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| <h2 id="index">Data Replication<a id="Data_Replication"></a></h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>There are times when you need to replicate data between datastores. In |
| many cases datastores themselves provide a means of doing this, however |
| if you want to avoid using datastore-specific functionality you can |
| utilise JDO to perform this task. JDO2 allows replication by use of |
| detach/attach functionality. We demonstrate this with an example</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>public class ElementHolder |
| { |
| long id; |
| private Set elements = new HashSet(); |
| |
| ... |
| } |
| |
| public class Element |
| { |
| String name; |
| |
| ... |
| } |
| |
| public class SubElement extends Element |
| { |
| double value; |
| |
| ... |
| }</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>so we have a 1-N unidirectional (Set) relation, and we define the |
| metadata like this</p> |
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| <div class="content"> |
| <pre><jdo> |
| <package name="org.apache.jdo.test"> |
| <class name="ElementHolder" identity-type="application" detachable="true"> |
| <inheritance strategy="new-table"/> |
| <field name="id" primary-key="true"/> |
| <field name="elements" persistence-modifier="persistent"> |
| <collection element-type="Element"/> |
| <join/> |
| </field> |
| </class> |
| |
| <class name="Element" identity-type="application" detachable="true"> |
| <inheritance strategy="new-table"/> |
| <field name="name" primary-key="true"/> |
| </class> |
| |
| <class name="SubElement"> |
| <inheritance strategy="new-table"/> |
| <field name="value"/> |
| </class> |
| </package> |
| </jdo></pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>and so in our application we create some objects in <em>datastore1</em>, like |
| this</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>PersistenceManagerFactory pmf1 = JDOHelper.getPersistenceManagerFactory("jdo.1.properties"); |
| PersistenceManager pm1 = pmf1.getPersistenceManager(); |
| Transaction tx1 = pm1.currentTransaction(); |
| Object holderId = null; |
| try |
| { |
| tx1.begin(); |
| |
| ElementHolder holder = new ElementHolder(101); |
| holder.addElement(new Element("First Element")); |
| holder.addElement(new Element("Second Element")); |
| holder.addElement(new SubElement("First Inherited Element")); |
| holder.addElement(new SubElement("Second Inherited Element")); |
| pm1.makePersistent(holder); |
| |
| tx1.commit(); |
| holderId = JDOHelper.getObjectId(holder); |
| } |
| finally |
| { |
| if (tx1.isActive()) |
| { |
| tx1.rollback(); |
| } |
| pm1.close(); |
| }</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>and now we want to replicate these objects into <em>datastore2</em>, so we |
| detach them from <em>datastore1</em> and attach them to <em>datastore2</em>, like this</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>// Detach the objects from "datastore1" |
| ElementHolder detachedHolder = null; |
| pm1 = pmf1.getPersistenceManager(); |
| tx1 = pm1.currentTransaction(); |
| try |
| { |
| pm1.getFetchPlan().setGroups(new String[] {FetchPlan.DEFAULT, FetchPlan.ALL}); |
| pm1.getFetchPlan().setMaxFetchDepth(-1); |
| |
| tx1.begin(); |
| |
| ElementHolder holder = (ElementHolder) pm1.getObjectById(holderID); |
| detachedHolder = (ElementHolder) pm1.detachCopy(holder); |
| |
| tx1.commit(); |
| } |
| finally |
| { |
| if (tx1.isActive()) |
| { |
| tx1.rollback(); |
| } |
| pm1.close(); |
| } |
| |
| // Attach the objects to datastore2 |
| PersistenceManagerFactory pmf2 = JDOHelper.getPersistenceManagerFactory("jdo.2.properties"); |
| PersistenceManager pm2 = pmf2.getPersistenceManager(); |
| Transaction tx2 = pm2.currentTransaction(); |
| try |
| { |
| tx2.begin(); |
| |
| pm2.makePersistent(detachedHolder); |
| |
| tx2.commit(); |
| } |
| finally |
| { |
| if (tx2.isActive()) |
| { |
| tx2.rollback(); |
| } |
| pm2.close(); |
| }</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>These objects are now replicated into <em>datastore2</em>. Clearly you can |
| extend this basic idea and replicate large amounts of data.</p> |
| </div> |
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