JDO : Datastore Replication

Many applications make use of multiple datastores. It is a common requirement to be able to replicate parts of one datastore in another datastore. Obviously, depending on the datastore, you could make use of the datastores own capabilities for replication. DataNucleus provides its own extension to JDO to allow replication from one datastore to another. This extension doesn't restrict you to using 2 datastores of the same type. You could replicate from RDBMS to XML for example, or from MySQL to HSQLDB.

The following sample code will replicate all objects of type Product and Employee from PMF1 to PMF2. These PMFs are created in the normal way so, as mentioned above, PMF1 could be for a MySQL datastore, and PMF2 for XML. By default this will replicate the complete object graphs reachable from these specified types.

import org.datanucleus.api.jdo.JDOReplicationManager;

...

JDOReplicationManager replicator = new JDOReplicationManager(pmf1, pmf2);
replicator.replicate(new Class[]{Product.class, Employee.class});
            


Example without using the JDOReplicationManager helper

If we just wanted to use pure JDO, we would handle replication like this. Let's take an example

public class ElementHolder
{
    long id;
    private Set elements = new HashSet();

    ...
}

public class Element
{
    String name;

    ...
}

public class SubElement extends Element
{
    double value;

    ...
}

so we have a 1-N unidirectional (Set) relation, and we define the metadata like this

<jdo>
    <package name="org.datanucleus.samples">
        <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="org.datanucleus.samples.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>

and so in our application we create some objects in datastore1 , like this

PersistenceManagerFactory pmf1 = JDOHelper.getPersistenceManagerFactory("dn.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();
}

and now we want to replicate these objects into datastore2 , so we detach them from datastore1 and attach them to datastore2 , like this

// 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("dn.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();
}

That's all there is. These objects are now replicated into datastore2 . Clearly you can extend this basic idea and replicate large amounts of data.