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<section id="queue-state-replication">
  <title>
    Queue State Replication
  </title>

  <section role="h2" id="queuestatereplication-AsynchronousReplicationofQueueState">
    <title>
      Asynchronous
      Replication of Queue State
    </title>

    <section role="h3" id="queuestatereplication-Overview">
      <title>
	Overview
      </title>
      <para>
	There is support in qpidd for selective asynchronous replication
	of queue state. This is achieved by:
      </para>
      <para>
	(a) enabling event generation for the queues in question
      </para>
      <para>
	(b) loading a plugin on the 'source' broker to encode those
	events as messages on a replication queue (this plugin is
	called
	replicating_listener.so)
      </para>
      <para>
	(c) loading a custom exchange plugin on the 'backup' broker (this
	plugin is called replication_exchange.so)
      </para>
      <para>
	(d) creating an instance of the replication exchange type on the
	backup broker
      </para>
      <para>
	(e) establishing a federation bridge between the replication
	queue on the source broker and the replication exchange on the
	backup broker
      </para>
      <para>
	The bridge established between the source and backup brokers for
	replication (step (e) above) should have acknowledgements turned
	on (this may be done through the --ack N option to qpid-route).
	This ensures that replication events are not lost if the bridge
	fails.
      </para>
      <para>
	The replication protocol will also eliminate duplicates to ensure
	reliably replicated state. Note though that only one bridge per
	replication exchange is supported. If clients try to publish to
	the replication exchange or if more than a the single required
	bridge from the replication queue on the source broker is
	created, replication will be corrupted. (Access control may be
	used to restrict access and help prevent this).
      </para>
      <para>
	The replicating event listener plugin (step (b) above) has the
	following options:
      </para>
      <programlisting>
Queue Replication Options:
  --replication-queue QUEUE                      Queue on which events for
                                                 other queues are recorded
  --replication-listener-name NAME (replicator)  name by which to register the
                                                 replicating event listener
  --create-replication-queue                     if set, the replication will
                                                 be created if it does not
                                                 exist
      </programlisting>
      <para>
	The name of the queue is required. It can either point to a
	durable queue whose definition has been previously recorded, or
	the --create-replication-queue option can be specified in which
	case the queue will be created a simple non-durable queue if it
	does not already exist.
      </para>
      <!--h3-->
    </section>

    <section role="h3" id="queuestatereplication-UsewithClustering">
      <title>
	Use with
	Clustering
      </title>
      <para>
	The source and/or backup brokers may also be clustered brokers.
	In this case the federated bridge will be re-established between
	replicas should either of the originally connected nodes fail.
	There are however the following limitations at present:
      </para>
      <itemizedlist>
	<listitem>
          <para>The backup site does not process membership updates after it
	  establishes the first connection. In order for newly added
	  members on a source cluster to be eligible as failover targets,
	  the bridge must be recreated after those members have been added
	  to the source cluster.
	  </para>
	</listitem>
      </itemizedlist>
      <itemizedlist>
	<listitem>
          <para>New members added to a backup cluster will not receive
	  information about currently established bridges. Therefore in
	  order to allow the bridge to be re-established from these members
	  in the event of failure of older nodes, the bridge must be
	  recreated after the new members have joined.
	  </para>
	</listitem>
      </itemizedlist>
      <itemizedlist>
	<listitem>
          <para>Only a single URL can be passed to create the initial link
	  from backup site to the primary site. this means that at the time
	  of creating the initial connection the initial node in the
	  primary site to which the connection is made needs to be running.
	  Once connected the backup site will receive a membership update
	  of all the nodes in the primary site, and if the initial
	  connection node in the primary fails, the link will be
	  re-established on the next node that was started (time) on the
	  primary site.
	  </para>
	</listitem>
      </itemizedlist>
      <para>
	Due to the acknowledged transfer of events over the bridge (see
	note above) manual recreation of the bridge and automatic
	re-establishment of te bridge after connection failure (including
	failover where either or both ends are clustered brokers) will
	not result in event loss.
      </para>
      <!--h3-->
    </section>

    <section role="h3" id="queuestatereplication-OperationsonBackupQueues">
      <title>
	Operations
	on Backup Queues
      </title>
      <para>
	When replicating the state of a queue to a backup broker it is
	important to recognise that any other operations performed
	directly on the backup queue may break the replication.
      </para>
      <para>
	If the backup queue is to be an active (i.e. accessed by clients
	while replication is on) only enqueues should be selected
	for
	replication. In this mode, any message enqueued on the source
	brokers copy of the queue will also be enqueued on the backup
	brokers copy. However not attempt will be made to remove messages
	from the backup queue in response to removal of messages from the
	source queue.
      </para>
      <!--h3-->
    </section>

    <section role="h3" id="queuestatereplication-SelectingQueuesforReplication">
      <title>
	Selecting
	Queues for Replication
      </title>
      <para>
	Queues are selected for replication by specifying the types of
	events they should generate (it is from these events that the
	replicating plugin constructs messages which are then pulled and
	processed by the backup site). This is done through options
	passed to the initial queue-declare command that creates the
	queue and may be done either through qpid-config or similar
	tools, or by the application.
      </para>
      <para>
	With qpid-config, the --generate-queue-events options is used:
      </para>
      <programlisting>
    --generate-queue-events N
                         If set to 1, every enqueue will generate an event that can be processed by
                         registered listeners (e.g. for replication). If set to 2, events will be
                         generated for enqueues and dequeues
      </programlisting>
      <para>
	From an application, the arguments field of the queue-declare
	AMQP command is used to convey this information. An entry should
	be added to the map with key 'qpid.queue_event_generation' and an
	integer value of 1 (to replicate only enqueue events) or 2 (to
	replicate both enqueue and dequeue events).
      </para>
      <para>
	Applications written using the c++ client API may fine the
	qpid::client::QueueOptions class convenient. This has a
	enableQueueEvents() method on it that can be used to set the
	option (the instance of QueueOptions is then passed as the value
	of the arguments field in the queue-declare command. The boolean
	option to that method should be set to true if only enequeue
	events should be replicated; by default it is false meaning that
	both enqueues and dequeues will be replicated. E.g.
      </para>
      <programlisting>
    QueueOptions options;
    options.enableQueueEvents(false);
    session.queueDeclare(arg::queue="my-queue", arg::arguments=options);
      </programlisting>
      <!--h3-->
    </section>

    <section role="h3" id="queuestatereplication-Example">
      <title>
	Example
      </title>
      <para>
	Lets assume we will run the primary broker on host1 and the
	backup on host2, have installed qpidd on both and have the
	replicating_listener and replication_exchange plugins in qpidd's
	module directory(*1).
      </para>
      <para>
	On host1 we start the source broker and specifcy that a queue
	called 'replication' should be used for storing the events until
	consumed by the backup. We also request that this queue be
	created (as transient) if not already specified:
      </para>
      <programlisting>
    qpidd --replication-queue replication-queue --create-replication-queue true --log-enable info+
      </programlisting>
      <para>
	On host2 we start up the backup broker ensuring that the
	replication exchange module is loaded:
      </para>
      <programlisting>
    qpidd
      </programlisting>
      <para>
	We can then create the instance of that replication exchange that
	we will use to process the events:
      </para>
      <programlisting>
    qpid-config -a host2 add exchange replication replication-exchange
      </programlisting>
      <para>
	If this fails with the message "Exchange type not implemented:
	replication", it means the replication exchange module was
	not
	loaded. Check that the module is installed on your system and if
	necessary provide the full path to the library.
      </para>
      <para>
	We then connect the replication queue on the source broker with
	the replication exchange on the backup broker using the
	qpid-route command:
      </para>
      <programlisting>
    qpid-route --ack 50 queue add host2 host1 replication-exchange replication-queue
</programlisting>
          <para>
            The example above configures the bridge to acknowledge messages
            in batches of 50.
          </para>
          <para>
            Now create two queues (on both source and backup brokers), one
            replicating both enqueues and dequeues (queue-a) and the
            other
            replicating only dequeues (queue-b):
          </para>
            <programlisting>
    qpid-config -a host1 add queue queue-a --generate-queue-events 2
    qpid-config -a host1 add queue queue-b --generate-queue-events 1

    qpid-config -a host2 add queue queue-a
    qpid-config -a host2 add queue queue-b
	    </programlisting>
	    <para>
	      We are now ready to use the queues and see the replication.
	    </para>
	    <para>
	      Any message enqueued on queue-a will be replicated to the backup
	      broker. When the message is acknowledged by a client connected to
	      host1 (and thus dequeued), that message will be removed from the
	      copy of the queue on host2. The state of queue-a on host2 will
	      thus mirror that of the equivalent queue on host1, albeit with a
	      small lag. (Note
	      however that we must not have clients connected to host2 publish
	      to-or consume from- queue-a or the state will fail to replicate
	      correctly due to conflicts).
	    </para>
	    <para>
	      Any message enqueued on queue-b on host1 will also be enqueued on
	      the equivalent queue on host2. However the acknowledgement and
	      consequent dequeuing of messages from queue-b on host1 will have
	      no effect on the state of queue-b on host2.
	    </para>
	    <para>
	      (*1) If not the paths in the above may need to be modified. E.g.
	      if using modules built from a qpid svn checkout, the following
	      would be added to the command line used to start qpidd on host1:
	    </para>
            <programlisting>
    --load-module &lt;path-to-qpid-dir&gt;/src/.libs/replicating_listener.so
	    </programlisting>
	    <para>
	      and the following for the equivalent command line on host2:
	    </para>
            <programlisting>
    --load-module &lt;path-to-qpid-dir&gt;/src/.libs/replication_exchange.so
	    </programlisting>
	    <!--h3-->
    </section>
    <!--h2-->
  </section>
</section>