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<?xml version="1.0" encoding="utf-8"?>
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<section>
<title>Queue Replication with the HA module</title>
<para>
As well as support for an active-passive cluster, the <filename>ha</filename> module
also allows you to replicate individual queues. The <firstterm>original</firstterm>
queue is used as normal. The <firstterm>replica</firstterm> queue is updated
automatically as messages are added to or removed from the original queue.
</para>
<para>
To create a replica you need the HA module to be loaded on both the orignal and replica
brokers. Note that it is not safe to modify the replica queue other than via the
automatic updates from the original. Adding or removing messages on the replica queue
will make replication inconsistent and may cause message loss. The HA module does
<emphasis>not</emphasis> enforce restricted access to the replica queue (as it does in
the case of a cluster) so it is up to the application to ensure the replca is not used
until it has been disconnected from the original.
</para>
<para>
Suppose that <command>myqueue</command> is a queue on <command>node1</command> and
we want to create a replica of <command>myqueue</command> on <command>node2</command>
(where both brokers are using the default AMQP port.) This is accomplished by the command:
<programlisting>
qpid-config --broker=node2 add queue --start-replica node1 myqueue
</programlisting>
</para>
<para>
If <command>myqueue</command> already exists on the replica broker you can start replication from the original queue like this:
<programlisting>
qpid-ha replicate -b node2 node1 myqueue
</programlisting>
</para>
</section>
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