| <div class="docbook"><div class="navheader"><table summary="Navigation header" width="100%"><tr><th align="center" colspan="3">9.7. Consumers</th></tr><tr><td align="left" width="20%"><a accesskey="p" href="Java-Broker-Runtime-Flow-To-Disk.html">Prev</a> </td><th align="center" width="60%">Chapter 9. Runtime</th><td align="right" width="20%"> <a accesskey="n" href="Java-Broker-Runtime-Background-Recovery.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="Java-Broker-Runtime-Consumers"></a>9.7. Consumers</h2></div></div></div><p>A Consumer is created when an AMQP connection wishes to receive messages from a message source (such as a | |
| Queue). The standard behaviours of consumers are defined by the respective AMQP specification, however in addition | |
| to the standard behaviours a number of Qpid specific enhancements are available</p><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="Java-Broker-Runtime-Consumers-Prioirty"></a>9.7.1. Priority</h3></div></div></div><p>By default, when there are multiple competing consumers attached to the same message source, the Broker | |
| attempts to distribute messages from the queue in a "fair" manner. Some use cases require allocation of messages | |
| to consumers to be based on the "priority" of the consumer. Where there are multiple consumers having differing | |
| priorities, the Broker will always attempt to deliver a message to a higher priority consumer before attempting | |
| delivery to a lower priority consumer. That is, a lower priority consumer will only receive a message if no | |
| higher priority consumers currently have credit available to consume the message, or those consumers have declined | |
| to accept the message (for instance because it does not meet the criteria of any selectors associated with the | |
| consumer).</p><p>Where a consumer is created with no explicit priority provided, the consumer is given the highest possible | |
| priority.</p><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="d0e6102"></a>9.7.1.1. Creating a Consumer with a non-standard priority</h4></div></div></div><p> | |
| In AMQP 0-9 and 0-9-1 the priority of the consumer can be set by adding an entry into the table provided as the | |
| <code class="literal">arguments</code> field (known as the <code class="literal">filter</code> field on AMQP 0-9) of the | |
| <code class="literal">basic.consume</code> method. The key for the entry must be the literal short string | |
| <code class="literal">x-priority</code>, and the value of the entry must be an integral number in the range | |
| -2<sup>31</sup> to 2<sup>31</sup>-1. | |
| </p><p> | |
| In AMQP 0-10 the priority of the consumer can be set in the map provided as the <code class="literal">arguments</code> | |
| field of the <code class="literal">message.subscribe</code> method. The key for the entry must be the literal string | |
| <code class="literal">x-priority</code>, and the value of the entry must be an integral number in the range | |
| -2<sup>31</sup> to 2<sup>31</sup>-1. | |
| </p><p> | |
| In AMQP 1.0 the priority of the consumer is set in the <code class="literal">properties</code> map of the | |
| <code class="literal">attach</code> frame where the broker side of the link represents the sending side of the link. | |
| The key for the entry must be the literal string <code class="literal">priority</code>, and the value of the entry must | |
| be an integral number in the range -2<sup>31</sup> to 2<sup>31</sup>-1. | |
| </p><p> | |
| When using the Qpid JMS client for AMQP 0-9/0-9-1/0-10 the consumer priority can be set in the address being | |
| used for the Destination object. | |
| </p><div class="table"><a id="d0e6161"></a><p class="title"><strong>Table 9.1. Setting the consumer priority</strong></p><div class="table-contents"><table border="1" summary="Setting the consumer priority"><colgroup><col /><col /></colgroup><thead><tr><th>Syntax</th><th>Example</th></tr></thead><tbody><tr><td>Addressing</td><td>myqueue : { link : { x-subscribe: { arguments : { x-priority : '10' } } } }</td></tr><tr><td>Binding URL</td><td>direct://amq.direct/myqueue/myqueue?x-qpid-replay-priority='10'</td></tr></tbody></table></div></div><p><br class="table-break" /> | |
| </p></div></div></div><div class="navfooter"><hr /><table summary="Navigation footer" width="100%"><tr><td align="left" width="40%"><a accesskey="p" href="Java-Broker-Runtime-Flow-To-Disk.html">Prev</a> </td><td align="center" width="20%"><a accesskey="u" href="Java-Broker-Runtime.html">Up</a></td><td align="right" width="40%"> <a accesskey="n" href="Java-Broker-Runtime-Background-Recovery.html">Next</a></td></tr><tr><td align="left" valign="top" width="40%">9.6. Flow to Disk </td><td align="center" width="20%"><a accesskey="h" href="Apache-Qpid-Broker-J-Book.html">Home</a></td><td align="right" valign="top" width="40%"> 9.8. Background Recovery</td></tr></table></div></div> |