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<div class="sect1">
<h1 class="sect1"><font face="arial, helvetica, sans-serif" color="#330099">About Data Partitioning Policies</font></h1>
<p>Data partitioning allows for an application to scale its data across more than a single database machine. EclipseLink supports data partitioning at the Entity level to allow a different set of entity instances for the same class to be stored in a different physical database or different node within a database cluster. Both regular databases and clustered databases are supported. Data can be partitioned both horizontally and vertically.</p>
<p>Partitioning can be enabled on an entity, a relationship, a query, or a persistence unit.</p>
<p>To configure data partitioning, use the <code>@Partitioned</code> annotation and one or more partitioning policy annotations. The annotations for defining the different kinds of policies are:</p>
<ul>
<li>
<p><code>@HashPartitioning</code>; Partitions access to a database cluster by the hash of a field value from the object, such as the object's ID, location, or tenant. The hash indexes into the list of connection pools/nodes. All write or read requests for objects with that hash value are sent to the same server. If a query does not include the hash field as a parameter, it can be sent to all servers and unioned, or it can be left to the session's default behavior.</p>
</li>
<li>
<p><code>@PinnedPartitioning</code>; Pins requests to a single connection pool/node. This allows for vertical partitioning.</p>
</li>
<li>
<p><code>@RangePartitioning</code>; Partitions access to a database cluster by a field value from the object, such as the object's ID, location, or tenant. Each server is assigned a range of values. All write or read requests for objects with that value are sent to the same server. If a query does not include the field as a parameter, then it can either be sent to all servers and unioned, or left to the session's default behavior.</p>
</li>
<li>
<p><code>@ReplicationPartitioning</code>; Sends requests to a set of connection pools/nodes. This policy is for replicating data across a cluster of database machines. Only modification queries are replicated.</p>
</li>
<li>
<p><code>@RoundRobinPartitioning</code>; Sends requests in a round-robin fashion to the set of connection pools/nodes. It is for load balancing read queries across a cluster of database machines. It requires that the full database be replicated on each machine, so it does not support partitioning. The data should either be read-only, or writes should be replicated.</p>
</li>
<li>
<p><code>@UnionPartitioning</code>; Sends queries to all connection pools and unions the results. This is for queries or relationships that span partitions when partitioning is used, such as on a <code>ManyToMany</code> cross partition relationship.</p>
</li>
<li>
<p><code>@ValuePartitioning</code>; Partitions access to a database cluster by a field value from the object, such as the object's location or tenant. Each value is assigned a specific server. All write or read requests for objects with that value are sent to the same server. If a query does not include the field as a parameter, then it can be sent to all servers and unioned, or it can be left to the session's default behavior.</p>
</li>
<li>
<p><code>@Partitioning</code>; Partitions access to a database cluster by a custom partitioning policy. A <code>PartitioningPolicy</code> class must be provided and implemented.</p>
</li>
</ul>
<p>Partitioning policies are globally-named objects in a persistence unit and are reusable across multiple descriptors or queries. This improves the usability of the configuration, specifically with JPA annotations and metadata.</p>
<p>The persistence unit properties support adding named connection pools in addition to the existing configuration for read/write/sequence. A named connection pool must be defined for each node in the database cluster.</p>
<p>If a transaction modifies data from multiple partitions, JTA should be used to ensure 2-phase commit of the data. An exclusive connection can also be configured in the <code>EntityManager</code> to ensure only a single node is used for a single transaction.</p>
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