver` doing?

The best way to illuminate the black box that is a `Solver`, is to play with the logging level:

- ERROR: When something is wrong, Planner fails fast: it throws a subclass of `RuntimeException` with a detailed message to the calling code, but does not log an error message to avoid duplicate log messages. If the calling code doesn't catch and eat that `RuntimeException`, a `Thread`'s default `ExceptionHandler` will log it as an error anyway.

- WARN: Log suspicous circumstances.

- INFO: Log every phase and the solver itself.

- DEBUG: Log every step of every phase.

- TRACE: Log every move of every step of every phase.

For example, set it to `DEBUG` logging, to see when the phases end and how fast steps are taken:

    INFO  Sovling started: time spend (0), score (null), new best score (null), random seed (0).
    DEBUG     Step index (0), time spend (1), score (0), initialized planning entity (col2@row0).
    DEBUG     Step index (1), time spend (3), score (0), initialized planning entity (col1@row2).
    DEBUG     Step index (2), time spend (4), score (0), initialized planning entity (col3@row3).
    DEBUG     Step index (3), time spend (5), score (-1), initialized planning entity (col0@row1).
    INFO  Phase construction heuristic finished: step total (4), time spend (6), best score (-1).
    DEBUG     Step index (0), time spend (10), score (-1),     best score (-1), accepted move size (12) for picked step (col1@row2 => row3).
    DEBUG     Step index (1), time spend (12), score (0), new best score (0), accepted move size (12) for picked step (col3@row3 => row2).
    INFO  Phase local search ended: step total (2), time spend (13), best score (0).
    INFO  Solving ended: time spend (13), best score (0), average calculate count per second (4846).

All time spends are in milliseconds.

Everything is logged to SLF4J, which is a simple logging facade that can delegate any log to Logback, Apache Commons Logging, Log4j or java.util.logging. Add a dependency to the logging adaptor for your logging framework of choice. If you're not using any logging framework yet, you can use Logback by adding this Maven dependency:

        <dependency>
          <groupId>ch.qos.logback</groupId>
          <artifactId>logback-classic</artifactId>
          <version>1.x</version>
        </dependency>

Configure the logging level on the package `org.drools.planner`. For example:

In Logback, configure it in your `logback.xml` file:

    <configuration>

      <logger name="org.drools.planner" level="debug"/>

      ...

    <configuration>

In Log4J, configure it in your `log4j.xml` file:

    <log4j:configuration xmlns:log4j="http://jakarta.apache.org/log4j/">

      <category name="org.drools.planner">
        <priority value="debug" />
      </category>

      ...

    </log4j:configuration>

## Chapter 4. Score calculation with a rule engine

## 4.1. Rule based score calculation

The score calculation (or fitness function) of a planning problem is based on constraints (such as hard constraints, soft constraints, rewards, ...). A rule engine, such as Drools Expert, makes it easy to implement those constraints as score rules.

Adding more constraints is easy and scalable (once you understand the DRL syntax). This allows you to add a bunch of soft constraint score rules on top of the hard constraints score rules with little effort and at a reasonable performance cost. For example, for a freight routing problem you could add a soft constraint to avoid the certain flagged highways during rush hour.

## 4.2. Choosing a Score implementation

### 4.2.1. The ScoreDefinition interface

The `ScoreDefinition` interface defines the score representation. The score must be a `Score` instance and the instance type (for example `DefaultHardAndSoftScore`) must be stable throughout the solver runtime.

The solver aims to find the solution with the highest score. The best solution is the solution with the highest score that it has encountered during its solving.

## Note

Most planning problems use negative scores, because they use negative constraints. The score is usually the sum of the weight of the negative constraints being broken, with an impossible perfect score of 0. This explains why the score of a solution of 4 queens is the negative of the number of queen couples which can attack each other.

Configure a `ScoreDefinition` in the solver configuration. You can implement a custom `ScoreDefinition`, although the build-in score definitions should suffice for most needs:

### 4.2.2. SimpleScore

The `SimpleScoreDefinition` defines the `Score` as a `SimpleScore` which has a single int value, for example `-123`.

        <scoreDefinition>

            <scoreDefinitionType>SIMPLE</scoreDefinitionType>

        </scoreDefinition>

### 4.2.3. HardAndSoftScore

The `HardAndSoftScoreDefinition` defines the `Score` as a `HardAndSoftScore` which has a hard int value and a soft int value, for example `-123hard/-456soft`.

        <scoreDefinition>

            <scoreDefinitionType>HARD_AND_SOFT</scoreDefinitionType>

        </scoreDefinition>

### 4.2.4. Implementing a custom Score

To implement a custom Score, you 'll also need to implement a custom `ScoreDefinition`. Extend `AbstractScoreDefinition` (preferable by copy pasting `HardAndSoftScoreDefinition` or `SimpleScoreDefinition`) and start from there.

Then hook you custom `ScoreDefinition` in your `SolverConfig.xml`:

        <scoreDefinition>

            <scoreDefinitionClass>org.drools.planner.examples.my.score.definition.MyScoreDefinition</scoreDefinitionClass>

        </scoreDefinition>

## 4.3. Defining the score rules source

There are 2 ways to define where your score rules live.

### 4.3.1. A scoreDrl resource on the classpath

This is the simplest way: the score rule live in a DRL file which is a resource on the classpath. Just add your score rules `*.drl` file in the solver configuration, for example:

        <scoreDrl>/org/drools/planner/examples/nqueens/solver/nQueensScoreRules.drl</scoreDrl>

You can add multiple `<scoreDrl>` entries if needed, but normally you 'll define all your score rules in 1 file.

### 4.3.2. A RuleBase (possibly defined by Guvnor)

If you prefer to build the `RuleBase` yourself or if you're combining Planner with Guvnor, you can set the `RuleBase` on the `XmlSolverFactory` before building the `Solver`:

        solverFactory.getSolverConfig().setRuleBase(ruleBase);

## 4.4. Implementing a score rule

The score calculation of a planning problem is based on constraints (such as hard constraints, soft constraints, rewards, ...). A rule engine, such as Drools, makes it easy to implement those constraints as score rules.

Here's an example of a constraint implemented as a score rule in such a DRL file:

    rule "multipleQueensHorizontal"
        when
            $q1 : Queen($id : id, $y : y);
            $q2 : Queen(id > $id, y == $y);
        then
            insertLogical(new UnweightedConstraintOccurrence("multipleQueensHorizontal", $q1, $q2));
    end

This score rule will fire once for every 2 queens with the same `y`. The `(id > $id)` condition is needed to assure that for 2 queens A and B, it can only fire for (A, B) and not for (B, A), (A, A) or (B, B). Let's take a closer look at this score rule on this solution of 4 queens:

In this solution the multipleQueensHorizontal score rule will fire for 6 queen couples: (A, B), (A, C), (A, D), (B, C), (B, D) and (C, D). Because none of the queens are on the same vertical or diagonal line, this solution will have a score of `-6`. An optimal solution of 4 queens has a score of `0`.

## Note

Notice that every score rule will relate to at least 1 planning entity class (directly or indirectly though a logically inserted fact).

This is normal: it would be a waste of time to write a score rule that only relates to problem facts, as the consequence will never change during planning, no matter what the possible solution.

## 4.5. Aggregating the score rules into the `Score`

A `ScoreCalculator` instance is asserted into the `WorkingMemory` as a global called `scoreCalculator`. Your score rules need to (direclty or indirectly) update that instance. Usually you 'll make a single rule as an aggregation of the other rules to update the score:

    global SimpleScoreCalculator scoreCalculator;

    rule "multipleQueensHorizontal"
        when
            $q1 : Queen($id : id, $y : y);
            $q2 : Queen(id > $id, y == $y);
        then
            insertLogical(new UnweightedConstraintOccurrence("multipleQueensHorizontal", $q1, $q2));
    end

    // multipleQueensVertical is obsolete because it is always 0

    rule "multipleQueensAscendingDiagonal"
        when
            $q1 : Queen($id : id, $ascendingD : ascendingD);
            $q2 : Queen(id > $id, ascendingD == $ascendingD);
        then
            insertLogical(new UnweightedConstraintOccurrence("multipleQueensAscendingDiagonal", $q1, $q2));
    end

    rule "multipleQueensDescendingDiagonal"
        when
            $q1 : Queen($id : id, $descendingD : descendingD);
            $q2 : Queen(id > $id, descendingD == $descendingD);
        then
            insertLogical(new UnweightedConstraintOccurrence("multipleQueensDescendingDiagonal", $q1, $q2));
    end

    rule "hardConstraintsBroken"
        when
            $occurrenceCount : Number() from accumulate(
                $unweightedConstraintOccurrence : UnweightedConstraintOccurrence(),
                count($unweightedConstraintOccurrence)
            );
        then
            scoreCalculator.setScore(- $occurrenceCount.intValue());
    end

Most use cases will also weigh their constraints differently, by multiplying the count of each score rule with its weight. For example in freight routing, you can make 5 broken "avoid crossroads" soft constraints count as much as 1 broken "avoid highways at rush hour" soft constraint. This allows your business analysts to easily tweak the score function as they see fit.

Here's an example from CurriculumCourse, where assiging a `Lecture` to a `Room` which is missing 2 seats is weighted equally bad as having 1 isolated `Lecture` in a `Curriculum`:

    // RoomCapacity: For each lecture, the number of students that attend the course must be less or equal
    // than the number of seats of all the rooms that host its lectures.
    // Each student above the capacity counts as 1 point of penalty.
    rule "roomCapacity"
        when
            ...
        then
            insertLogical(new IntConstraintOccurrence("roomCapacity", ConstraintType.NEGATIVE_SOFT,
                    ($studentSize - $capacity),
                    ...));
    end

    // CurriculumCompactness: Lectures belonging to a curriculum should be adjacent
    // to each other (i.e., in consecutive periods).
    // For a given curriculum we account for a violation every time there is one lecture not adjacent
    // to any other lecture within the same day.
    // Each isolated lecture in a curriculum counts as 2 points of penalty.
    rule "curriculumCompactness"
        when
            ...
        then
            insertLogical(new IntConstraintOccurrence("curriculumCompactness", ConstraintType.NEGATIVE_SOFT,
                    2,
                    ...));
    end

    // Accumulate soft constraints
    rule "softConstraintsBroken"
            salience -1 // Do the other rules first (optional, for performance)
        when
            $softTotal : Number() from accumulate(
                IntConstraintOccurrence(constraintType == ConstraintType.NEGATIVE_SOFT, $weight : weight),
                sum($weight)
            )
        then
            scoreCalculator.setSoftConstraintsBroken($softTotal.intValue());
    end

## 4.6. Delta based score calculation

It's recommended to use Drools in forward-chaining mode (which is the default behaviour), because this will create the effect of a delta based score calculation, instead of a full score calculation on each solution evaluation. For example, if a single queen A moves from y `0` to `3`, it won't bother to recalculate the "multiple queens on the same horizontal line" constraint between 2 queens if neither of those involved queens is queen A.

This is a huge performance and scalibility gain. Drools Planner gives you this huge scalibility gain without forcing you to write a very complicated delta based score calculation algorithm. Just let the Drools rule engine do the hard work.

Figure 4.1. Delta based score calculation for the 4 queens puzzle

The speedup due to delta based score calculation is huge, because the speedup is relative to the size of your planning problem (your n). By using score rules, you get that speedup without writing any delta code.

## 4.7. Tips and tricks

- If you know a certain constraint can never be broken, don't bother writing a score rule for it. For example in n queens, there is no "multipleQueensVertical" rule because a `Queen`'s `column` never changes and each `Solution` build puts each `Queen` on a different `column`. This tends to give a huge performance gain, not just because the score function is faster, but mainly because most `Solver` implementations will spend less time evaluating unfeasible solutions.

- Be watchfull for score traps. A score trap is a state in which several moves need to be done to resolve or lower the weight of a single constraint occurrence. Some examples of score traps:

  - If you need 2 doctors at each table, but you're only moving 1 doctor at a time, then the solver has no insentive to move a doctor to a table with no doctors. Punish a table with no doctors more then a table with only 1 doctor in your score function.

  - If you only add the table as a cause of the ConstraintOccurrence and forget the jobType (which is doctor or politician), then the solver has no insentive to move a docter to table which is short of a doctor and a politician.

- If you use tabu search, combine it with a `minimalAcceptedSelection` selector. Take some time to tweak the value of `minimalAcceptedSelection`.

- Verify that your score calculation happens in the correct `Number` type. If you're making the sum of integer values, don't let drools use Double's or your performance will hurt. The `Solver` will usually spend most of its execution time running the score function.

- Always remember that premature optimization is the root of all evil. Make sure your design is flexible enough to allow configuration based tweaking.

- Currently, don't allow drools to backward chain instead of forward chain, so avoid query's. It kills delta based score calculation (so it kills scalibility).

- Currently, don't allow drools to switch to MVEL mode, for performance.

- For optimal performance, use at least java 1.6 and always use server mode (`java -server`). We have seen performance increases of 30% by switching from java 1.5 to 1.6 and 50% by turning on server mode.

- If you're doing performance tests, always remember that the JVM needs to warm up. First load your `Solver` and do a short run, before you start benchmarking it.

In case you haven't figured it out yet: performance (and scalability) is very important for solving planning problems well. What good is a real-time freight routing solver that takes a day to find a feasible solution? Even small and innocent looking problems can hide an enormous problem size. For example, they probably still don't know the optimal solution of the traveling tournament problem for as little as 12 traveling teams.

## Chapter 5. Optimization algorithms

## 5.1. The size of real world problems

In number of possible solutions for a planning problem can be mind blowing. For example:

- 4 queens has `256` possible solutions (`4 ^ 4`) and 2 optimal solutions.

- 5 queens has `3125` possible solutions (`5 ^ 5`) and 1 optimal solution.

- 8 queens has `16777216` possible solutions (`8 ^ 8`) and 92 optimal solutions.

- 64 queens has more than `10^115` possible solutions (`64 ^ 64`).

- Most real-life planning problems have an incredible number of possible solutions and only 1 or a few optimal solutions.

For comparison: the minimal number of atoms in the known universe (10^80). As a planning problem gets bigger, the search space tends to blow up really fast. Adding only 1 extra planning entity or planning value can heavily multiply the running time of some algorithms.

An algorithm that checks every possible solution (even with pruning) can easily run for billions of years on a single real-life planning problem. What we really want is to find the best solution in the limited time at our disposal. Planning competitions (such as the International Timetabling Competition) show that local search variations (tabu search, simulated annealing, ...) usually perform best for real-world problems given real-world time limitations.

## 5.2. The secret sauce of Drools Planner

Drools Planner is the first framework to combine optimization algorithms (metaheuristics, ...) with score calculation by a rule engine such as Drools Expert. This combination turns out to be a very efficient, because:

- A rule engine such as Drools Expert is great for calculating the score of a solution of a planning problem. It make it easy and scalable to add additional soft or hard constraints such as "a teacher shouldn't teach more then 7 hours a day". It does delta based score calculation without any extra code. However it tends to be not suited to use to actually find new solutions.

- An optimization algorithm is great at finding new improving solutions for a planning problem, without necessarily brute-forcing every possibility. However it needs to know the score of a solution and offers no support in calculating that score efficiently.

## 5.3. Optimization algorithms overview

Table 5.1. Optimization algorithms overview

Algorithm | Scalable? | Optimal solution? | Needs little configuration? | Highly configurable? | Requires initialized solution?
--- | --- | --- | --- | --- | ---
Exact algorithms |  |  |  |  |
Brute force | 0/5 | 5/5 - Guaranteed | 5/5 | 0/5 | No
Branch and bound | 0/5 | 5/5 - Guaranteed | 4/5 | 1/5 | No
Construction heuristics |  |  |  |  |
First Fit | 5/5 | 1/5 - Stops after initialization | 5/5 | 1/5 | No
First Fit Decreasing | 5/5 | 2/5 - Stops after initialization | 4/5 | 2/5 | No
Best Fit | 5/5 | 2/5 - Stops after initialization | 4/5 | 2/5 | No
Best Fit Decreasing | 5/5 | 2/5 - Stops after initialization | 4/5 | 2/5 | No
Cheapest Insertion | 3/5 | 2/5 - Stops after initialization | 5/5 | 2/5 | No
Metaheuristics |  |  |  |  |
Local search |  |  |  |  |
Hill-climbing | 4/5 | 2/5 - Gets stuck in local optima | 3/5 | 3/5 | Yes
Tabu search | 4/5 | 4/5 | 3/5 | 5/5 | Yes
Simulated annealing | 4/5 | 4/5 | 2/5 | 5/5 | Yes
Evolutionary algorithms |  |  |  |  |
Evolutionary strategies | 4/5 | ?/5 | ?/5 | ?/5 | Yes
Genetic algorithms | 4/5 | ?/5 | ?/5 | ?/5 | Yes

If you want to learn more about metaheuristics, read the free book Essentials of Metaheuristics or Clever Algorithms.

## 5.4. Which optimization algorithms should I use?

The best optimization algorithms configuration for your use case depends heavily on your use case. Nevertheless, this vanilla recipe will get you into the game with a pretty good configuration, probably much better than what you're used to.

Start with a quick configuration that involves little or no configuration and optimization code:

1. First Fit

Next, implement planning entity difficulty comparison and turn it into:

1. First Fit Decreasing

Next, implement moves and add tabu search behind it:

1. First Fit Decreasing

2. Tabu search (use property tabu or move tabu)

At this point the free lunch is over. The return on invested time lowers. The result is probably already more than good enough.

But you can do even better, at a lower return on invested time. Use the Benchmarker and try a couple of simulated annealing configurations:

1. First Fit Decreasing

2. Simulated annealing (try several starting temperatures)

And combine them with tabu search:

1. First Fit Decreasing

2. Simulated annealing (relatively long time)

3. Tabu search (relatively short time)

If you have time, continue experimenting even further. Blog about your experiments!

## 5.5. `SolverPhase`

A `Solver` can use multiple optimization algorithms in sequence. Each optimization algorithm is represented by a `SolverPhase`. There is never more than 1 `SolverPhase` solving at the same time.

## Note

Some `SolverPhase` implementations can combine techniques from multiple optimization algorithms, but they are still just 1 `SolverPhase`. For example: a local search `SolverPhase` can do simulated annealing with property tabu.

Here's a configuration that runs 3 phases in sequence:

    <solver>
      ...
      <constructionHeuristic>
        ... <!-- First phase: First Fit decreasing -->
      </constructionHeuristic>
      <localSearch>
        ... <!-- Second phase: Simulated annealing -->
      </localSearch>
      <localSearch>
        ... <!-- Third phase: Tabu search -->
      </localSearch>
    </solver>

The solver phases are run in the order defined by solver configuration. When the first phase terminates, the second phase starts, and so on. When the last phase terminates, the `Solver` terminates.

Some phases (especially construction heuristics) will terminate automatically. Other phases (especially metaheuristics) will only terminate if the phase is configured to terminate:

    <solver>
      ...
      <termination><!-- Solver termination -->
        <maximumSecondsSpend>90</maximumSecondsSpend>
      </termination>
      <localSearch>
        <termination><!-- Phase termination -->
          <maximumSecondsSpend>60</maximumSecondsSpend><!-- Give the next phase a chance to run too, before the Solver terminates -->
        </termination>
        ...
      </localSearch>
      <localSearch>
        ...
      </localSearch>
    </solver>

If the `Solver` terminates (before the last phase terminates itself), the current phase is terminated and all subsequent phases won't run.

## 5.6. Termination

Not all phases terminate automatically and sometimes you don't want to wait that long anyway. A `Solver` can be terminated synchronously by up-front configuration or asynchronously from another thread.

Especially metaheuristics phases will need to be told when to stop solving. This can be because of a number of reasons: the time is up, the perfect score has been reached, ... The only thing you can't depend on is on finding the optimal solution (unless you know the optimal score), because a metaheuristics algorithm generally doesn't know it when it finds the optimal solution. For real-life problems this doesn't turn out to be much of a problem, because finding the optimal solution could take billions of years, so you 'll want to terminate sooner anyway. The only thing that matters is finding the best solution in the available time.

For synchronous termination, configure a `Termination` on a `Solver` or a `SolverPhase` when it needs to stop. You can implement your own `Termination`, but the build-in implementations should suffice for most needs. Every `Termination` can calculate a time gradient (needed for some optimization algorithms), which is a ratio between the time already spend solving and the estimated entire solving time of the `Solver` or `SolverPhase`.

### 5.6.1. TimeMillisSpendTermination

Terminates when an amount of time has been reached:

        <termination>

            <maximumTimeMillisSpend>500</maximumTimeMillisSpend>

        </termination>

        <termination>

            <maximumSecondsSpend>10</maximumSecondsSpend>

        </termination>

        <termination>

            <maximumMinutesSpend>5</maximumMinutesSpend>

        </termination>

        <termination>

            <maximumHoursSpend>1</maximumHoursSpend>

        </termination>

## Note

If you use this `Termination`, you will most likely sacrifice perfect reproducibility (even with `environmentMode` `REPRODUCIBLE`) because of an available CPU time difference:

- The available CPU time influences the number of steps that can be taken, which might be a few more or less.

- The `Termination` might produce slightly different time gradient values, which will send time gradient based algorithms (such as simulated annealing) on a radically different path.

### 5.6.2. ScoreAttainedTermination

Terminates when a certain score has been reached. You can use this `Termination` if you know the perfect score, for example for 4 queens:

        <termination>

            <scoreAttained>0</scoreAttained>

        </termination>

For a planning problem with hard and soft constraints, it could look like this:

        <termination>

            <scoreAttained>0hard/-5000soft</scoreAttained>

        </termination>

You can use this `Termination` to terminate once it reaches a feasible solution.

### 5.6.3. StepCountTermination

Terminates when an amount of steps has been reached:

        <termination>

            <maximumStepCount>100</maximumStepCount>

        </termination>

This `Termination` can only be used for a `SolverPhase`, not for the `Solver` itself.

### 5.6.4. UnimprovedStepCountTermination

Terminates when the best score hasn't improved in a number of steps:

        <termination>

            <maximumUnimprovedStepCount>100</maximumUnimprovedStepCount>

        </termination>

If it hasn't improved recently, it's probably not going to improve soon anyway and it's not worth the effort to continue. We have observed that once a new best solution is found (even after a long time of no improvement on the best solution), the next few step tend to improve the best solution too.

### 5.6.5. Combining Terminations

Terminations can be combined, for example: terminate after 100 steps or if a score of 0 has been reached:

        <termination>

            <terminationCompositionStyle>OR</terminationCompositionStyle>

            <maximumStepCount>100</maximumStepCount>

            <scoreAttained>0</scoreAttained>

        </termination>

Alternatively you can use AND, for example: terminate after reaching a feasible score of at least -100 and no improvements in 5 steps:

        <termination>

            <terminationCompositionStyle>AND</terminationCompositionStyle>

            <maximumUnimprovedStepCount>5</maximumUnimprovedStepCount>

            <scoreAttained>-100</scoreAttained>

        </termination>

This example ensures it doesn't just terminate after finding a feasible solution, but also completes any obvious improvements on that solution before terminating.

### 5.6.6. Asynchronous termination from another thread

Sometimes you 'll want to terminate a Solver early from another thread, for example because a user action or a server restart. That cannot be configured by a `Termination` as it's impossible to predict when and if it will occur. Therefor the `Solver` interface has these 2 thread-safe methods:

    public interface Solver {

        // ...

        boolean terminateEarly();

        boolean isTerminateEarly();

    }

If you call the `terminateEarly()` method from another thread, the `Solver` will terminate at its earliest convenience and the `solve()` method will return in the original `Solver` thread.

## 5.7. SolverEventListener

Each time a new best solution is found, the `Solver` fires a `BestSolutionChangedEvent`.

To listen to such events, add a `SolverEventListener` to the `Solver`:

    public interface Solver {

        // ...

        void addEventListener(SolverEventListener eventListener);

        void removeEventListener(SolverEventListener eventListener);

    }

## 5.8. Custom SolverPhase

Between phases or before the first phase, you might want to execute a custom action on the `Solution` to get a better score. Yet you'll still want to reuse the score calculation. For example, to implement a custom construction heuristic without implementing an entire `SolverPhase`.

## Note

Most of the time, a custom construction heuristic is not worth the hassle. The supported constructions heuristics are configurable (so you can tweak them with the `Benchmarker`), `Termination` aware and support partially initialized solutions too.

Implement the `CustomSolverPhaseCommand` interface:

    public interface CustomSolverPhaseCommand {

        void changeWorkingSolution(SolutionDirector solutionDirector);

    }

For example:

    public class ExaminationSolutionInitializer implements CustomSolverPhaseCommand {

        public void changeWorkingSolution(SolutionDirector solutionDirector) {
            Examination examination = (Examination) solutionDirector.getWorkingSolution();
            for (Exam exam : examination.getExamList()) {
                Score unscheduledScore = solutionDirector.calculateScoreFromWorkingMemory();
                ...
                for (Period period : examination.getPeriodList()) {
                    exam.setPeriod(period)
                    workingMemory.update(examHandle, exam);
                    Score score = solutionDirector.calculateScoreFromWorkingMemory();
                    ...
                }
                ...
            }
        }

    }

## Warning

Any change on the planning entities in a `CustomSolverPhaseCommand` must be told to the `WorkingMemory` of `solutionDirector.getWorkingMemory()`.

## Warning

Do not change any of the planning facts in a `CustomSolverPhaseCommand`. That will corrupt the `Solver` because any previous score or solution was for a different problem. If you want to do that, see repeated planning and real-time planning instead.

And configure it like this:

    <solver>
      ...
      <customSolverPhase>
        <customSolverPhaseCommandClass>org.drools.planner.examples.examination.solver.solution.initializer.ExaminationSolutionInitializer</customSolverPhaseCommandClass>
      </customSolverPhase>
      ... <!-- Other phases -->
    </solver>

It's possible to configure multiple `customSolverPhaseCommandClass` instances, which will be run in sequence.

## Note

If the changes of a `CustomSolverPhaseCommand` don't result in a better score, the best solution won't be changed (so effectively nothing will have changed for the next `SolverPhase` or `CustomSolverPhaseCommand`). TODO: we might want to change this behaviour?

## Note

If the `Solver` or `SolverPhase` wants to terminate while a `CustomSolverPhaseCommand` is still running, it will wait to terminate until the `CustomSolverPhaseCommand` is done.

## Chapter 6. Exact methods

## 6.1. Overview

Exact methods will always find the global optimum and recognize it too. That being said, they don't scale (not even beyond toy problems) and are therefor mostly useless.

## 6.2. Brute Force

### 6.2.1. Algorithm description

The brute force algorithm creates and evaluates every possible solution.

Notice that it creates a search tree that explodes as the problem size increases. Brute force is mostly unusable for a real-world problem due to time limitations.

### 6.2.2. Configuration

Using the brute force algorithm is easy:

    <solver>
      ...
      <bruteForce>
      </bruteForce>
    </solver>

## 6.3. Branch and bound

### 6.3.1. Algorithm description

Branch and bound is an improvement over brute force, as it prunes away subsets of solutions which cannot have a better solution than the best solution already found at that point.

Notice that it (like brute force) creates a search tree that explodes (but less than brute force) as the problem size increases. Branch and bound is mostly unusable for a real-world problem due to time limitations.

It can determine a lower bound of problem. A lower bound is a score which is proven to be higher than the optimal score of a problem. So it gives an indication of the quality of any best solution found for that problem: the closer to best score is to the lower bound, the better.

### 6.3.2. Configuration

Branch and bound is not yet implemented in Drools Planner. Patches welcome.

## Chapter 7. Construction heuristics

## 7.1. Overview

A construction heuristic builds a pretty good initial solution in a finite length of time. Its solution isn't always feasible, but it finds it fast and metaheuristics can finish the job.

Construction heuristics terminate automatically, so there's usually no need to configure a `Termination` on the construction heuristic phase specifically.

## 7.2. First Fit

### 7.2.1. Algorithm description

The First Fit algorithm cycles through all the planning entity (in default order), initializing 1 planning entity at a time. It assigns the planning entity to the best available planning value, taking the already initialized planning entities into account. It terminates when all planning entities have been initialized. It never changes a planning entity after it has been assigned.

Notice that it starts with putting `Queen` A into row 0 (and never moving it later), which makes it impossible reach the optimal solution. Suffixing this construction heuristic with metaheurstics can remedy that.

### 7.2.2. Configuration

Configure this `SolverPhase`:

      <constructionHeuristic>
        <constructionHeuristicType>FIRST_FIT</constructionHeuristicType>
        <!-- Speedup that can be applied to most, but not all use cases: -->
        <!-- <constructionHeuristicPickEarlyType>FIRST_LAST_STEP_SCORE_EQUAL_OR_IMPROVING</constructionHeuristicPickEarlyType> -->
      </constructionHeuristic>

## Note

The `constructionHeuristicPickEarlyType` of `FIRST_LAST_STEP_SCORE_EQUAL_OR_IMPROVING` is a big speedup, which should be applied when initializing a planning entity can only make the score lower or equal. So if:

- There are no positive constraints.

- There is no negative constraint that can stop been broken by adding a planning entity (except if another negative constraint gets broken which outweighs it the first negative constraint).

If that is not the case, then it can still be good to apply it in some cases, but not in most cases. Use the `Benchmarker` to decide.

## 7.3. First Fit Decreasing

### 7.3.1. Algorithm description

Like `First Fit`, but assigns the more difficult planning entities first, because they are less likely to fit in the leftovers. So it sorts the planning entities on decreasing difficulty.

Requires the model to support planning entity difficulty comparison.

## Note

One would expect that this algorithm always performs better than `First Fit`. That's not always the case, but usually is.

### 7.3.2. Configuration

Configure this `SolverPhase`:

      <constructionHeuristic>
        <constructionHeuristicType>FIRST_FIT_DECREASING</constructionHeuristicType>
        <!-- Speedup that can be applied to most, but not all use cases: -->
        <!-- <constructionHeuristicPickEarlyType>FIRST_LAST_STEP_SCORE_EQUAL_OR_IMPROVING</constructionHeuristicPickEarlyType> -->
      </constructionHeuristic>

## 7.4. Best Fit

### 7.4.1. Algorithm description

Like `First Fit`, but uses the weaker planning values first, because the strong planning values are more likely to be able to accomodate later planning entities. So it sorts the planning values on increasing strength.

Requires the model to support planning value strength comparison.

## Note

One would expect that this algorithm always performs better than `First Fit`. That's not always the case.

### 7.4.2. Configuration

Configure this `SolverPhase`:

      <constructionHeuristic>
        <constructionHeuristicType>BEST_FIT</constructionHeuristicType>
        <!-- Speedup that can be applied to most, but not all use cases: -->
        <!-- <constructionHeuristicPickEarlyType>FIRST_LAST_STEP_SCORE_EQUAL_OR_IMPROVING</constructionHeuristicPickEarlyType> -->
      </constructionHeuristic>

## 7.5. Best Fit Decreasing

### 7.5.1. Algorithm description

Combines `First Fit Decreasing` and `Best Fit`. So it sorts the planning entities on decreasing difficulty and the planning values on increasing strength.

Requires the model to support planning entity difficulty comparison and planning value strength comparison.

## Note

One would expect that this algorithm always performs better than `First Fit`, `First Fit Decreasing` and `Best Fit`. That's not always the case.

### 7.5.2. Configuration

Configure this `SolverPhase`:

      <constructionHeuristic>
        <constructionHeuristicType>BEST_FIT_DECREASING</constructionHeuristicType>
        <!-- Speedup that can be applied to most, but not all use cases: -->
        <!-- <constructionHeuristicPickEarlyType>FIRST_LAST_STEP_SCORE_EQUAL_OR_IMPROVING</constructionHeuristicPickEarlyType> -->
      </constructionHeuristic>

## 7.6. Cheapest insertion

### 7.6.1. Algorithm description

TODO

### 7.6.2. Configuration

TODO Not implemented yet.

## Chapter 8. Local search solver

## 8.1. Overview

Local search starts from an initial solution and evolves that single solution into a mostly better and better solution. It uses a single search path of solutions, not a search tree. At each solution in this path it evaluates a number of moves on the solution and applies the most suitable move to take the step to the next solution. It does that for high number of iterations until its terminated (usually because its time has run out).

Local search acts a lot like a human planner: it uses a single search path and moves facts around to find a good feasible solution. Therefore it's pretty natural to implement.

Local search needs to start from an initialized solution, therefor it's recommended to configure a construction heuristic solver phase before it.

## 8.2. Hill climbing (simple local search)

### 8.2.1. Algorithm description

Hill climbing can easily get stuck in a local optima, but improvements (such as tabu search and simulated annealing) address this problem.

## 8.3. Tabu search

### 8.3.1. Algorithm description

Like hill climbing, but maintains a tabu list to avoid getting stuck in local optima. See Tabu Search accepter below.

## 8.4. Simulated annealing

### 8.4.1. Algorithm description

See Simulated Annealing accepter below.

## 8.5. About neighborhoods, moves and steps

### 8.5.1. A move

A move is the change from a solution A to a solution B. For example, below you can see a single move on the starting solution of 4 queens that moves a single queen to another row:

Figure 8.1. A single move (4 queens example)

A move can have a small or large impact. In the above example, the move of queen C0 to C2 is a small move. Some moves are the same move type. These are some possibilities for move types in n queens:

- Move a single queen to another row. This is a small move. For example, move queen C0 to C2.

- Move all queens a number of rows down or up. This a big move.

- Move a single queen to another column. This is a small move. For example, move queen C2 to A0 (placing it on top of queen A0).

- Add a queen to the board at a certain row and column.

- Remove a queen from the board.

Because we have decided that all queens will be on the board at all times and each queen has an appointed column (for performance reasons), only the first 2 move types are usable in our example. Furthermore, we 're only using the first move type in the example because we think it gives the best performance, but you are welcome to prove us wrong.

Each of your move types will be an implementation of the `Move` interface:

    public interface Move {

        boolean isMoveDoable(EvaluationHandler evaluationHandler);

        Move createUndoMove(EvaluationHandler evaluationHandler);

        void doMove(EvaluationHandler evaluationHandler);

    }

Let's take a look at the `Move` implementation for 4 queens which moves a queen to a different row:

    public class RowChangeMove implements Move {

        private Queen queen;

        private Row toRow;

        public RowChangeMove(Queen queen, Row toRow) {

            this.queen = queen;

            this.toRow = toRow;

        }

        // ... see below

    }

An instance of `RowChangeMove` moves a queen from its current row to a different row.

Drools Planner calls the `doMove(WorkingMemory)` method to do a move. The `Move` implementation must notify the working memory of any changes it does on the solution facts:

        public void doMove(WorkingMemory workingMemory) {

            FactHandle queenHandle = workingMemory.getFactHandle(queen);

            queen.setRow(toRow);

            workingMemory.update(queenHandle, queen); // after changes are made

        }

You need to call the `workingMemory.update(FactHandle, Object)` method after modifying the fact. Note that you can alter multiple facts in a single move and effectively create a big move (also known as a coarse-grained move).

Drools Planner automatically filters out non doable moves by calling the `isDoable(WorkingMemory)` method on a move. A non doable move is:

- A move that changes nothing on the current solution. For example, moving queen B0 to row 0 is not doable, because it is already there.

- A move that is impossible to do on the current solution. For example, moving queen B0 to row 10 is not doable because it would move it outside the board limits.

In the n queens example, a move which moves the queen from its current row to the same row isn't doable:

        public boolean isMoveDoable(WorkingMemory workingMemory) {

            return !ObjectUtils.equals(queen.getRow(), toRow);

        }

Because we won't generate a move which can move a queen outside the board limits, we don't need to check it. A move that is currently not doable can become doable on a later solution.

Each move has an undo move: a move (usually of the same type) which does the exact opposite. In the above example the undo move of C0 to C2 would be the move C2 to C0. An undo move can be created from a move, but only before the move has been done on the current solution.

        public Move createUndoMove(WorkingMemory workingMemory) {

            return new RowChangeMove(queen, queen.getRow());

        }

Notice that if C0 would have already been moved to C2, the undo move would create the move C2 to C2, instead of the move C2 to C0.

The local search solver can do and undo a move more than once, even on different (successive) solutions.

A move must implement the `equals()` and `hashcode()` methods. 2 moves which make the same change on a solution, must be equal.

        public boolean equals(Object o) {

            if (this == o) {

                return true;

            } else if (o instanceof RowChangeMove) {

                RowChangeMove other = (RowChangeMove) o;

                return new EqualsBuilder()

                        .append(queen, other.queen)

                        .append(toRow, other.toRow)

                        .isEquals();

            } else {

                return false;

            }

        }

        public int hashCode() {

            return new HashCodeBuilder()

                    .append(queen)

                    .append(toRow)

                    .toHashCode();

        }

In the above example, the `Queen` class uses the default `Object` `equal()` and `hashcode()` implementations. Notice that it checks if the other move is an instance of the same move type. This is important because a move will be compared to a move with another move type if you're using more then 1 move type.

It's also recommended to implement the `toString()` method as it allows you to read Drools Planner's logging more easily:

        public String toString() {

            return queen + " => " + toRow;

        }

Now that we can make a single move, let's take a look at generating moves.

### 8.5.2. Move generation

At each solution, local search will try all possible moves and pick the best move to change to the next solution. It's up to you to generate those moves. Let's take a look at all the possible moves on the starting solution of 4 queens:

Figure 8.2. Possible moves at step 0 (4 queens example)

As you can see, not all the moves are doable. At the starting solution we have 12 doable moves (`n * (n - 1)`), one of which will be move which changes the starting solution into the next solution. Notice that the number of possible solutions is 256 (`n ^ n`), much more that the amount of doable moves. Don't create a move to every possible solution. Instead use moves which can be sequentially combined to reach every possible solution.

It's highly recommended that you verify all solutions are connected by your move set. This means that by combining a finite number of moves you can reach any solution from any solution. Otherwise you're already excluding solutions at the start. Especially if you're using only big moves, you should check it. Just because big moves outperform small moves in a short test run, it doesn't mean that they will outperform them in a long test run.

You can mix different move types. Usually you're better off preferring small (fine-grained) moves over big (course-grained) moves because the score delta calculation will pay off more. However, as the traveling tournament example proves, if you can remove a hard constraint by using a certain set of big moves, you can win performance and scalability. Try it yourself: run both the simple (small moves) and the smart (big moves) version of the traveling tournament example. The smart version evaluates a lot less unfeasible solutions, which enables it to outperform and outscale the simple version.

Move generation currently happens with a `MoveFactory`:

    public class NQueensMoveFactory extends CachedMoveListMoveFactory {

        public List<Move> createMoveList(Solution solution) {

            NQueens nQueens = (NQueens) solution;

            List<Move> moveList = new ArrayList<Move>();

            for (Queen queen : nQueens.getQueenList()) {

                for (int y : nQueens.getRowList()) {

                    moveList.add(new YChangeMove(queen, y));

                }

            }

            return moveList;

        }

    }

Future versions might also support move generation by DRL.

### 8.5.3. Generic MoveFactory

To get started quickly, Planner comes with a few build-in `MoveFactory` implementations:

- `GenericChangeMoveFactory`: A `GenericChangeMove` changes 1 planning variable of 1 planning entity to another planning value. For example: Given course C1 in room R1 and period P1, change its room to room R2.

- `GenericSwapMoveFactory`: A `GenericSwapMove` swaps all the planning variables of 2 planning entities. For example: Given course C1 in room R1 and period P1 and course C2 in room R2 and period P2, put course C1 in room R2 and period P2 and put course C2 in room R1 and period P1.

- `GenericSwapPillarMoveFactory`: A `GenericSwapPillarMove` swaps all the planning variables of 2 pillars. A pillar is a set of planning entities that have the same planning values for all those planning variables. For example: Given course C10, course 11 and course 12 in room R1 and period P1 and course C20 in room R2 and period P2, put course C10, course 11 and course 12 in room R2 and period P2 and put course C20 in room R1 and period P1.

To use one or multiple build-in `MoveFactory` implementations, configure it as a `Selector`:

      <localSearch>
        <selector>
          <selector>
            <moveFactoryClass>org.drools.planner.core.move.generic.GenericChangeMoveFactory</moveFactoryClass>
          </selector>
          <selector>
            <moveFactoryClass>org.drools.planner.core.move.generic.GenericSwapMoveFactory</moveFactoryClass>
          </selector>
        </selector>
        ...
      </localSearch>

They are slightly slower than a custom implementation, but equally scalable.

### 8.5.4. A step

A step is the winning move. The local search solver tries every move on the current solution and picks the best accepted move as the step:

Figure 8.3. Decide the next step at step 0 (4 queens example)

Because the move B0 to B3 has the highest score (`-3`), it is picked as the next step. Notice that C0 to C3 (not shown) could also have been picked because it also has the score `-3`. If multiple moves have the same highest score, one is picked randomly, in this case B0 to B3.

The step is made and from that new solution, the local search solver tries all the possible moves again, to decide the next step after that. It continually does this in a loop, and we get something like this:

Figure 8.4. All steps (4 queens example)

Notice that the local search solver doesn't use a search tree, but a search path. The search path is highlighted by the green arrows. At each step it tries all possible moves, but unless it's the step, it doesn't investigate that solution further. This is one of the reasons why local search is very scalable.

As you can see, the local search solver solves the 4 queens problem by starting with the starting solution and make the following steps sequentially:

1. B0 to B3

2. D0 to B2

3. A0 to B1

If we turn on `DEBUG` logging for the category `org.drools.planner`, then those steps are shown into the log:

    INFO  Solving started: time spend (0), score (-6), new best score (-6), random seed (0).
    DEBUG     Step index (0), time spend (20), score (-3), new best score (-3), accepted move size (12) for picked step (col1@row0 => row3).
    DEBUG     Step index (1), time spend (31), score (-1), new best score (-1), accepted move size (12) for picked step (col0@row0 => row1).
    DEBUG     Step index (2), time spend (40), score (0), new best score (0), accepted move size (12) for picked step (col3@row0 => row2).
    INFO  Phase local search ended: step total (3), time spend (41), best score (0).
    INFO  Solving ended: time spend (41), best score (0), average calculate count per second (1780).

Notice that the logging uses the `toString()` method of our `Move` implementation: `col1@row0 => row3`.

The local search solver solves the 4 queens problem in 3 steps, by evaluating only 37 possible solutions (3 steps with 12 moves each + 1 starting solution), which is only fraction of all 256 possible solutions. It solves 16 queens in 31 steps, by evaluating only 7441 out of 18446744073709551616 possible solutions. Note: with construction heurstistics it's even a lot more efficient.

### 8.5.5. Getting stuck in local optima

A hill climber always takes improving moves. This may seem like a good thing, but it's not. It suffers from a number of problems:

- It can get stuck in a local optimum. For example if it reaches a solution X with a score -1 and there is no improving move, it is forced to take a next step that leads to a solution Y with score -2, after that however, it's very real that it will pick the step back to solution X with score -1. It will then start looping between solution X and Y.

- It can start walking in its own footsteps, picking the same next step at every step.

Of course Drools Planner implements better local searches, such as tabu search and simulated annealing which can avoid these problems. We recommend to never use a hill climber, unless you're absolutely sure there are no local optima in your planning problem.

## 8.6. Deciding the next step

The local search solver decides the next step with the aid of 3 configurable components:

- A selector which selects (or generates) the possible moves of the current solution.

- An acceptor which filters out unacceptable moves. It can also weigh a move it accepts.

- A forager which gathers all accepted moves and picks the next step from them.

Figure 8.5. Decide the next step at step 0 (4 queens example)

In the above example the selector generated the moves shown with the blue lines, the acceptor accepted all of them and the forager picked the move B0 to B3.

If we turn on `TRACE` logging for the category `org.drools.planner`, then the decision making is shown in the log:

    INFO  Solver started: time spend (0), score (-6), new best score (-6), random seed (0).
    TRACE         Ignoring not doable move (col0@row0 => row0).
    TRACE         Move score (-4), accepted (true) for move (col0@row0 => row1).
    TRACE         Move score (-4), accepted (true) for move (col0@row0 => row2).
    TRACE         Move score (-4), accepted (true) for move (col0@row0 => row3).
    ...
    TRACE         Move score (-3), accepted (true) for move (col1@row0 => row3).
    ...
    TRACE         Move score (-3), accepted (true) for move (col2@row0 => row3).
    ...
    TRACE         Move score (-4), accepted (true) for move (col3@row0 => row3).
    DEBUG     Step index (0), time spend (6), score (-3), new best score (-3), accepted move size (12) for picked step (col1@row0 => row3).
    ...

Because the last solution can degrade (especially in tabu search and simulated annealing), the `Solver` remembers the best solution it has encountered through the entire search path. Each time the current solution is better than the last best solution, the current solution is cloned and referenced as the new best solution.

### 8.6.1. Selector

A selector is currently based on a `MoveFactory`.

      <selector>

        <moveFactoryClass>org.drools.planner.examples.nqueens.solver.NQueensMoveFactory</moveFactoryClass>

      </selector>

You're not obligated to generate the same set of moves at each step. It's generally a good idea to use several selectors, mixing fine grained moves and course grained moves:

      <selector>

        <selector>

          <moveFactoryClass>org.drools.planner.examples.nurserostering.solver.move.factory.EmployeeChangeMoveFactory</moveFactoryClass>

        </selector>

        <selector>

          <moveFactoryClass>org.drools.planner.examples.nurserostering.solver.move.factory.ShiftAssignmentSwitchMoveFactory</moveFactoryClass>

        </selector>

        <selector>

          <moveFactoryClass>org.drools.planner.examples.nurserostering.solver.move.factory.ShiftAssignmentPillarPartSwitchMoveFactory</moveFactoryClass>

        </selector>

      </selector>

### 8.6.2. Acceptor

An acceptor is used (together with a forager) to active tabu search, simulated annealing, great deluge, ... For each move it checks whether it is accepted or not.

You can implement your own `Acceptor`, although the build-in acceptors should suffice for most needs. You can also combine multiple acceptors.

#### 8.6.2.1. Tabu search acceptor

When tabu search takes steps it creates tabu's. It does not accept a move as the next step if that move breaks tabu. Drools Planner implements several tabu types:

- Solution tabu makes recently visited solutions tabu. It does not accept a move that leads to one of those solutions. If you can spare the memory, don't be cheap on the tabu size.

          <acceptor>

              <solutionTabuSize>1000</solutionTabuSize>

          </acceptor>

- Move tabu makes recent steps tabu. It does not accept a move equal to one of those steps.

          <acceptor>

              <moveTabuSize>7</moveTabuSize>

          </acceptor>

- Undo move tabu makes the undo move of recent steps tabu.

          <acceptor>

              <undoMoveTabuSize>7</undoMoveTabuSize>

          </acceptor>

- Property tabu makes a property of recent steps tabu. For example, it can make the queen tabu, so that a recently moved queen can't be moved.

          <acceptor>

              <propertyTabuSize>5</propertyTabuSize>

          </acceptor>

  To use property tabu, your moves must implement the `TabuPropertyEnabled` interface, for example:

      public class YChangeMove implements Move, TabuPropertyEnabled {

          private Queen queen;

          private Row toRow;

          // ...

          public List<? extends Object> getTabuPropertyList() {

              return Collections.singletonList(queen);

          }

      }

  You can also make multiple properties tabu (with OR or AND semantics):

          public List<? extends Object> getTabuPropertyList() {

              // tabu with other moves that contain the same leftExam OR the same rightExam

              return Arrays.asList(leftExam, rightExam);

          }

          public List<? extends Object> getTabuPropertyList() {

              // tabu with other moves that contain the same exam AND the same toPeriod (but not necessary the same toRoom)

              return Collections.singletonList(Arrays.asList(exam, toPeriod));

          }

You can even combine tabu types:

        <acceptor>

            <solutionTabuSize>1000</solutionTabuSize>

            <moveTabuSize>7</moveTabuSize>

        </acceptor>

If you pick a too small tabu size, your solver can still get stuck in a local optimum. On the other hand, with the exception of solution tabu, if you pick a too large tabu size, your solver can get stuck by bouncing of the walls. Use the benchmarker to fine tweak your configuration. Experiments teach us that it is generally best to use a prime number for the move tabu, undo move tabu or property tabu size.

A tabu search acceptor should be combined with a high or no subset selection.

#### 8.6.2.2. Simulated annealing acceptor

Simulated annealing does not always pick the move with the highest score, neither does it evaluate many moves per step. At least at first. Instead, it gives unimproving moves also a chance to be picked, depending on its score and the time gradient of the `Termination`. In the end, it gradually turns into a hill climber, only accepting improving moves.

In many use cases, simulated annealing surpasses tabu search. By changing a few lines of configuration, you can easily switch from tabu search to simulated annealing and back.

Start with a `simulatedAnnealingStartingTemperature` set to the maximum score delta a single move can cause. Use the `Benchmarker` to tweak the value.

        <acceptor>

          <simulatedAnnealingStartingTemperature>2hard/100soft</simulatedAnnealingStartingTemperature>

        </acceptor>

        <forager>

            <minimalAcceptedSelection>4</minimalAcceptedSelection>

        </forager>

A simulated annealing acceptor should be combined with a low subset selection. The classic algorithm uses a `minimalAcceptedSelection` of `1`, but usually `4` performs better.

You can even combine it with a tabu acceptor at the same time. Use a lower tabu size than in a pure tabu search configuration.

        <acceptor>

          <simulatedAnnealingStartingTemperature>10.0</simulatedAnnealingStartingTemperature>

          <propertyTabuSize>5</propertyTabuSize>

        </acceptor>

        <forager>

            <minimalAcceptedSelection>4</minimalAcceptedSelection>

        </forager>

This differs from phasing, another powerful technique, where first simulated annealing is used, followed by tabu search.

### 8.6.3. Forager

A forager gathers all accepted moves and picks the move which is the next step. Normally it picks the accepted move with the highest score. If several accepted moves have the highest score, one is picked randomly.

You can implement your own `Forager`, although the build-in forager should suffice for most needs.

#### 8.6.3.1. Subset selection

When there are many possible moves, it becomes inefficient to evaluate all of them at every step. To evaluate only a random subset of all the moves, use:

- An `minimalAcceptedSelection` integer, which specifies how many accepted moves should have be evaluated during each step. By default it is positive infinity, so all accepted moves are evaluated at every step.

        <forager>

          <minimalAcceptedSelection>1000</minimalAcceptedSelection>

        </forager>

Unlike the n queens problem, real world problems require the use of subset selection. Start from an `minimalAcceptedSelection` that takes a step in less then 2 seconds. Turn on INFO logging to see the step times. Use the `Benchmarker` to tweak the value.

#### 8.6.3.2. Pick early type

A forager can pick a move early during a step, ignoring subsequent selected moves. There are 3 pick early types:

- `NEVER`: A move is never picked early: all accepted moves are evaluated that the selection allows. This is the default.

          <forager>

              <pickEarlyType>NEVER</pickEarlyType>

          </forager>

- `FIRST_BEST_SCORE_IMPROVING`: Pick the first accepted move that improves the best score. If none improve the best score, it behaves exactly like the pickEarlyType NEVER.

          <forager>

              <pickEarlyType>FIRST_BEST_SCORE_IMPROVING</pickEarlyType>

          </forager>

- `FIRST_LAST_STEP_SCORE_IMPROVING`: Pick the first accepted move that improves the last step score. If none improve the last step score, it behaves exactly like the pickEarlyType NEVER.

          <forager>

              <pickEarlyType>FIRST_LAST_STEP_SCORE_IMPROVING</pickEarlyType>

          </forager>

## 8.7. Using a custom Selector, Acceptor, Forager or Termination

It is easy to plug in a custom `Selector`, `Acceptor`, `Forager` or `Termination` by extending the abstract class and also the config class.

For example, to use a custom `Selector`, extend the `AbstractSelector` class (see `AllMovesOfOneExamSelector`), extend the `SelectorConfig` class (see `AllMovesOfOneExamSelectorConfig`) and configure it in the configuration XML:

        <selector class="org.drools.planner.examples.examination.solver.selector.AllMovesOfOneExamSelectorConfig"/>

If you build a better implementation that's not domain specific, consider contributing it back as a pull request on github and we'll take it along in future refactors and optimize it.

## Chapter 9. Evolutionary algorithms

## 9.1. Overview

Evolutionary algorithms work on a population of solutions and evolve that population.

## 9.2. Evolutionary Strategies

This algorithm has not been implemented yet.

## 9.3. Genetic algorithms

This algorithm has not been implemented yet.

## Chapter 10. Benchmarking and tweaking

## 10.1. Finding the best configuration

Drools Planner supports several optimization algorithms (as solver phases), but you're probably wondering which is the best one? Although some optimization algorithms generally perform better then others, it really depends on your problem domain. Most solver phases have settings which can be tweaked. Those settings can influence the results a lot, although most solver phases work pretty well out-of-the-box.

Luckily, Drools Planner includes a benchmarker, which allows you to play out different solver phases with different settings against each other, so you can pick the best configuration for your planning problem.

## 10.2. Building a benchmarker

### 10.2.1. Adding the extra dependency

The benchmarker is current in the drools-planner-core modules, but it requires an extra dependency on the JFreeChart library.

If you use maven, add a dependency in your `pom.xml` file:

        <dependency>
          <groupId>jfree</groupId>
          <artifactId>jfreechart</artifactId>
          <version>1.0.13</version>
        </dependency>

This is similar for gradle, ivy and buildr.

If you use ANT, you've probably already copied the required jars from the download zip's `binaries` directory.

### 10.2.2. Building a `PlannerBenchmark`

You can build a `PlannerBenchmark` instance with the `XmlPlannerBenchmarkFactory`. Configure it with a benchmark configuration xml file:

        XmlPlannerBenchmarkFactory plannerBenchmarkFactory = new XmlPlannerBenchmarkFactory();

        plannerBenchmarkFactory.configure("/org/drools/planner/examples/nqueens/benchmark/nqueensBenchmarkConfig.xml");

        PlannerBenchmark plannerBenchmark = benchmarkFactory.buildPlannerBenchmark();

        plannerBenchmark.benchmark();

A basic benchmark configuration file looks something like this:

    <?xml version="1.0" encoding="UTF-8"?>

    <plannerBenchmark>

      <benchmarkDirectory>local/data/nqueens</benchmarkDirectory>

      <warmUpSecondsSpend>30</warmUpSecondsSpend>

      <inheritedSolverBenchmark>

        <problemBenchmarks>

          <xstreamAnnotatedClass>org.drools.planner.examples.nqueens.domain.NQueens</xstreamAnnotatedClass>

          <inputSolutionFile>data/nqueens/unsolved/unsolvedNQueens32.xml</inputSolutionFile>

          <inputSolutionFile>data/nqueens/unsolved/unsolvedNQueens64.xml</inputSolutionFile>

          <problemStatisticType>BEST_SOLUTION_CHANGED</problemStatisticType>

        </problemBenchmarks>

        <solver>

          <solutionClass>org.drools.planner.examples.nqueens.domain.NQueens</solutionClass>

          <planningEntityClass>org.drools.planner.examples.nqueens.domain.Queen</planningEntityClass>

          <scoreDrl>/org/drools/planner/examples/nqueens/solver/nQueensScoreRules.drl</scoreDrl>

          <scoreDefinition>

            <scoreDefinitionType>SIMPLE</scoreDefinitionType>

          </scoreDefinition>

          <termination>

            <maximumSecondsSpend>20</maximumSecondsSpend>

          </termination>

          <constructionHeuristic>

            <constructionHeuristicType>FIRST_FIT_DECREASING</constructionHeuristicType>

            <constructionHeuristicPickEarlyType>FIRST_LAST_STEP_SCORE_EQUAL_OR_IMPROVING</constructionHeuristicPickEarlyType>

          </constructionHeuristic>

        </solver>

      </inheritedSolverBenchmark>

      <solverBenchmark>

        <name>Solution tabu</name>

        <solver>

          <localSearch>

            <selector>

              <moveFactoryClass>org.drools.planner.examples.nqueens.solver.move.factory.RowChangeMoveFactory</moveFactoryClass>

            </selector>

            <acceptor>

              <solutionTabuSize>1000</solutionTabuSize>

            </acceptor>

            <forager>

              <pickEarlyType>NEVER</pickEarlyType>

            </forager>

          </localSearch>

        </solver>

      </solverBenchmark>

      <solverBenchmark>

        <name>Move tabu</name>

        <solver>

          <localSearch>

            <selector>

              <moveFactoryClass>org.drools.planner.examples.nqueens.solver.move.factory.RowChangeMoveFactory</moveFactoryClass>

            </selector>

            <acceptor>

              <moveTabuSize>5</moveTabuSize>

            </acceptor>

            <forager>

              <pickEarlyType>NEVER</pickEarlyType>

            </forager>

          </localSearch>

        </solver>

      </solverBenchmark>

      <solverBenchmark>

        <name>Property tabu</name>

        <solver>

          <localSearch>

            <selector>

              <moveFactoryClass>org.drools.planner.examples.nqueens.solver.move.factory.RowChangeMoveFactory</moveFactoryClass>

            </selector>

            <acceptor>

              <propertyTabuSize>5</propertyTabuSize>

            </acceptor>

            <forager>

              <pickEarlyType>NEVER</pickEarlyType>

            </forager>

          </localSearch>

        </solver>

      </solverBenchmark>

    </plannerBenchmark>

This `PlannerBenchmark` will try 3 configurations (1 solution tabu, 1 move tabu and 1 property tabu) on 2 data sets (32 and 64 queens), so it will run 6 solvers.

Every `solverBenchmark` entity contains a solver configuration (for example with a local search solver phase) and one or more `inputSolutionFile` elements. It will run the solver configuration on each of those unsolved solution files. A `name` is optional and generated if absent.

The common part of multiple `solverBenchmark` entities can be extracted to the `inheritedSolverBenchmark` entity, but that can still be overwritten per `solverBenchmark` entity. Note that inherited solver phases such as `<constructionHeuristic>` or `<localSearch>` are not overwritten but instead are added to the head of the solver phases list.

You need to specify a `benchmarkDirectory` (relative to the working directory). The best solution of each `Solver` run and a handy overview HTML file will be written in that directory.

### 10.2.3. ProblemIO: input and output of Solution files

#### 10.2.3.1. `ProblemIO` interface

The benchmarker needs to be able to read the input files to contain a `Solution` write the best `Solution` of each benchmark to an output file. For that it uses a class that implements the `ProblemIO` interface:

    public interface ProblemIO {

        String getFileExtension();

        Solution read(File inputSolutionFile);

        void write(Solution solution, File outputSolutionFile);

    }

#### 10.2.3.2. `XStreamProblemIO`

By default, a `XStreamProblemIO` instance is used and you just need to configure your `Solution` class as being annotated with XStream:

        <problemBenchmarks>
          <xstreamAnnotatedClass>org.drools.planner.examples.nqueens.domain.NQueens</xstreamAnnotatedClass>
          <inputSolutionFile>data/nqueens/unsolved/unsolvedNQueens32.xml</inputSolutionFile>
          ...
        </problemBenchmarks>

However, your input files need to have been written with a `XStreamProblemIO` instance.

#### 10.2.3.3. Custom `ProblemIO`

Alternatively, you can implement your own `ProblemIO` implementation and configure it with the `problemIOClass` element:

        <problemBenchmarks>
          <problemIOClass>org.drools.planner.examples.machinereassignment.persistence.MachineReassignmentProblemIO</problemIOClass>
          <inputSolutionFile>data/machinereassignment/input/model_a1_1.txt</inputSolutionFile>
          ...
        </problemBenchmarks>

### 10.2.4. Warming up the hotspot compiler

Without a warm up, the results of the first (or first few) benchmarks are not reliable, because they will have lost CPU time on hotspot JIT compilation (and possibly DRL compilation too).

The avoid that distortion, the benchmarker can run some of the benchmarks for a specified amount of time, before running the real benchmarks. Generally, a warm up of 30 seconds suffices:

    <plannerBenchmark>
      ...
      <warmUpSecondsSpend>30</warmUpSecondsSpend>
      ...
    </plannerBenchmark>

## 10.3. Summary statistics

### 10.3.1. Best score summary

Several summary statistics of the `PlannerBenchmark` run will be written in the `benchmarkDirectory`. Here is an example of a summary statistic:

Figure 10.1. Best score summary statistic

## 10.4. Statistics per data set (graph and CSV)

The benchmarker supports outputting statistics as graphs and CSV (comma separated values) files to the `benchmarkDirectory`.

To configure graph and CSV output of a statistic, just add a `problemStatisticType` line:

    <plannerBenchmark>

      <benchmarkDirectory>local/data/nqueens/solved</benchmarkDirectory>

      <inheritedSolverBenchmark>

        <problemBenchmarks>

          ...

          <problemStatisticType>BEST_SOLUTION_CHANGED</problemStatisticType>

          <problemStatisticType>CALCULATE_COUNT_PER_SECOND</problemStatisticType>

        </problemBenchmarks>

        ...

      </inheritedSolverBenchmark>

      ...

    </plannerBenchmark>

Multiple `problemStatisticType` elements are allowed. Some statistic types might influence performance noticeably. The following types are supported:

### 10.4.1. Best score over time statistic (graph and CSV)

To see how the best score evolves over time, add `BEST_SOLUTION_CHANGED` as `a problemStatisticType`.

Figure 10.2. Best score over time statistic

The best score over time statistic is very useful to detect abnormalities, such as score traps.

## Note

Don't be fooled by the simulated annealing line in this graph. If you give simulated annealing only 5 minutes, it might still be better than 5 minutes of tabu search. That's because this simulated annealing implementation automatically determines it's velocity based on the amount of time that can be spend. On the other hand, for the tabu search, what you see is what you'd get.

### 10.4.2. Calculate count per second statistic (graph and CSV)

To see how fast the scores are calculated, add `CALCULATE_COUNT_PER_SECOND` as a `problemStatisticType`.

Figure 10.3. Calculate count per second statistic

## Note

The initial high calculate count is typical during solution initialization. In this example, it's far easier to calculate the score of a solution if only a handful exams have been added, in contrast to all of them. After those few seconds of initialization, the calculate count is relatively stable, apart from an occasional stop-the-world garbage collector disruption.

### 10.4.3. Memory use statistic (graph and CSV)

To see how much memory is used, add `MEMORY_USE` as `a problemStatisticType`.

Figure 10.4. Memory use statistic

## Chapter 11. Repeated planning

## 11.1. Introduction to repeated planning

The world constantly changes. The planning facts used to create a solution, might change before or during the execution of that solution. There are 3 types of situations:

- Unforeseen fact changes: For example: an employee assigned to a shift calls in sick, an airplane scheduled to take off has a technical delay, one of the machines or vehicles break down, ... Use backup planning.

- Unknown long term future facts: For example: The hospital admissions for the next 2 weeks are reliable, but those for week 3 and 4 are less reliable and for week 5 and beyond are not worth planning yet. Use continuous planning.

- Constantly changing planning facts: Use real-time planning.

Waiting to start planning - to lower the risk of planning facts changing - usually isn't a good way to deal with that. More CPU time means a better planning solution. An incomplete plan is better then no plan.

Luckily, the Drools Planner algorithms support planning a solution that's already (partially) planned, known as repeated planning.

## 11.2. Backup planning

Backup planning is the technique of adding extra score constraints to create space in the planning for when things go wrong. That creates a backup plan in the plan. For example: try to assign an employee as the spare employee (1 for every 10 shifts at the same time), keep 1 hospital bed open in each department, ...

Then, when things go wrong (one of the employees calls in sick), change the planning facts on the original solution (delete the sick employee leave his/her shifts unassigned) and just restart the planning, starting from that solution, which has a different score now. The construction heuristics will fill in the newly created gaps (probably with the spare employee) and the metaheuristics will even improve it further.

## 11.3. Continuous planning (windowed planning)

Continuous planning is the technique of planning one or more upcoming planning windows at the same time and repeating that process every week (or every day). Because time infinite, there are an infinite future windows, so planning all future windows is impossible. Instead we plan only a number of upcoming planning windows.

Past planning windows are immutable. The first upcoming planning window is considered stable (unlikely to change), while later upcoming planning windows are considered draft (likely to change during the next planning effort). Distant future planning windows are not planned at all.

Past planning windows have locked planning entities: the planning entities can no longer be changed (they are locked in place), but some of them are still needed in the working memory, as they might affect some of the score constraints that apply on the upcoming planning entities. For example: when an employee should not work more than 5 days in a row, he shouldn't work today and tomorrow if he worked the past 4 days already.

Sometimes some planning entities are semi-locked: they can be changed, but occur a certain score penalty if they differ from their original place. For example: avoid rescheduling hospital beds less than 2 days before the patient arrives (unless it's really worth it), avoid changing the airplane gate (or worse, the terminal) during the 2 hours before boarding, ...

Figure 11.1. Continuous planning diagram

Notice the difference between the original planning of November 1th and the new planning of November 5th: some planning facts (F, H, I, J, K) changed, which results in unrelated planning entities (G) changing too.

## 11.4. Real-time planning (event based planning)

To do real-time planning, first combine backup planning and continuous planning with short planning windows to lower the burden of real-time planning. Don't configure any `Termination`, just terminate early when you need the results or subscribe to the `BestSolutionChangedEvent` (the latter doesn't guarantee yet that every `ProblemFactChange` has been processed already).

While the `Solver` is solving, an outside event might want to change one of the problem facts, for example an airplane is delayed and needs the runway at a later time. Do not change the problem fact instances used by the `Solver` while it is solving, as that will corrupt it. Instead, add a `ProblemFactChange` to the `Solver` which it will execute as soon as the timing is right.

    public interface Solver {

       ...

       boolean addProblemFactChange(ProblemFactChange problemFactChange);

       ...

    }

    public interface ProblemFactChange {

        void doChange(SolutionDirector solutionDirector);

    }

Here's an example:

        public void deleteComputer(final CloudComputer cloudComputer) {
            solver.addProblemFactChange(new ProblemFactChange() {
                public void doChange(SolutionDirector solutionDirector) {
                    CloudBalance cloudBalance = (CloudBalance) solutionDirector.getWorkingSolution();
                    WorkingMemory workingMemory = solutionDirector.getWorkingMemory();
                    // First remove the planning fact from all planning entities that use it
                    for (CloudProcessAssignment cloudProcessAssignment : cloudBalance.getCloudProcessAssignmentList()) {
                        if (ObjectUtils.equals(cloudProcessAssignment.getCloudComputer(), cloudComputer)) {
                            FactHandle cloudProcessAssignmentHandle = workingMemory.getFactHandle(cloudProcessAssignment);
                            cloudProcessAssignment.setCloudComputer(null);
                            workingMemory.retract(cloudProcessAssignmentHandle);
                        }
                    }
                    // Next remove it the planning fact itself
                    for (Iterator<CloudComputer> it = cloudBalance.getCloudComputerList().iterator(); it.hasNext(); ) {
                        CloudComputer workingCloudComputer = it.next();
                        if (ObjectUtils.equals(workingCloudComputer, cloudComputer)) {
                            FactHandle cloudComputerHandle = workingMemory.getFactHandle(workingCloudComputer);
                            workingMemory.retract(cloudComputerHandle);
                            it.remove(); // remove from list
                            break;
                        }
                    }
                }
            });
        }

## Warning

Any change on the problem facts or planning entities in a `ProblemFactChange` must be done on the instances of the `Solution` of `solutionDirector.getWorkingSolution()`.

## Warning

Any change on the problem facts or planning entities in a `ProblemFactChange` must be told to the `WorkingMemory` of `solutionDirector.getWorkingMemory()`.

## Note

Many types of changes can leave a planning entity uninitialized, resulting in a partially initialized solution. That's fine, as long as the first solver phase can handle it. All construction heuristics solver phases can handle that, so it's recommended to configure such a `SolverPhase` as the first phase.

In essence, the `Solver` will stop, run the `ProblemFactChange` and restart. Each `SolverPhase` will be run again. Each configured `Termination` (except `terminateEarly`) will reset.

## Index