| <div class="wiki-content maincontent"><p>This is much simpler and more powerful in ActiveMQ 4.0 - you can configure different <a shape="rect" href="dispatch-policies.xml">Dispatch Policies</a> directly. </p> |
| |
| <p>For now in ActiveMQ 3.x we have a <a shape="rect" href="what-is-the-prefetch-limit-for.xml">PrefetchPolicy configuration</a>. This allows a certain number of messages to be dispatched to a consumer before they are acknowledged. This feature is to achieve high performance; the higher the value the less likely it is that a client has to wait for messages to arrive before processing the message and so the higher the throughput.</p> |
| |
| <p>e.g. under heavy load of a busy system you probably want to dispatch 1000 or so messages to a consumer, so as soon as its ready to receive a message, there are a bunch of them in memory waiting to be used - rather than having a slow request-response RPC each time.</p> |
| |
| <p>However the effect of this is that when you send a small number of messages, they tend to all go to one consumer unless you've lots of messages. Note that this issue can be avoided in 4.x as there is now a configurable DispatchPolicy so that you can enforce, say, round robin dispatch policy to ensure that messages are dispatched fairly to avoid this issue. </p> |
| |
| <p>In ActiveMQ 3.x or later a work around for this issue is to reconfigure the PrefetchPolicy on the ConnectionFactory/Connection so that the broker only prefetches a single message per consumer; though this will greatly reduce the performance and throughput of the system.</p> |
| |
| <h3 id="HowdoIchangedispatchpolicy-ConfiguringthedispatchpolicyinActiveMQ4.x">Configuring the dispatch policy in ActiveMQ 4.x</h3> |
| |
| <p>In AMQ 4.x, the dispatch policy can now be configured on the xml configuration file (ie. activemq.xml)<br clear="none"> |
| ie.</p> |
| |
| <div class="code panel pdl" style="border-width: 1px;"><div class="codeContent panelContent pdl"> |
| <script class="brush: java; gutter: false; theme: Default" type="syntaxhighlighter"><![CDATA[ |
| <destinationPolicy> |
| <policyMap><policyEntries> |
| <policyEntry topic="COMMAND.>"> |
| <dispatchPolicy> |
| <strictOrderDispatchPolicy /> |
| </dispatchPolicy> |
| <subscriptionRecoveryPolicy> |
| <noSubscriptionRecoveryPolicy /> |
| </subscriptionRecoveryPolicy> |
| </policyEntry> |
| </policyEntries></policyMap> |
| </destinationPolicy> |
| ]]></script> |
| </div></div></div> |
| |