# Messaging Concepts

Apache ActiveMQ Artemis is an asynchronous messaging system, an example of
[Message Oriented
Middleware](https://en.wikipedia.org/wiki/Message-oriented_middleware) , we'll
just call them messaging systems in the remainder of this book.

We'll first present a brief overview of what kind of things messaging systems
do, where they're useful and the kind of concepts you'll hear about in the
messaging world.

If you're already familiar with what a messaging system is and what it's
capable of, then you can skip this chapter.

## General Concepts

Messaging systems allow you to loosely couple heterogeneous systems together,
whilst typically providing reliability, transactions and many other features.

Unlike systems based on a [Remote Procedure
Call](https://en.wikipedia.org/wiki/Remote_procedure_call) (RPC) pattern,
messaging systems primarily use an asynchronous message passing pattern with no
tight relationship between requests and responses. Most messaging systems also
support a request-response mode but this is not a primary feature of messaging
systems.

Designing systems to be asynchronous from end-to-end allows you to really take
advantage of your hardware resources, minimizing the amount of threads blocking
on IO operations, and to use your network bandwidth to its full capacity. With
an RPC approach you have to wait for a response for each request you make so
are limited by the network round trip time, or *latency* of your network. With
an asynchronous system you can pipeline flows of messages in different
directions, so are limited by the network *bandwidth* not the latency. This
typically allows you to create much higher performance applications.

Messaging systems decouple the senders of messages from the consumers of
messages. The senders and consumers of messages are completely independent and
know nothing of each other. This allows you to create flexible, loosely coupled
systems.

Often, large enterprises use a messaging system to implement a message bus
which loosely couples heterogeneous systems together. Message buses often form
the core of an [Enterprise Service
Bus](https://en.wikipedia.org/wiki/Enterprise_service_bus). (ESB). Using a
message bus to de-couple disparate systems can allow the system to grow and
adapt more easily. It also allows more flexibility to add new systems or retire
old ones since they don't have brittle dependencies on each other.

## Messaging styles

Messaging systems normally support two main styles of asynchronous messaging:
[message queue](https://en.wikipedia.org/wiki/Message_queue) messaging (also
known as *point-to-point messaging*) and [publish
subscribe](https://en.wikipedia.org/wiki/Publish_subscribe) messaging.  We'll
summarise them briefly here:

### Point-to-Point

With this type of messaging you send a message to a queue. The message is then
typically persisted to provide a guarantee of delivery, then some time later
the messaging system delivers the message to a consumer.  The consumer then
processes the message and when it is done, it acknowledges the message. Once
the message is acknowledged it disappears from the queue and is not available
to be delivered again. If the system crashes before the messaging server
receives an acknowledgement from the consumer, then on recovery, the message
will be available to be delivered to a consumer again.

With point-to-point messaging, there can be many consumers on the queue but a
particular message will only ever be consumed by a maximum of one of them.
Senders (also known as *producers*) to the queue are completely decoupled from
receivers (also known as *consumers*) of the queue - they do not know of each
other's existence.

A classic example of point to point messaging would be an order queue in a
company's book ordering system. Each order is represented as a message which is
sent to the order queue. Let's imagine there are many front end ordering
systems which send orders to the order queue. When a message arrives on the
queue it is persisted - this ensures that if the server crashes the order is
not lost. Let's also imagine there are many consumers on the order queue - each
representing an instance of an order processing component - these can be on
different physical machines but consuming from the same queue. The messaging
system delivers each message to one and only one of the ordering processing
components.  Different messages can be processed by different order processors,
but a single order is only processed by one order processor - this ensures
orders aren't processed twice.

As an order processor receives a message, it fulfills the order, sends order
information to the warehouse system and then updates the order database with
the order details. Once it's done that it acknowledges the message to tell the
server that the order has been processed and can be forgotten about. Often the
send to the warehouse system, update in database and acknowledgement will be
completed in a single transaction to ensure
[ACID](https://en.wikipedia.org/wiki/ACID) properties.

### Publish-Subscribe

With publish-subscribe messaging many senders can send messages to an entity on
the server, often called a *topic* (e.g. in the JMS world).

There can be many *subscriptions* on a topic, a subscription is just another
word for a consumer of a topic. Each subscription receives a *copy* of *each*
message sent to the topic. This differs from the message queue pattern where
each message is only consumed by a single consumer.

Subscriptions can optionally be *durable* which means they retain a copy of
each message sent to the topic until the subscriber consumes them - even if the
server crashes or is restarted in between. Non-durable subscriptions only last
a maximum of the lifetime of the connection that created them.

An example of publish-subscribe messaging would be a news feed. As news
articles are created by different editors around the world they are sent to a
news feed topic. There are many subscribers around the world who are interested
in receiving news items - each one creates a subscription and the messaging
system ensures that a copy of each news message is delivered to each
subscription.

## Delivery guarantees

A key feature of most messaging systems is *reliable messaging*. With reliable
messaging the server gives a guarantee that the message will be delivered once
and only once to each consumer of a queue or each durable subscription of a
topic, even in the event of system failure. This is crucial for many
businesses; e.g. you don't want your orders fulfilled more than once or any of
your orders to be lost.

In other cases you may not care about a once and only once delivery guarantee
and are happy to cope with duplicate deliveries or lost messages - an example
of this might be transient stock price updates - which are quickly superseded
by the next update on the same stock. The messaging system allows you to
configure which delivery guarantees you require.

## Transactions

Messaging systems typically support the sending and acknowledgement of multiple
messages in a single local transaction. Apache ActiveMQ Artemis also supports
the sending and acknowledgement of message as part of a large global
transaction - using the Java mapping of XA: JTA.

## Durability

Messages are either durable or non durable. Durable messages will be persisted
in permanent storage and will survive server failure or restart. Non durable
messages will not survive server failure or restart. Examples of durable
messages might be orders or trades, where they cannot be lost. An example of a
non durable message might be a stock price update which is transitory and
doesn't need to survive a restart.

## Messaging APIs and protocols

How do client applications interact with messaging systems in order to send and
consume messages?

Several messaging systems provide their own proprietary APIs with which the
client communicates with the messaging system.

There are also some standard ways of operating with messaging systems and some
emerging standards in this space.

Let's take a brief look at these:

### Java Message Service (JMS)

[JMS](https://en.wikipedia.org/wiki/Java_Message_Service) is part of Oracle's
Java EE specification. It's a Java API that encapsulates both message queue and
publish-subscribe messaging patterns. JMS is a lowest common denominator
specification - i.e. it was created to encapsulate common functionality of the
already existing messaging systems that were available at the time of its
creation.

JMS is a very popular API and is implemented by most messaging systems.  JMS is
only available to clients running Java.

JMS does not define a standard wire format - it only defines a programmatic API
so JMS clients and servers from different vendors cannot directly interoperate
since each will use the vendor's own internal wire protocol.

Apache ActiveMQ Artemis provides a fully compliant [JMS 1.1 and JMS 2.0 client
implementation](using-jms.md).

### System specific APIs

Many systems provide their own programmatic API for which to interact with the
messaging system. The advantage of this it allows the full set of system
functionality to be exposed to the client application. API's like JMS are not
normally rich enough to expose all the extra features that most messaging
systems provide.

Apache ActiveMQ Artemis provides its own core client API for clients to use if
they wish to have access to functionality over and above that accessible via
the JMS API.

Please see [Core](core.md) for using the Core API with Apache ActiveMQ Artemis.

### RESTful API

[REST](https://en.wikipedia.org/wiki/Representational_State_Transfer)
approaches to messaging are showing a lot interest recently.

It seems plausible that API standards for cloud computing may converge on a
REST style set of interfaces and consequently a REST messaging approach is a
very strong contender for becoming the de-facto method for messaging
interoperability.

With a REST approach messaging resources are manipulated as resources defined
by a URI and typically using a simple set of operations on those resources,
e.g. PUT, POST, GET etc. REST approaches to messaging often use HTTP as their
underlying protocol.

The advantage of a REST approach with HTTP is in its simplicity and the fact
the internet is already tuned to deal with HTTP optimally.

Please see [Rest Interface](rest.md) for using Apache ActiveMQ Artemis's
RESTful interface.

### AMQP

[AMQP](https://en.wikipedia.org/wiki/AMQP) is a specification for interoperable
messaging. It also defines a wire format, so any AMQP client can work with any
messaging system that supports AMQP. AMQP clients are available in many
different programming languages.

Apache ActiveMQ Artemis implements the [AMQP
1.0](https://www.oasis-open.org/committees/tc_home.php?wg_abbrev=amqp)
specification. Any client that supports the 1.0 specification will be able to
interact with Apache ActiveMQ Artemis.

Please see [AMQP](amqp.md) for using AMQP with Apache ActiveMQ Artemis.

### MQTT

[MQTT](https://mqtt.org/) is a lightweight connectivity protocol.  It is
designed to run in environments where device and networks are constrained.  Out
of the box Apache ActiveMQ Artemis supports version MQTT 3.1.1.  Any client
supporting this version of the protocol will work against Apache ActiveMQ
Artemis.

Please see [MQTT](mqtt.md) for using MQTT with Apache ActiveMQ Artemis.

### STOMP

[Stomp](https://stomp.github.io/) is a very simple text protocol for
interoperating with messaging systems. It defines a wire format, so
theoretically any Stomp client can work with any messaging system that
supports Stomp. Stomp clients are available in many different
programming languages.

Please see [Stomp](stomp.md) for using STOMP with Apache ActiveMQ Artemis.

### OpenWire

ActiveMQ 5.x defines it's own wire protocol: OpenWire.  In order to support
ActiveMQ 5.x clients, Apache ActiveMQ Artemis supports OpenWire.  Any ActiveMQ
5.12.x or higher can be used with Apache ActiveMQ Artemis.

Please see [OpenWire](openwire.md) for using OpenWire with Apache ActiveMQ
Artemis.

## High Availability

High Availability (HA) means that the system should remain operational after
failure of one or more of the servers. The degree of support for HA varies
between various messaging systems.

Apache ActiveMQ Artemis provides automatic failover where your sessions are
automatically reconnected to the backup server on event of live server failure.

For more information on HA, please see [High Availability and Failover](ha.md).

## Clusters

Many messaging systems allow you to create groups of messaging servers called
*clusters*. Clusters allow the load of sending and consuming messages to be
spread over many servers. This allows your system to scale horizontally by
adding new servers to the cluster.

Degrees of support for clusters varies between messaging systems, with some
systems having fairly basic clusters with the cluster members being hardly
aware of each other.

Apache ActiveMQ Artemis provides very configurable state-of-the-art clustering
model where messages can be intelligently load balanced between the servers in
the cluster, according to the number of consumers on each node, and whether
they are ready for messages.

Apache ActiveMQ Artemis also has the ability to automatically redistribute
messages between nodes of a cluster to prevent starvation on any particular
node.

For full details on clustering, please see [Clusters](clusters.md).

## Bridges and routing

Some messaging systems allow isolated clusters or single nodes to be bridged
together, typically over unreliable connections like a wide area network (WAN),
or the internet.

A bridge normally consumes from a queue on one server and forwards messages to
another queue on a different server. Bridges cope with unreliable connections,
automatically reconnecting when the connections becomes available again.

Apache ActiveMQ Artemis bridges can be configured with filter expressions to
only forward certain messages, and transformation can also be hooked in.

Apache ActiveMQ Artemis also allows routing between queues to be configured in
server side configuration. This allows complex routing networks to be set up
forwarding or copying messages from one destination to another, forming a
global network of interconnected brokers.

For more information please see [Core Bridges](core-bridges.md) and [Diverting
and Splitting Message Flows](diverts.md).
