6.3. vmCursor on Destination

In Red Hat JBoss A-MQ, a cursor is used to hold a batch of messages in memory, while those messages are waiting to be sent to a destination. By default, the batch of messages is pulled out of the message store and then held in the cursor (this is the store cursor).

JBoss A-MQ has another cursor implementation, the VM cursor, which can be significantly faster in some cases. With the VM cursor, incoming messages are inserted directly into the cursor, bypassing the message store (the messages are also, concurrently, inserted into the message store). This works well if the consumers are fast and are able to keep up with the flow of messages. On the other hand, for slow consumers this strategy does not work so well, because the VM cursor fills up with a backlog of messages and then it becomes necessary to invoke flow control to throttle messages from the producer.

To configure a broker to use the vmCursor for all topics and queues, add the following lines to your broker configuration:

<broker ... >
  ...
  <destinationPolicy>
    <policyMap>
      <policyEntries>
        <policyEntry topic=">">
          <pendingSubscriberPolicy>
            <vmCursor />
          </pendingSubscriberPolicy>
        </policyEntry>
        <policyEntry queue=">">
          <deadLetterStrategy>
            <individualDeadLetterStrategy queuePrefix="Test.DLQ."/>
          </deadLetterStrategy>
          <pendingQueuePolicy>
            <vmQueueCursor />
          </pendingQueuePolicy>
        </policyEntry>
      </policyEntries>
    </policyMap>
  </destinationPolicy>
  ...
</broker>

Where both the topic and queue entries specify the wildcard, >, that matches all destination names. You could also specify a more selective destination pattern, so that the VM cursor would be enabled only for those destinations where you are sure that consumers can keep up with the message flow.

For more information about the Red Hat JBoss A-MQ cursor implementations—, and the advantages and disadvantages of each one see Configuring Broker Persistence.