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<h1 class="sect1"><font face="arial, helvetica, sans-serif" color="#330099">Database Event Notification and Oracle CQN</font></h1>
<p>Some databases and database products allow events to be raised from the database when rows are updated or deleted.</p>
<p>EclipseLink supports an API to allow the database to notify EclipseLink of database changes, so the changed objects can be invalidated in the EclipseLink shared cache. This allows a shared cache to be used, and stale data to be avoided, even if other applications access the same data in the database. EclipseLink supports integration with the Oracle Database feature for Database Change Notification CQN. A custom <code>DatabaseEventListener</code> may be provided for other databases and products that support database events.</p>
<p>There are also other solutions to caching in a shared environment, including:</p>
<ul>
<li>
<p>Disable the shared cache (through setting <code>@Cacheable(false)</code>, or <code>@Cache(isolation=ISOLATED)</code>).</p>
</li>
<li>
<p>Only cache read-only objects.</p>
</li>
<li>
<p>Set a cache invalidation timeout to reduce stale data.</p>
</li>
<li>
<p>Use refreshing on objects/queries when fresh data is required.</p>
</li>
<li>
<p>Use optimistic locking to ensure write consistency (writes on stale data will fail, and will automatically invalidate the cache).</p>
</li>
</ul>
<p>The JPA Cache API and the EclipseLink <code>JpaCache</code> API can also be used directly to invalidate objects in the shared cache by the application. EclipseLink cache coordination could also be used to send invalidation messages to a cluster of EclipseLink persistence units.</p>
<p>Database events can reduce the chance of an application getting stale data, but do not eliminate the possibility. Optimistic locking should still be used to ensure data integrity. Even in a single server application stale data is still possible within a persistence context unless pessimistic locking is used. Optimistic (or pessimistic) locking is always required to ensure data integrity in any multi-user system.</p>
<div class="sect2"><a id="sthref61" name="sthref61"></a>
<h2 class="sect2"><font face="arial, helvetica, sans-serif" color="#330099">Oracle Continuous Query Notification</font></h2>
<p>The Oracle database released a Continuous Query Notification (CQN) feature in the 10.2 release. CQN allows for database events to be raised when the rows in a table are modified. The JDBC API for CQN was not complete until 11.2, so 11.2 is required for EclipseLink's integration.</p>
<p>EclipseLink CQN support is enabled by the <code>OracleChangeNotificationListener</code> listener which integrates with Oracle JDBC to received database change events. Use the <code>eclipselink.cache.database-event-listener</code> property to configure the full class name of the listener.</p>
<p>By default all tables in the persistence unit are registered for change notification, but this can be configured using the <code>databaseChangeNotificationType</code> attribute of the <code>@Cache</code> annotation to selectively disable change notification for certain classes.</p>
<p>Oracle CQN uses the <code>ROWID</code> to inform of row level changes. This requires EclipseLink to include the <code>ROWID</code> in all queries for a CQN enabled class. EclipseLink must also select the object's <code>ROWID</code> after an insert operation. EclipseLink must maintain a cache index on the <code>ROWID</code>, in addition to the object's Id. EclipseLink also selects the database transaction Id once each transaction to avoid invalidating the cache on the server that is processing the transaction.</p>
<p>EclipseLink's CQN integration has the following limitations:</p>
<ul>
<li>
<p>Changes to an object's secondary tables will not trigger it to be invalidate unless a version is used and updated in the primary table.</p>
</li>
<li>
<p>Changes to an object's OneToMany, ManyToMany, and ElementCollection relationships will not trigger it to be invalidate unless a version is used and updated in the primary table.</p>
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