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<h1 class="sect1"><font face="arial, helvetica, sans-serif" color="#330099">About Explicit Query Refreshes</font></h1>
<p>Some distributed systems require only a small number of objects to be consistent across the servers in the system. Conversely, other systems require that several specific objects must always be guaranteed to be up-to-date, regardless of the cost. If you build such a system, you can explicitly refresh selected objects from the database at appropriate intervals, without incurring the full cost of distributed cache coordination.</p>
<p>To implement this type of strategy, do the following:</p>
<ol>
<li>
<p>Configure a set of queries that refresh the required objects.</p>
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<li>
<p>Establish an appropriate refresh policy.</p>
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<li>
<p>Invoke the queries as required to refresh the objects.</p>
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<p>The <code>@Cache</code> annotation provides the <code>alwaysRefresh</code> and <code>refreshOnlyIfNewer</code> attributes which force all queries that go to the database to refresh the cache. The cache is only actually refreshed if the optimistic lock value in the database is newer than in the cache. For more information, see "Refreshing the Cache" in <em>Solutions Guide for EclipseLink</em>.</p>
<p>When you execute a query, if the required objects are in the cache, EclipseLink returns the cached objects without checking the database for a more recent version. This reduces the number of objects that EclipseLink must build from database results, and is optimal for noncoordinated cache environments. However, this may not always be the best strategy for a coordinated cache environment.</p>
<p>To override this behavior, set the <code>alwaysRefresh</code> attribute to specify that the objects from the database always take precedence over objects in the cache. This updates the cached objects with the data from the database.</p>
<p>You can implement this type of refresh policy on each EclipseLink entity, or just on certain queries, depending upon the nature of the application.</p>
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