How to use Castor in a J2EE cluster

# How to use Castor in a J2EE cluster

## Introduction

With release 0.9.9, several cache providers have been added that are distributed caches per se or can be configured to operate in such a mode. This effectively allows Castor JDO to be used in a clustered J2EE (multi-JVM) environment, where Castor JDO runs on each of the cluster instances, and where cache state is automatically synchronized between these instances.

In such an environment, Castor JDO will make use of the underlying cache provider to replicate/distribute the content of a specific cache between the various JDOManager instances. Through the distribution mechanism of the cache provider, a client of a Castor JDO instance on one JVM will see any updates made to domain objects performed against any other JVM/JDO instance.

## Intended Audience

Anyone who wants to use Castor JDO in a J2EE cluster.

The example given describes the use of the Coherence cache provider to enable distributed caching.

## Prerequisites

You should have a valid class mapping for a Java class, similar to the following one:

    <mapping>
        <class name="com.xyz.MyOtherObject" identity="id">
            <field name="id" type="integer">
                   ...
            </field>
            <field name="description" type="string">
                   ...
            </field>
        </class>
    </mapping>

## Steps

Here is how to proceed.

### Add <cache-type> element to class mapping

Add a `<cache-type>` element as shown below, specifying the cache provider to use in the '`type`' attribute.

    <mapping>
        <class name="com.xyz.MyOtherObject" identity="id">
            <cache-type type="coherence"/>
            <field name="id" type="integer">
                   ...
            </field>
            <field name="description" type="string">
                   ...
            </field>
        </class>
    </mapping>

This instructs Castor JDO to use the '`coherence`' cache provider for objects of type com.xyz.MyOtherObject. It is the cache provider that is responsible to distribute any changes to the cache state to all other Castor JDO instances within the same cluster.

### Add Coherence JARs to CLASSPATH

Add the Coherence JARs to the class path of your e.g. web application by putting the JARs into the `WEB-INF/lib` folder of your web application.

## References

- Caching

- Caching and clustered environments

- Tangosol Coherence