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== Introduction ==
[http://http://en.wikipedia.org/wiki/Load_balancing_(computing) Load balancing] to distribute workload across multiple nodes in a cluster works with RAP out of the box since version 1.0. One thing to mind though when setting up a load balancer for RAP applications is that it must be configured to use sticky sessions (aka session affinity). This entails that once a session is started, the same server serves all subsequent requests for that session. For information on how to set up load balancing, refer to the servlet container documentation. E.g. [http://tomcat.apache.org/tomcat-7.0-doc/balancer-howto.html Tomcat]

If your application provides critical servies you may want to make it [http://en.wikipedia.org/wiki/Failover fail safe] by running it in a [http://en.wikipedia.org/wiki/High-availability_cluster high-availability cluster]. A high availability cluster operates by having redundant nodes. So that if one node crashes, the service as a whole is unaffected as other nodes take over the work.

Throughout this document, the term ''cluster'' refers to high-availablility clusters.

With version 1.5 M1, RAP can be configured for ''transparent session failover'' so that applications can run in a high-availability cluster. This means that two or more servlet engines form a cluster, where a session, and its contents, can be moved from one node of the cluster to another node.

In §7.7.2 (Distributed Environments), the servlet specification requests for distributable applications that all session data must implement the <code>Serializable</code> interface. This results in two main obstacles to overcome with the current implementation:
* All RWT objects that live in the session scope (i.e. are directly or indirectly stored in a session attribute) must be serializable. This will mainly affect <code>Display</code>, <code>Widget</code> and derived classes and <code>Resource</code> and derived classes.
* The RWTLifeCycle which creates a ''UI Thread'' that lives until the session is terminated. The UI thread is necessary to enable blocking dialogs and so ensures compatibility with SWT. The downside of the UI thread of course is that sessions (which hold a reference to the UI thread) cannot be migrated as - naturally - a thread cannot be serialized.

Serializable RWT sessions can also be used to swap inactive sessions onto disk and thereby releasing the memory consumed by them. This allows for long to infinite session timeout settings that result in user friendly web applications that don't bother users with 'session has expried' messages and on the other hand don't consume more memory than actually needed. This feature is e.g. supported by [http://tomcat.apache.org/tomcat-7.0-doc/config/manager.html Tomcat]

== State of Development ==
The core features that enable RWT and JFace to run in a high-availability cluster are implemented. 
{{bug|341761}} is used as a meta-bug to capture the problem and track the solution.

=== JEE Compatible Operation Mode  ===

An alternative operation mode is available that does not use an extra thread. As a consequence thereof [[#Blocking Dialogs|blocking dialogs]] aren't possible with this operation mode. This operation mode also makes it easier to access security or transaction contexts and is conform to the [http://oracle.com/technetwork/java/javaee/tech/index.html JEE Specification]. Even if you don't have a need for high availability clustering, if you don’t use blocking dialogs or workbench features we recommend to use this operation mode.

=== Serializable Session Data ===
The core classes with session scope are serializable. This includes <code>Device</code> and <code>Display</code>, all widgets, layouts and layout data, <code>Dialog</code> and its subclasses, <code>Accessible</code> and its associated classes, drag & drop classes, resources, data structures (e.g.<code>Point</code>, <code>Rectangle</code>, etc.), listeners and their respective events, <code>ISessionStoreListener</code>.

If a session attribute changes, the servlet engine must be told to replicate the change. Unfortunately the Servlet specs do not specify how this should be done. The most common way is to call <code>HttpSession.setAttribute()</code> to flag the object as changed (see [http://java.sun.com/developer/technicalArticles/J2EE/clustering/ J2EE clustering, Part 2, section ''Session-storage guidelines'']).
In RWT, after each request the session attribute that holds the <code>ISessionStore</code> is re-set into the session. This means that as long as application code stores data in an attribute of the <code>ISessionStore</code> or with the help of SessionSingletonBase, all changes will be replicated.

Each serializable class has a [http://download.oracle.com/javase/1,5.0/docs/api/java/io/Serializable.html version] associated, which is used during deserialization to ensure that the loaded class is compatible with the serialized object with respect to serialization.
A class can state the version of its ''serialized form'' explicitly by declaring a field named <code>serialVersionUID</code> that holds the verion number. If a class does not explicitly declare a <code>serialVersionUID</code>, then a version will be calculated at runtime.
Even though it is strongly recommended to explicitly declare the <code>serialVersionUID</code>, we decided against for these reasons (<del>{{bug|351641}}</del>):
* The serialized form is stored for a short time only. Currently, it is hardly imaginable that in between serialization and deserialization, a new application is deployed (and with it the version of the serialized form changes)
* Maintaining an explicit <code>serialVersionUID</code> is hard. With each field that is added or removed  from a serializable class, it must be considered whether the serial version is affected. Thre is no (automated) quality assurance established to ensure this and installing one would be a considerable amount of work.
* The clutter of <code>serialVersionUID</code> declarations reduces the readability of the source code.
If the future shows that explicit <code>serialVersionUID</code> become necessary, they can still be added.

=== Blocking Dialogs ===
For application code to still be able to use dialogs we introduced the <code>DialogUtil</code> helper class. It allows to open dialogs in a non-blocking way and informs you via the supplied <code>DialogCallback</code> when the dialog was closed.
<source lang="java">
MessageBox messageBox = new MessageBox( parent, SWT.YES | SWT.NO );
messageBox.setMessage( "Are you OK?" );
DialogUtil.open( messageBox, new DialogCallback() {
  public void dialogClosed( int returnCode ) {
    if( returnCode == SWT.YES ) {
      // ...
    } 
  }
} );
</source>
Please consult the JavaDoc for further details.

=== Background Threads and Display#(a)syncExec() ===
<code>UICallback#activate()</code> and <code>#deactivate()</code> - RWTs [[RAP/UI_Callback|server push]] mechanism - is not affected when running in a cluster. They continue to work as before. 

However, <code>UICallback#activate()</code> is usually used to propagate changes from background threads to the UI.. <code>Display#(a)syncExec()</code> can be used to propagate UI changes from background threads. The passed in runnable is stored in a queue of the Display and executed when the event loop is running for the next time.

In a cluster environment, this has several implications. Passing a runnable to <code>(a)syncExec()</code> effectively alters the session. But as this usually happens outside of a request, the servlet engine has no chance to replicate the changed session.

Each display has a ''Synchronizer'' that manages the queue of async runnables for that display. At application start, the default synchronizer for a display can be altered by calling <code>Display#setSynchronizer()</code>.
Each ''Synchronizer'' is notified whenever a runnable is added by calling either <code>asyncExec()</code> or <code>syncExec()</code>. 
The <code>ClusterSynchronizer</code> is provided to replicate changes to the queue of async runnables of a display. 
It sends a notification-request to the session to which the display belongs that in turn gives the servlet engine the opportunity to replicate the session.

A further problem and one that cannot be solved by RWT is the background thread itself. Threads - naturally - aren't serializable and thus cannot be replicated among cluster nodes. This means that the application is responsible to implement the background thread in a fail-safe way.

=== JFace ===
The RAP port of [[JFace|JFace]] was altered to work with transparent session failover. All classes from <code>org.eclipse.rap.jface</code> except those from the <code>org.eclipse.jface.bindings.*</code> packages are serializable.

=== OSGi ===
Running a clustered RWT application in OSGi is currently not possible. See {{bug|353117}} for more details and a list of blocking issues.

=== Known Issues ===
Images that were created with one of the <code>Graphics#getImage()</code> methods cannot be serialized (<del>{{bug|357769}}</del>).

The graphics context (<code>GC</code>) is not meant to be serialized because it should only be used within the scope of a request. I.e. it is created and destroyed within the <code>paintControl()</code> method of a <code>PaintListener</code>.

As a consequence of the [[#JEE_Compatible_Life_Cycle|JEE compatible life cycle]], <code>Display#sleep()</code> cannot be implemented and throws an <code>UnsupportedOperationException</code>. This affects several other places:
* <code>Browser#execute()</code> and <code>evaluate()</code> will not work, these methods call <code>sleep()</code> to wait for the result of the Javascript execution.
* Attempts to call <code>Windows#open()</code> when <code>setBlockOnOpen()</code> was set to <code>true</code> (which is the default) will not work. Blocking dialogs use a nested event loop to block execution flow. See the  [[#Blocking_Dialogs|Blocking Dialogs]] section on how to work around this issue.
* Entry points that whish to use the JEE compatible life cycle must not contain the usual event loop.

=== Test Infrastructure ===
To be able to run tests from JUnit, there are helper classes with which an arbitrary number of embedded serlvet engines can be created. Either standalone or clustered. Currently the tests can be run with [http://www.eclipse.org/jetty/downloads.php Jetty 7.4] and [http://tomcat.apache.org/index.html Tomcat 7].

The tests reside the [http://eclipse.org/rap/source source code repository] under <code>org.eclipse.rap.rwt.cluster.test</code> and the fixture can be found in <code>org.eclipse.rap.rwt.cluster.testfixture</code>. Tests for the fixture in turn are in <code>org.eclipse.rap.rwt.cluster.testfixture.test</code>.

== How to setup RWT in a cluster ==
Cluster support in RWT requires the servlet 3.0 API. Setup a cluster with a servlet engine that conforms to the servlet 3.0 specification. There are short tutorials that help to set up a cluster with [[Jetty/Feature/Session_Clustering_Using_a_Database|Jetty]] or [http://tomcat.apache.org/tomcat-7.0-doc/cluster-howto.html Tomcat].

The <code>web.xml</code> should look like the one below. The <code>distributable</code> element tells the servlet engine to replicate the session among the nodes in the cluster.
<source lang="xml">
<web-app xmlns="http://java.sun.com/xml/ns/javaee" 
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
  version="2.5">

  <distributable/>

  <context-param>
    <param-name>org.eclipse.rwt.Configurator</param-name>
    <param-value>com.example.ExampleConfigurator</param-value>
  </context-param>

  <listener>
    <listener-class>org.eclipse.rwt.engine.RWTServletContextListener</listener-class>
  </listener>

  <servlet>
    <servlet-name>rwtDelegate</servlet-name>
    <servlet-class>org.eclipse.rwt.engine.RWTServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>rwtDelegate</servlet-name>
    <url-pattern>/example</url-pattern>
  </servlet-mapping>

</web-app>
</source>

The class specified in the <code>org.eclipse.rwt.Configurator</code> parameter must implement the <code>org.eclipse.rwt.application.ApplicationConfigurator</code> interface. 
Its <code>configure()</code> method is called just before the application starts. It should be used to add the entry point(s) that are available to the application and configure other aspects of the application.
To work in a cluster environment, the <code>ApplicationConfigurator</code> implementation must also select the <code>SESSION_FAILOVER</code> ''operation mode''.
The <code>ExampleConfigurator</code> should look like this:
<source lang="java">
public class ExampleConfigurator implements ApplicationConfigurator {
  public void configure( ApplicationConfiguration configuration ) {
    configuration.setOperationMode( OperationMode.SESSION_FAILOVER );
    configuration.addEntryPoint( "default", ExampleEntryPoint.class );
  }
}
</source>

As the cluster-enabled life cycle does not support <code>Display#sleep()</code>, the event loop must be removed from the entry point. The <code>ExampleEntryPoint</code> should look like the one below.
<source lang="java">
public class ExampleEntryPoint implements IEntryPoint {
  public int createUI() {
    Display display = new Display();
    Shell shell = new Shell( display );
    shell.setText( "Hello World" );
    // create more widgets...
    shell.open();
    // omit the event loop
    return 0;
  }
}
</source>

If the application is going to use <code>Display#(a)syncExec()</code> to [[RAP/UI_Callback|propagate UI changes from background threads]], an appropriate ''Synchronizer'' must be attached to the Display.
<source lang="java">
Display display = new Display();
display.setSynchronizer( new ClusteredSynchronizer( display ) );
</source>

When deploying the application in the cluster, it must be ensured that each node receives the exact same version of the application.

== Performance Comparison ==
To get an idea what the performance impact of clustering a web application means, we did basic load tests. 

A simple web application that consists of a shell and several buttons was used. This application was deployed to a [http://tomcat.apache.org Tomcat 7.0] and a [http://www.eclipse.org/jetty Jetty 7.4] servlet engine. Each once configured as a single engine and once clustered.

With the help of [http://jakarta.apache.org/jmeter JMeter], a web session was recorded in which several of the buttons were selected. The entire recording consists of 24 requests. This session was then played back, simulating 500 concurrent users with a ramp up period of 60 Seconds that use the web application.

Even though the performance impact is significant, the response times are sufficiently fast for Ajax-driven web applications
''Manual'' sessions that were opened while the load tests were running, also proved that.
The table below shows the response times for each of the scenarios in Milliseconds.

{|
|            
!Average
!Median
!90% Line
|-
|Jetty single
|align="right" | 4.8 ms
|align="right" | 1 ms
|align="right" | 4 ms
|-
|Jetty cluster
|align="right" | 13.3 ms
|align="right" | 6 ms
|align="right" | 29 ms
|-
|Tomcat single
|align="right" | 1.5 ms
|align="right" | 1 ms
|align="right" | 2 ms
|-
|Tomcat cluster
|align="right" | 3.1 ms
|align="right" | 2 ms
|align="right" | 6 ms
|}


[[Image:RAPClusterPerformance.png|Performance comparison between single and clustered servlet engines]]

== Development Snapshots ==
Development takes place in HEAD, if you are interested you may want to directly check out from the [http://eclipse.org/rap/source source code repository]. Nightly builds are also available and can be obtained from the [http://eclipse.org/rap/downloads/ downloads page].

If you whish to run the cluster tests, you will have to add [http://download.eclipse.org/jetty/ Jetty 8.x], the [http://www.h2database.com/html/main.html H2] database (version 1.1) and the [http://jcp.org/aboutJava/communityprocess/final/jsr315/index.html Servlet API] (version 3.0) to your target platform. The [http://download.eclipse.org/tools/orbit/downloads/ Eclipse Orbit] project offers bundled versions of H2 (<code>org.h2</code>) and the Servlet API (<code>javax.servlet</code>). In addition [http://tomcat.apache.org/index.html Tomcat 7.0.12] is needed, please use the pre-bundled version from the [http://eclipse.org/rap/source RAP source code repository] at {{Git|rap|org.apache.tomcat.git}}.

Please note that these tests (like all other RWT tests) are neither part of nightly builds nor milestone builds.


[[Category:RAP]]