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         <ul id="-path-navigation"><li><a href="/index.html">Home</a></li><li><a href="/releases/index.html">Releases</a></li><li><a href="/releases/qpid-jms-amqp-0-x-6.3.1/index.html">Qpid JMS AMQP 0-x 6.3.1</a></li><li><a href="/releases/qpid-jms-amqp-0-x-6.3.1/jms-amqp-0-8-book/index.html">Apache Qpid JMS AMQP 0-8/0-9/0-9-1</a></li><li>5.4.&#160;Session</li></ul>

         <div id="-middle-content">
           <div class="docbook"><div class="navheader"><table summary="Navigation header" width="100%"><tr><th align="center" colspan="3">5.4.&#160;Session</th></tr><tr><td align="left" width="20%"><a accesskey="p" href="JMS-Client-0-8-Client-Understanding-Connection.html">Prev</a>&#160;</td><th align="center" width="60%">Chapter&#160;5.&#160;Understanding the Client</th><td align="right" width="20%">&#160;<a accesskey="n" href="JMS-Client-0-8-Client-Understanding-MessageProducer.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="JMS-Client-0-8-Client-Understanding-Session"></a>5.4.&#160;Session</h2></div></div></div><p>A Session object is a single-threaded context for producing and consuming messages.</p><p>Session objects are created from the Connection. Whilst Session objects are relatively
       lightweight, patterns utilising a single Session per message are not recommended.</p><p>The number of sessions open per connection at any one time is limited. This value is
       negotiated when the connection is made. It defaults to 256.</p><p>Qpid JMS Sessions have the ability to prefetch messages to improve consumer performance.
       This feature is described next.</p><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="JMS-Client-0-8-Client-Understanding-Session-Prefecth"></a>5.4.1.&#160;Prefetch</h3></div></div></div><p>Prefetch specifies how many messages the client will optimistically cache for delivery
         to a consumer. This is a useful parameter to tune that can improve the throughput of an
         application. The prefetch buffer is scoped per <span class="emphasis"><em>Session</em></span>.</p><p>The size of the prefetch buffer can be tuned per Connection using the connection url
         option <a class="link" href="JMS-Client-0-8-Connection-URL.html#JMS-Client-0-8-Connection-URL-ConnectionOptions-Maxprefetch"><code class="literal">maxprefetch</code></a> (or JVM wide using the system property <a class="link" href="JMS-Client-0-8-System-Properties.html#JMS-Client-0-8-System-Properties-Maxprefetch"><code class="literal">max_prefetch</code></a>). By default, prefetch defaults to 500.</p><p>There are situations when you may wish to consider reducing the size of prefetch:</p><p>
         </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>When using a <a class="link" href="http://www.eaipatterns.com/CompetingConsumers.html" target="_top">Competing Consumers</a> pattern, prefetch can give the appearance of unequal
               division of work. This will be apparent on startup when the queue has messages. The
               first consumer started will cache prefetch size number of messages, possibly leaving
               the other consumers with no initial work.</p></li><li class="listitem"><p>When using special queue types (such as LVQs, Sorted Queue and Priority Queues).
               For these queue types the special delivery rules apply whilst the message resides on
               the Broker. As soon as the message is sent to the client it delivery order is then
               fixed. For example, if using a priority queue, and a prefetch of 100, and 100 messages
               arrive with priority 2, the broker will send these to the client. If then a new
               message arrives with priority 1, the broker cannot leap frog messages of the lower
               priority. The priority 1 message will be delivered at the front of the next
               batch.</p></li><li class="listitem"><p>When message size is large and you do not wish the memory footprint of the
               application to grow (or suffer an OutOfMemoryError).</p></li></ol></div><p>
       </p><p>Finally, if using multiple MessageConsumers on a single Session, keep in mind that
         unless you keep polling <span class="emphasis"><em>all</em></span> consumers, it is possible for some traffic
         patterns to result in consumer starvation and an application level deadlock. For example, if
         prefetch is 100, and 100 hundred messages arrive suitable for consumer A, those messages
         will be prefetched by the session, entirely filling the prefetch buffer. Now if the
         application performs a blocking <a class="link" href="http://docs.oracle.com/javaee/6/api/javax/jms/MessageConsumer.html#receive()" target="_top">MessageConsumer#receive()</a> for Consumer B on the same Session, the application
         will hang indefinitely as even if messages suitable for B arrive at the Broker. Those
         messages can never be sent to the Session as no space is available in prefetch. </p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>Please note, when the acknowledgement mode
             <span class="emphasis"><em>Session#SESSION_TRANSACTED</em></span> or
             <span class="emphasis"><em>Session#CLIENT_ACKNOWLEDGE</em></span> is set on a consuming session, the
           prefetched messages are released from the prefetch buffer on transaction commit/rollback
           (in case of acknowledgement mode <span class="emphasis"><em>Session#SESSION_TRANSACTED</em></span> ) or
           acknowledgement of the messages receipt (in case of acknowledgement mode
             <span class="emphasis"><em>Session#CLIENT_ACKNOWLEDGE</em></span> ). If the consuming application does not
           commit/rollback the receiving transaction (for example, due to mistakes in application
           exception handling logic), the prefetched messages continue to remain in the prefetch
           buffer preventing the delivery of the following messages. As result, the application might
           stop the receiving of the messages until the transaction is committed/rolled back (for
             <span class="emphasis"><em>Session#SESSION_TRANSACTED</em></span> ) or received messages are acknowledged
           (for <span class="emphasis"><em>Session#CLIENT_ACKNOWLEDGE</em></span>).</p></div><p>
           Settings maxprefetch to 0 ( either globally via JVM system property
           <a class="link" href="JMS-Client-0-8-System-Properties.html#JMS-Client-0-8-System-Properties-Maxprefetch"><code class="literal">max_prefetch</code></a>
           or on a connection level as a connection option
           <a class="link" href="JMS-Client-0-8-Connection-URL.html#JMS-Client-0-8-Connection-URL-ConnectionOptions-Maxprefetch"><code class="literal">maxprefetch</code></a> )
           switches off the pre-fetching functionality. With maxprefetch=0 messages are fetched one by one without caching on the client.
       </p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p> Setting maxprefetch to 0 is recommended in Spring-JMS based applications whenever
             <span class="emphasis"><em>DefaultMassgeListenerContainer</em></span> is configured with a
             <span class="emphasis"><em>CachingConnectionFactory</em></span> that has <span class="emphasis"><em>cacheLevel</em></span>
           set to either <span class="emphasis"><em>CACHE_CONSUMER</em></span> or <span class="emphasis"><em>CACHE_SESSION</em></span>.
           In these configurations the Qpid JMS <span class="emphasis"><em>Session</em></span> objects remain open in
           Spring's dynamically scaled pools. If maxprefetch is not 0, any prefetched messages held
           by the <span class="emphasis"><em>Session</em></span> and any new ones subsequently sent to it (in the
           background until prefetch is reached) will be effectively by 'stuck' (unavailable to the
           application) until Spring decides to utilise the cached Session again. This can give the
           impression that message delivery has stopped even though messages remain of the queue.
           Setting maxprefetch to 0 prevents this problem from occurring.</p><p> If using maxprefetch &gt; 0 <span class="emphasis"><em>SingleConnectionFactory</em></span> must be
           used. SingleConnectionFactory does not have the same session/consumer caching behaviour so
           does not exhibit the same problem. </p></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="JMS-Client-0-8-Client-Understanding-Session-TemporaryQueues"></a>5.4.2.&#160;TemporaryQueues</h3></div></div></div><p>Temporary queues are exposed to Management in the same way as normal queues. Temporary
         queue names take the form string <code class="literal">TempQueue</code> followed by a random
         UUID.</p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="JMS-Client-0-8-Client-Understanding-Session-CreateQueue"></a>5.4.3.&#160;CreateQueue</h3></div></div></div><p>In the Client, <a class="link" href="http://docs.oracle.com/javaee/6/api/javax/jms/Session.html#createQueue(java.lang.String)" target="_top">Session#createQueue()</a> accepts either a queue name, or a Binding URL. If only name
         is specified the destination will be resolved into binding URL:
         direct://amq.direct//&lt;queue name&gt;?routingkey='&lt;queue name&gt;'&amp;durable='true'. </p><p>Calling Session#createQueue() has no effect on the Broker.</p><p>Reiterating the advice from the JMS javadoc, it is suggested that this method is not
         generally used. Instead, application should lookup Destinations declared within JNDI.</p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="JMS-Client-0-8-Client-Understanding-Session-CreateTopic"></a>5.4.4.&#160;CreateTopic</h3></div></div></div><p>In the Client, <a class="link" href="http://docs.oracle.com/javaee/6/api/javax/jms/Session.html#createTopic(java.lang.String)" target="_top">Session#createTopic()</a> accepts either a topic name, or a Binding URL. If only name
         is specified the destination will be resolved into binding URL: topic://amq.topic//&lt;topic
         name&gt;?routingkey='&lt;topic name&gt;'.</p><p>Calling Session#createTopic() has no effect on the Broker.</p><p>Reiterating the advice from the JMS javadoc, it is suggested that this method is not
         generally used. Instead, application should lookup Destinations declared within JNDI.</p></div></div><div class="navfooter"><hr /><table summary="Navigation footer" width="100%"><tr><td align="left" width="40%"><a accesskey="p" href="JMS-Client-0-8-Client-Understanding-Connection.html">Prev</a>&#160;</td><td align="center" width="20%"><a accesskey="u" href="JMS-Client-0-8-Client-Understanding.html">Up</a></td><td align="right" width="40%">&#160;<a accesskey="n" href="JMS-Client-0-8-Client-Understanding-MessageProducer.html">Next</a></td></tr><tr><td align="left" valign="top" width="40%">5.3.&#160;Connection&#160;</td><td align="center" width="20%"><a accesskey="h" href="JMS-Client-Book.html">Home</a></td><td align="right" valign="top" width="40%">&#160;5.5.&#160;MessageProducer</td></tr></table></div></div>

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	<div id="-middle-content">
	<div class="docbook"><div class="navheader"><table summary="Navigation header" width="100%"><tr><th align="center" colspan="3">5.4. Session</th></tr><tr><td align="left" width="20%"><a accesskey="p" href="JMS-Client-0-8-Client-Understanding-Connection.html">Prev</a> </td><th align="center" width="60%">Chapter 5. Understanding the Client</th><td align="right" width="20%"> <a accesskey="n" href="JMS-Client-0-8-Client-Understanding-MessageProducer.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="JMS-Client-0-8-Client-Understanding-Session"></a>5.4. Session</h2></div></div></div><p>A Session object is a single-threaded context for producing and consuming messages.</p><p>Session objects are created from the Connection. Whilst Session objects are relatively
	lightweight, patterns utilising a single Session per message are not recommended.</p><p>The number of sessions open per connection at any one time is limited. This value is
	negotiated when the connection is made. It defaults to 256.</p><p>Qpid JMS Sessions have the ability to prefetch messages to improve consumer performance.
	This feature is described next.</p><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="JMS-Client-0-8-Client-Understanding-Session-Prefecth"></a>5.4.1. Prefetch</h3></div></div></div><p>Prefetch specifies how many messages the client will optimistically cache for delivery
	to a consumer. This is a useful parameter to tune that can improve the throughput of an
	application. The prefetch buffer is scoped per <span class="emphasis"><em>Session</em></span>.</p><p>The size of the prefetch buffer can be tuned per Connection using the connection url
	option <a class="link" href="JMS-Client-0-8-Connection-URL.html#JMS-Client-0-8-Connection-URL-ConnectionOptions-Maxprefetch"><code class="literal">maxprefetch</code></a> (or JVM wide using the system property <a class="link" href="JMS-Client-0-8-System-Properties.html#JMS-Client-0-8-System-Properties-Maxprefetch"><code class="literal">max_prefetch</code></a>). By default, prefetch defaults to 500.</p><p>There are situations when you may wish to consider reducing the size of prefetch:</p><p>
	</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>When using a <a class="link" href="http://www.eaipatterns.com/CompetingConsumers.html" target="_top">Competing Consumers</a> pattern, prefetch can give the appearance of unequal
	division of work. This will be apparent on startup when the queue has messages. The
	first consumer started will cache prefetch size number of messages, possibly leaving
	the other consumers with no initial work.</p></li><li class="listitem"><p>When using special queue types (such as LVQs, Sorted Queue and Priority Queues).
	For these queue types the special delivery rules apply whilst the message resides on
	the Broker. As soon as the message is sent to the client it delivery order is then
	fixed. For example, if using a priority queue, and a prefetch of 100, and 100 messages
	arrive with priority 2, the broker will send these to the client. If then a new
	message arrives with priority 1, the broker cannot leap frog messages of the lower
	priority. The priority 1 message will be delivered at the front of the next
	batch.</p></li><li class="listitem"><p>When message size is large and you do not wish the memory footprint of the
	application to grow (or suffer an OutOfMemoryError).</p></li></ol></div><p>
	</p><p>Finally, if using multiple MessageConsumers on a single Session, keep in mind that
	unless you keep polling <span class="emphasis"><em>all</em></span> consumers, it is possible for some traffic
	patterns to result in consumer starvation and an application level deadlock. For example, if
	prefetch is 100, and 100 hundred messages arrive suitable for consumer A, those messages
	will be prefetched by the session, entirely filling the prefetch buffer. Now if the
	application performs a blocking <a class="link" href="http://docs.oracle.com/javaee/6/api/javax/jms/MessageConsumer.html#receive()" target="_top">MessageConsumer#receive()</a> for Consumer B on the same Session, the application
	will hang indefinitely as even if messages suitable for B arrive at the Broker. Those
	messages can never be sent to the Session as no space is available in prefetch. </p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>Please note, when the acknowledgement mode
	<span class="emphasis"><em>Session#SESSION_TRANSACTED</em></span> or
	<span class="emphasis"><em>Session#CLIENT_ACKNOWLEDGE</em></span> is set on a consuming session, the
	prefetched messages are released from the prefetch buffer on transaction commit/rollback
	(in case of acknowledgement mode <span class="emphasis"><em>Session#SESSION_TRANSACTED</em></span> ) or
	acknowledgement of the messages receipt (in case of acknowledgement mode
	<span class="emphasis"><em>Session#CLIENT_ACKNOWLEDGE</em></span> ). If the consuming application does not
	commit/rollback the receiving transaction (for example, due to mistakes in application
	exception handling logic), the prefetched messages continue to remain in the prefetch
	buffer preventing the delivery of the following messages. As result, the application might
	stop the receiving of the messages until the transaction is committed/rolled back (for
	<span class="emphasis"><em>Session#SESSION_TRANSACTED</em></span> ) or received messages are acknowledged
	(for <span class="emphasis"><em>Session#CLIENT_ACKNOWLEDGE</em></span>).</p></div><p>
	Settings maxprefetch to 0 ( either globally via JVM system property
	<a class="link" href="JMS-Client-0-8-System-Properties.html#JMS-Client-0-8-System-Properties-Maxprefetch"><code class="literal">max_prefetch</code></a>
	or on a connection level as a connection option
	<a class="link" href="JMS-Client-0-8-Connection-URL.html#JMS-Client-0-8-Connection-URL-ConnectionOptions-Maxprefetch"><code class="literal">maxprefetch</code></a> )
	switches off the pre-fetching functionality. With maxprefetch=0 messages are fetched one by one without caching on the client.
	</p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p> Setting maxprefetch to 0 is recommended in Spring-JMS based applications whenever
	<span class="emphasis"><em>DefaultMassgeListenerContainer</em></span> is configured with a
	<span class="emphasis"><em>CachingConnectionFactory</em></span> that has <span class="emphasis"><em>cacheLevel</em></span>
	set to either <span class="emphasis"><em>CACHE_CONSUMER</em></span> or <span class="emphasis"><em>CACHE_SESSION</em></span>.
	In these configurations the Qpid JMS <span class="emphasis"><em>Session</em></span> objects remain open in
	Spring's dynamically scaled pools. If maxprefetch is not 0, any prefetched messages held
	by the <span class="emphasis"><em>Session</em></span> and any new ones subsequently sent to it (in the
	background until prefetch is reached) will be effectively by 'stuck' (unavailable to the
	application) until Spring decides to utilise the cached Session again. This can give the
	impression that message delivery has stopped even though messages remain of the queue.
	Setting maxprefetch to 0 prevents this problem from occurring.</p><p> If using maxprefetch > 0 <span class="emphasis"><em>SingleConnectionFactory</em></span> must be
	used. SingleConnectionFactory does not have the same session/consumer caching behaviour so
	does not exhibit the same problem. </p></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="JMS-Client-0-8-Client-Understanding-Session-TemporaryQueues"></a>5.4.2. TemporaryQueues</h3></div></div></div><p>Temporary queues are exposed to Management in the same way as normal queues. Temporary
	queue names take the form string <code class="literal">TempQueue</code> followed by a random
	UUID.</p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="JMS-Client-0-8-Client-Understanding-Session-CreateQueue"></a>5.4.3. CreateQueue</h3></div></div></div><p>In the Client, <a class="link" href="http://docs.oracle.com/javaee/6/api/javax/jms/Session.html#createQueue(java.lang.String)" target="_top">Session#createQueue()</a> accepts either a queue name, or a Binding URL. If only name
	is specified the destination will be resolved into binding URL:
	direct://amq.direct//<queue name>?routingkey='<queue name>'&durable='true'. </p><p>Calling Session#createQueue() has no effect on the Broker.</p><p>Reiterating the advice from the JMS javadoc, it is suggested that this method is not
	generally used. Instead, application should lookup Destinations declared within JNDI.</p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="JMS-Client-0-8-Client-Understanding-Session-CreateTopic"></a>5.4.4. CreateTopic</h3></div></div></div><p>In the Client, <a class="link" href="http://docs.oracle.com/javaee/6/api/javax/jms/Session.html#createTopic(java.lang.String)" target="_top">Session#createTopic()</a> accepts either a topic name, or a Binding URL. If only name
	is specified the destination will be resolved into binding URL: topic://amq.topic//<topic
	name>?routingkey='<topic name>'.</p><p>Calling Session#createTopic() has no effect on the Broker.</p><p>Reiterating the advice from the JMS javadoc, it is suggested that this method is not
	generally used. Instead, application should lookup Destinations declared within JNDI.</p></div></div><div class="navfooter"><hr /><table summary="Navigation footer" width="100%"><tr><td align="left" width="40%"><a accesskey="p" href="JMS-Client-0-8-Client-Understanding-Connection.html">Prev</a> </td><td align="center" width="20%"><a accesskey="u" href="JMS-Client-0-8-Client-Understanding.html">Up</a></td><td align="right" width="40%"> <a accesskey="n" href="JMS-Client-0-8-Client-Understanding-MessageProducer.html">Next</a></td></tr><tr><td align="left" valign="top" width="40%">5.3. Connection </td><td align="center" width="20%"><a accesskey="h" href="JMS-Client-Book.html">Home</a></td><td align="right" valign="top" width="40%"> 5.5. MessageProducer</td></tr></table></div></div>

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