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<td align="left" valign="top"><font face="helvetica, arial, sans-serif"><b>EclipseLink Solutions Guide for EclipseLink</b><br />
Release 2.7</div></td>
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<h1 class="sect1"><font face="arial, helvetica, sans-serif" color="#330099">Introduction to the Solution</font></h1>
<p>EclipseLink applications that are deployed to an application server cluster benefit from cluster scalability, load balancing, and failover. These capabilities ensure that EclipseLink applications are highly available and scale as application demand increases. EclipseLink applications are deployed the same way in application server clusters as they are in standalone server environments. However, additional planning and configuration is required to ensure cache consistency in an application server cluster.</p>
<p>EclipseLink uses a shared (L2) object cache that avoids database access for objects and their relationships. The cache is enabled by default and enhances application performance. In an application server cluster, caching can result in consistency issues (such as stale data) because changes made on one server are not reflected on objects cached in other servers. Cache consistency is problematic only for objects that are frequently updated. Read-only objects are not affected by cache consistency. For more details about caching, see:</p>
<p><code><a href="http://wiki.eclipse.org/EclipseLink/UserGuide/JPA/Basic_JPA_Development/Caching/Caching_Overview">http://wiki.eclipse.org/EclipseLink/UserGuide/JPA/Basic_JPA_Development/Caching/Caching_Overview</a></code></p>
<p>Various options are available for addressing cache consistency:</p>
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<li>
<p>Use cache coordination. Cache coordination is a feature that broadcasts changes between the servers in the cluster to update or invalidate changed objects.</p>
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<li>
<p>Use distributed caching. EclipseLink Grid is an integration between EclipseLink and Oracle Coherence that addresses many cache consistency issues that result from operating in a distributed environment. For details on EclipseLink Grid, see</p>
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<li>
<p>Use optimistic locking. Optimistic locking is a feature that prevents updates to stale objects, and triggers the objects to be invalidated in the cache.</p>
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<p>Refresh the cache. Refreshing a cache loads that latest data in the cache.</p>
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<p>Disable the shared cache for highly volatile entities or limit the cache to read-only objects.</p>
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