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1.12. Replicating Queues with the HA module

As well as support for an active-passive cluster, the HA module allows you to replicate individual queues, even if the brokers are not in a cluster. The original queue is used as normal. The replica queue is updated automatically as messages are added to or removed from the original queue.

Warning

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 not 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 replica is not used until it has been disconnected from the original.

1.12.1. Replicating queues

To create a replica queue, the HA module must be loaded on both the original and replica brokers (it is loaded by default.) You also need to set the configuration option:

	ha-queue-replication=yes
      

to enable this feature on a stand-alone broker. It is automatically enabled for brokers that are part of a cluster.

Suppose that myqueue is a queue on node1 and we want to create a replica of myqueue on node2 (where both brokers are using the default AMQP port.) This is accomplished by the command:

	qpid-config --broker=node2 add queue --start-replica node1 myqueue
      

If myqueue already exists on the replica broker you can start replication from the original queue like this:

	qpid-ha replicate -b node2 node1 myqueue
      

1.12.2. Replicating queues between clusters

You can replicate queues between two standalone brokers, between a standalone broker and a cluster, or between two clusters (see Section 1.11, “Active-Passive Messaging Clusters”.) For failover in a cluster there are two cases to consider.

  1. When the original queue is on the active node of a cluster, failover is automatic. If the active node fails, the replication link will automatically reconnect and the replica will continue to be updated from the new primary.

  2. When the replica queue is on the active node of a cluster, there is no automatic failover. Automatic failover will be supported in the future, in the meantime you can use the workaround detailed in Section 1.11.12, “Failover of federation links and queue replication.”.