[[Distributable_Web_Applications]]
= Distributable Web Applications

In a standard web application, session state does not survive beyond the lifespan of the servlet container.
A distributable web application allows session state to survive beyond the lifespan of a single server, either via persistence or by replicating state to other nodes in the cluster.
A web application indicates its intention to be distributable via the `<distributable/>` element within the web application's deployment descriptor.

e.g.

./WEB-INF/web.xml:
[source,xml]
----
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0">
    <distributable/>
</web-app>
----

[[distributable-web-subsystem]]
== Distributable Web Subsystem

The distributable-web subsystem manages a set of session management profiles that encapsulate the configuration of a distributable session manager.
One of these profiles will be designated as the default profile (via the "default-session-management" attribute) and thus defines the default behavior of a distributable web application.

[source]
----
[standalone@embedded /] /subsystem=distributable-web:read-attribute(name=default-session-management)
{
    "outcome" => "success",
    "result" => "default"
}
----

The default session management stores web session data within an Infinispan cache.
We can introspect its configuration:

[source]
----
[standalone@embedded /] /subsystem=distributable-web/infinispan-session-management=default:read-resource
{
    "outcome" => "success",
    "result" => {
        "cache" => undefined,
        "cache-container" => "web",
        "granularity" => "SESSION",
        "affinity" => {"primary-owner" => undefined}
    }
}
----

cache-container::
This references a cache-container defined in the Infinispan subsystem into which session data will be stored.
cache::
This references a cache within associated cache-container upon whose configuration the web application's cache will be based.
If undefined, the default cache of the associated cache container will be used.
granularity::
This defines how the session manager will map a session into individual cache entries.
Possible values are:
SESSION:::
Stores all session attributes within a single cache entry.
This is generally more expensive than ATTRIBUTE granularity, but preserves any cross-attribute object references.
ATTRIBUTE:::
Stores each session attribute within a separate cache entry.
This is generally more efficient than SESSION granularity, but does not preserve any cross-attribute object references.

affinity::
This resource defines the affinity that a web request should have for a given server.
The affinity of the associated web session determines the algorithm for generating the route to be appended onto the session ID (within the JSESSIONID cookie, or when encoding URLs).
This annotation of the session ID is used by load balancers to advise how future requests for existing sessions should be directed.
Routing is designed to be opaque to application code such that calls to `HttpSession.getId()` always return an unmodified session ID.
This is only generated when creating/updating the JSESSIONID cookie, or when encoding URLs via `HttpServletResponse.encodeURL()` and `encodeRedirectURL()`.
Possible values are:
affinity=none:::
Web requests will have no affinity to any particular node.
This option is intended for use cases where web session state is not maintained within the application server.
affinity=local:::
Web requests will have an affinity to the server that last handled a request for a given session.
This option corresponds to traditional sticky session behavior.
affinity=primary-owner:::
Web requests will have an affinity to the primary owner of a given session.
This is the default affinity for this distributed session manager.
Behaves the same as affinity=local if the backing cache is not distributed nor replicated.
affinity=ranked:::
Web requests will have an affinity to the first available node in a ranked list comprised of: primary owner, backup nodes, local node (if not a primary nor backup owner).
Only for use with load balancers that support multiple routes.
Behaves the same as affinity=local if cache is not distributed nor replicated.

e.g. Creating a new session management profile, using ATTRIBUTE granularity with local session affinity:

[source]
----
[standalone@embedded /] /subsystem=distributable-web/infinispan-session-management=foo:add(cache-container=web, granularity=ATTRIBUTE)
{
    "outcome" => "success"
}
[standalone@embedded /] /subsystem=distributable-web/infinispan-session-management=foo/affinity=local:add(){allow-resource-service-restart=true}
{
    "outcome" => "success"
}
----

The subsystem also supports session management where session data is stored in a remote infinispan-server cluster via the HotRod protocol.

e.g.

[source]
----
[standalone@embedded /] /subsystem=distributable-web/hotrod-session-management=foo:add(remote-cache-container=datagrid, cache-configuration=foo, granularity=ATTRIBUTE)
{
    "outcome" => "success"
}
----

remote-cache-container::
This references a <<remote_cache_container,remote-cache-container>> defined in the Infinispan subsystem into which session data will be stored.
cache-configuration::
If a remote cache whose name matches the deployment name does not exist, this attribute defines a cache configuration within the remote infinispan server, from which an application-specific cache will be created.
granularity::
This defines how the session manager will map a session into individual cache entries.
Possible values are:
SESSION:::
Stores all session attributes within a single cache entry.
This is generally more expensive than ATTRIBUTE granularity, but preserves any cross-attribute object references.
ATTRIBUTE:::
Stores each session attribute within a separate cache entry.
This is generally more efficient than SESSION granularity, but does not preserve any cross-attribute object references.
affinity::
This resource defines the affinity that a web request should have for a given server.
The affinity of the associated web session determines the algorithm for generating the route to be appended onto the session ID (within the JSESSIONID cookie, or when encoding URLs).
This annotation of the session ID is used by load balancers to advise how future requests for existing sessions should be directed.
Routing is designed to be opaque to application code such that calls to `HttpSession.getId()` always return an unmodified session ID.
This is only generated when creating/updating the JSESSIONID cookie, or when encoding URLs via `HttpServletResponse.encodeURL()` and `encodeRedirectURL()`.
Possible values are:
affinity=none:::
Web requests will have no affinity to any particular node.
This option is intended for use cases where web session state is not maintained within the application server.
affinity=local:::
Web requests will have an affinity to the server that last handled a request for a given session.
This option corresponds to traditional sticky session behavior.

== Overriding default behavior

A web application can override the default distributable session management behavior in 1 of 2 ways:

. Reference a session-management profile by name
. Provide deployment-specific session management configuration

=== Referencing an existing session management profile

To use an existing distributed session management profile, a web application should include a distributable-web.xml deployment descriptor located within the application's /WEB-INF directory.

e.g.

./WEB-INF/distributable-web.xml
[source,xml]
----
<?xml version="1.0" encoding="UTF-8"?>
<distributable-web xmlns="urn:jboss:distributable-web:2.0">
    <session-management name="foo"/>
</distributable-web>
----

Alternatively, the target distributed session management profile can be defined within an existing jboss-all.xml deployment descriptor:

e.g.

./META-INF/jboss-all.xml
[source,xml]
----
<?xml version="1.0" encoding="UTF-8"?>
<jboss xmlns="urn:jboss:1.0">
    <distributable-web xmlns="urn:jboss:distributable-web:2.0">
        <session-management name="foo"/>
    </distributable-web>
</jboss>
----

=== Using a deployment-specific session management profile

If custom session management configuration will only be used by a single web application, you may find it more convenient to define the configuration within the deployment descriptor itself.
Ad hoc configuration looks identical to the configuration used by the distributable-web subsystem.

e.g.

./WEB-INF/distributable-web.xml
[source,xml]
----
<?xml version="1.0" encoding="UTF-8"?>
<distributable-web xmlns="urn:jboss:distributable-web:2.0">
    <infinispan-session-management cache-container="foo" cache="bar" granularity="SESSION">
        <primary-owner-affinity/>
    </infinispan-session-management>
</distributable-web>
----

Alternatively, session management configuration can be defined within an existing jboss-all.xml deployment descriptor:

e.g.

./META-INF/jboss-all.xml
[source,xml]
----
<?xml version="1.0" encoding="UTF-8"?>
<jboss xmlns="urn:jboss:1.0">
    <distributable-web xmlns="urn:jboss:distributable-web:2.0">
        <infinispan-session-management cache-container="foo" cache="bar" granularity="ATTRIBUTE">
            <local-affinity/>
        </infinispan-session-management>
    </distributable-web>
</jboss>
----

== Distributable Shared Sessions

WildFly supports the ability to share sessions across web applications within an enterprise archive.
In previous releases, WildFly always presumed distributable session management of shared sessions.
Version 2.0 of the shared-session-config deployment descriptor was updated to allow an EAR to opt-in to this behavior using the familiar `<distributable/>` element.
Additionally, you can customize the behavior of the distributable session manager used for session sharing via the same configuration mechanism described in the above sections.

e.g.

./META-INF/jboss-all.xml
[source,xml]
----
<?xml version="1.0" encoding="UTF-8"?>
<jboss xmlns="urn:jboss:1.0">
    <shared-session-config xmlns="urn:jboss:shared-session-config:2.0">
        <distributable/>
        <session-config>
            <cookie-config>
                <path>/</path>
            </cookie-config>
        </session-config>
    </shared-session-config>
    <distributable-web xmlns="urn:jboss:distributable-web:2.0">
        <session-management name="foo"/>
    </distributable-web>
</jboss>
----

== Optimizing performance of distributed web applications

One of the primary design goals of WildFly's distributed session manager was the parity of HttpSession semantics between distributable and non-distributable web applications.
In order to provide predictable behavior suitable for most web applications, the default distributed session manager configuration is quite conservative, generally favoring consistency over availability.
However, these defaults may not be appropriate for your application.
In general, the effective performance of the distributed session manager is constrained by:

. Replication/persistence payload size
. Locking/isolation of a given session

To optimize the configuration of the distributed session manager for your application, you can address the above constraints by tuning one or more of the following:

* <<session_granularity,Granularity>>
* <<session_concurrency,Concurrency>>
* <<session_attribute_immutability,Immutability>>
* <<session_attribute_marshalling,Marshalling>>

[[session_granularity]]
=== Session granularity

By default, WildFly's distributed session manager uses SESSION granularity, meaning that all session attributes are stored within a single cache entry.
While this ensures that any object references shared between session attributes are preserved following replication/persistence, it means that a change to a single attribute results in the replication/persistence of *all* attributes.

If your application does not share any object references between attributes, users are strongly advised to use ATTRIBUTE granularity.
Using ATTRIBUTE granularity, each session attribute is stored in a separate cache entry.
This means that a given request is only required to replicate/persist those attributes that were added/modified/removed/mutated in a given request.
For read-heavy applications, this can dramatically reduce the replication/persistence payload per request.

[[session_concurrency]]
=== Session concurrency

WildFly's default distributed session manager behavior is also conservative with respect to concurrent access to a given session.
By default, a request acquires exclusive access to its associated session for the duration of a request, and until any async child context is complete.
This maximizes the performance of a single request, as each request corresponds to a single cache transaction; allows for repeatable read semantics to the session; and ensures that subsequent requests are not prone to stale reads, even when handled by another cluster member.

However, if multiple requests attempt to access the same session concurrently, their processing will be effectively serialized.  This might not be feasible, especially for heavily asynchronous web applications.

Relaxing transaction isolation from REPEATABLE_READ to READ_COMMITTED on the associated cache configuration will allow concurrent requests to perform lock-free (but potentially stale) reads by deferring locking to the first attempt to write to the session.
This improves the throughput of requests for the same session for highly asynchronous web applications whose session access is read-heavy.

e.g.
[source]
----
/subsystem=infinispan/cache-container=web/distributed-cache=dist/component=locking:write-attribute(name=isolation, value=READ_COMMITTED)
----

For asynchronous web applications whose session access is write-heavy, merely relaxing transaction isolation is not likely to be sufficient.
These web applications will likely benefit from disabling cache transactions altogether.
When transactions are disabled, cache entries are locked and released for every write to the session, resulting in last-write-wins semantics.
For write-heavy applications, this typically improves the throughput of concurrent requests for the same session, at the cost of longer response times for individual requests.

[source]
----
/subsystem=infinispan/cache-container=web/distributed-cache=dist/component=transaction:write-attribute(name=mode, value=NONE)
----

NOTE: Relaxing transaction isolation currently prevents WildFly from enforcing that a given session is handled by one JVM at a time, a constraint dictated by the servlet specification.

[[session_attribute_immmutability]]
=== Session attribute immutability

In WildFly, distributed session attributes are presumed to be mutable objects, unless of a known immutable type, or unless otherwise specified.

Take the following session access pattern:
[source,java]
----
HttpSession session = request.getSession();
MutableObject object = session.getAttribute("...");
object.mutate();
----

By default, WildFly replicates/persists the mutable session attributes at the end of the request, ensuring that a subsequent request will read the mutated value, not the original value.
However, the replication/persistence of mutable session attributes at the end of the request happens whether or not these objects were actually mutated.
To avoid redundant session writes, users are strongly encouraged to store immutable objects in the session whenever possible.
This allows the application more control over when session attributes will replicate/persist, since immutable session attributes will only update upon explicit calls to `HttpSession.setAttribute(...)`.

WildFly can determine whether most JDK types are immutable, but any unrecognized/custom types are presumed to be mutable.
To indicate that a given session attribute of a custom type should be treated as immutable by the distributed session manager, annotate the class with one of the following annotations:

. `@org.wildfly.clustering.web.annotation.Immutable`
. `@net.jcip.annotations.Immutable`

e.g.
[source,java]
----
@Immutable
public class ImmutableClass implements Serializable {
    // ...
}
----

Alternatively, immutable classes can be enumerated via the distributable-web deployment descriptor.

e.g.
[source,xml]
----
<distributable-web xmlns="urn:jboss:distributable-web:2.0">
    <session-management>
        <immutable-class>foo.bar.ImmutableClass</immutable-class>
        <immutable-class>...</immutable-class>
    </session-management>
</distributable-web>
----

[[session_attribute_marshalling]]
=== Session attribute marshalling

Minimizing the replication/persistence payload for individual session attributes has a direct impact on performance by reducing the number of bytes sent over the network or persisted to storage.
A web application can optimize the marshalling of a given session attribute, either through custom JDK serialization logic, or by implementing a custom externalizer.
An externalizer is an implementation of the `org.wildfly.clustering.marshalling.Externalizer` interface, which dictates how a given class should be marshalled.
An externalizer reads/writes the state of an object directly from/to an input/output stream, but also:

. Allows an application to store an object in the session that does not implement `java.io.Serializable`
. Eliminates the need to serialize the class descriptor of an object along with its state

e.g.
[source,java]
----
public class MyObjectExternalizer implements org.wildfly.clustering.marshalling.Externalizer<MyObject> {

    @Override
    public Class<MyObject> getTargetClass() {
        return MyObject.class;
    }

    @Override
    public void writeObject(ObjectOutput output, MyObject object) throws IOException {
        // Write object state to stream
    }

    @Override
    public MyObject readObject(ObjectInput input) throws IOException, ClassNotFoundException {
        // Construct and read object state from stream
        return ...;
    }
}
----

Externalizers are dynamically loaded during deployment via the service loader mechanism.
Implementations should be enumerated within a file named:
`/META-INF/services/org.wildfly.clustering.marshalling.Externalizer`
