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diff --git a/qpid/doc/book/src/How-to-Use-SlowConsumerDisconnect.xml b/qpid/doc/book/src/How-to-Use-SlowConsumerDisconnect.xml new file mode 100644 index 0000000000..5f4c627430 --- /dev/null +++ b/qpid/doc/book/src/How-to-Use-SlowConsumerDisconnect.xml @@ -0,0 +1,280 @@ +<?xml version="1.0" encoding="utf-8"?> +<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN" + "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd"> +<!-- + + Licensed to the Apache Software Foundation (ASF) under one + or more contributor license agreements. See the NOTICE file + distributed with this work for additional information + regarding copyright ownership. The ASF licenses this file + to you under the Apache License, Version 2.0 (the + "License"); you may not use this file except in compliance + with the License. You may obtain a copy of the License at + + http://www.apache.org/licenses/LICENSE-2.0 + + Unless required by applicable law or agreed to in writing, + software distributed under the License is distributed on an + "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY + KIND, either express or implied. See the License for the + specific language governing permissions and limitations + under the License. + +--> +<section> + <title>Slow Consumer Disconnect - User Guide</title> + + <section> +<title>Introduction</title> + <para>Slow Consumer Disconnect (SCD) is a new feature in Qpid that provides a configurable + mechanism to prevent a single slow consumer from causing a back up of unconsumed messages on + the broker. </para> + + <para>This is most relevant where Topics are in use, since a published message is not removed + from the broker's memory until all subscribers have acknowledged that message. </para> + + <para>Cases where a consumer is 'slow' can arise due to one of the following: poor network + connectivity exists; a transient system issue affects a single client; a single subscriber + written by a client team is behaving incorrectly and not acknowledging messages; a + downstream resource such as a database is non-responsive. </para> + + <para>SCD will enable the application owner to configure limits for a given consumer's queue and + the behaviour to execute when those limits are reached. </para> + + </section> + + <section> +<title>What can it do?</title> + <para>SCD is only applicable to topics or durable subscriptions and can be configured on either + a topic or a subscription name. </para> + + <para>On triggering of a specified threshold the offending client will be disconnected from the + broker with a 506 error code wrapped in a JMSException returned to the client via the + ExceptionListener registered on the Connection object. </para> + + <para>Note that it is essential that an ExceptionListener be specified by the client on + creation of the connection and that exceptions coming back on that listener are handled + correctly. </para> + + </section> + + <section> +<title>Frequency of SCD Checking</title> + <section> +<title><emphasis role='bold'>Configuring Frequency</emphasis></title> + <para>You can configure the frequency with which the SCD process will check for slow consumers, + along with the unit of time used to specify that frequency. </para> + + <para>The <emphasis role="italic">virtualhosts.virtualhost.hostname.slow-consumer-detection</emphasis> + elements <emphasis role="italic">delay</emphasis> and <emphasis role="italic">timeunit</emphasis> + are used to specify the frequency and timeunit respectively in the virtualhosts.xml + file e.g. </para> + +<programlisting> +<virtualhosts> + <default>test</default> + <virtualhost> + <name>test</name> + <test> + <slow-consumer-detection> + <delay>60<delay/> + <timeunit>seconds<timeunit/> + <slow-consumer-detection/> + </test> + </virtualhost> +</virtualhosts> +</programlisting> + + </section> + + <section> +<title><emphasis role='bold'>SCD Log output</emphasis></title> + <para>When the SCD component finds a queue with a configured threshold to check, the operational + logging component (if enabled) will output the following line:</para> + + <programlisting> + SCD-1003 : Checking Status of Queue + </programlisting> + + </section> + + </section> + + <section> +<title>Client Exception<emphasis role='bold'>s</emphasis></title> + <para>When a Slow Consumer is disconnected, the client receives a 506 error from the broker + wrapped in a JMSException and the Session and Connection are closed:</para> + +<programlisting> +Dispatcher-Channel-1 2010-09-01 16:23:34,206 INFO [qpid.client.AMQSession.Dispatcher] + Dispatcher-Channel-1 thread terminating for channel 1:org.apache.qpid.client.AMQSession_0_8@1de8aa8 +pool-2-thread-3 2010-09-01 16:23:34,238 INFO [apache.qpid.client.AMQConnection] Closing AMQConnection due to + :org.apache.qpid.AMQChannelClosedException: Error: Consuming to slow. [error code 506: resource error] +javax.jms.JMSException: 506 +at org.apache.qpid.client.AMQConnection.exceptionReceived(AMQConnection.java:1396) +at org.apache.qpid.client.protocol.AMQProtocolHandler.exception(AMQProtocolHandler.java:329) +at org.apache.qpid.client.protocol.AMQProtocolHandler.methodBodyReceived(AMQProtocolHandler.java:536) +at org.apache.qpid.client.protocol.AMQProtocolSession.methodFrameReceived(AMQProtocolSession.java:453) +at org.apache.qpid.framing.AMQMethodBodyImpl.handle(AMQMethodBodyImpl.java:93) +at org.apache.qpid.client.protocol.AMQProtocolHandler$1.run(AMQProtocolHandler.java:462) +at org.apache.qpid.pool.Job.processAll(Job.java:110) +at org.apache.qpid.pool.Job.run(Job.java:149) +at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:885) +at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:907) +at java.lang.Thread.run(Thread.java:619) +Caused by: org.apache.qpid.AMQChannelClosedException: Error: Consuming to slow. [error code 506: resource error] +at org.apache.qpid.client.handler.ChannelCloseMethodHandler.methodReceived(ChannelCloseMethodHandler.java:96) +at org.apache.qpid.client.handler.ClientMethodDispatcherImpl.dispatchChannelClose(ClientMethodDispatcherImpl.java:163) +at org.apache.qpid.framing.amqp_8_0.ChannelCloseBodyImpl.execute(ChannelCloseBodyImpl.java:140) +at org.apache.qpid.client.state.AMQStateManager.methodReceived(AMQStateManager.java:112) +at org.apache.qpid.client.protocol.AMQProtocolHandler.methodBodyReceived(AMQProtocolHandler.java:511) +... 8 more +main 2010-09-01 16:23:34,316 INFO [apache.qpid.client.AMQSession] Closing session: + org.apache.qpid.client.AMQSession_0_8@ffeef1 +</programlisting> + + </section> + + <section> +<title>Disconnection Thresholds</title> + <section> +<title>Topic Subscriptions</title> + <para>One key feature of SCD is the disconnection of a consuming client when a specified + threshold is exceeded. For a pub-sub model using topics, this means that messages will no + longer be delivered to the private queue which was associated with that consuming client, + thus reducing any associated backlog in the broker. </para> + + </section> + + <section> +<title>Durable Topic Subscriptions</title> + <para>For durable subscriptions, simply disconnecting the consuming client will not suffice + since the associated queue is by definition durable and messages would continue to flow to + it after disconnection, potentially worsening any backing up of data on the broker. </para> + + <para>The solution is to configure durable subscriptions to delete the underlying queue on + disconnection. This means that messages will no longer be delivered to the private queue + associated with the subscription, thus preventing any backlog. </para> + + <para>Full details of how to configure the thresholds are provided below. </para> + + </section> + + <section> +<title>Message Age Threshold</title> + <para>You can configure SCD to be triggered on a topic or subscription when the oldest message + in the associated private queue for the consumer ages beyond the specified value, in + milliseconds. </para> + + </section> + + <section> +<title>Queue Depth Threshold</title> + <para>You can opt to use the depth of the queue in bytes as a threshold. SCD will be triggered + by a queue depth greater than the threshold specified i.e. when a broker receives a + message that takes the queue depth over the threshold. </para> + + </section> + + <section> +<title>Message Count Threshold</title> + <para>You can use the message count for the consumer's queue as the trigger, where a count + higher than that specified will trigger disconnection. </para> + + </section> + + <section> +<title><emphasis role='bold'>Delete Policy</emphasis></title> + <para>You can configure the policy you wish to apply in your broker configuration. There are + currently 2 policies available: </para> + + <para> +<emphasis role='bold'>Delete Temporary Queues Only</emphasis> + </para> + + <para>If you do not specify a <topicDelete/> element in your configuration, then only temporary + queues associated with a topic subscription will be deleted on client disconnect. This is + the default behaviour. </para> + <para/> + + <para> +<emphasis role='bold'>Delete Durable Subscription Queues</emphasis> + </para> + + <para>If you add the <topicDelete/> element with the sub-element + <delete-persistent/> to your config, then the persistent queue which is associated + with durable subscriptions to a topic will also be deleted. This is an important + consideration since without deleting the underlying queue the client's unconsumed data + will grow indefinitely while they will be unable to reconnect to that queue due to the SCD + threshold configured, potentially having an adverse effect on the application or broker in + use.</para> + <para/> + + <para><emphasis role="bold"> Example Topic Configuration </emphasis></para> + + <para/> + + <para> +The following steps are required to configure SCD: + </para> + +<itemizedlist> + <listitem> + <para>Enable SCD checking for your virtual host</para> + </listitem> + <listitem> + <para>Specify frequency for SCD checking</para> + </listitem> + <listitem> + <para>Define thresholds for the topic</para> + </listitem> + <listitem> + <para>Define the policy to apply on trigger </para> + </listitem> +</itemizedlist> + + <para>The example below shows a simple definition, with all three thresholds specified and a + simple disconnection, with deletion of any temporary queue, defined. </para> + + <para>For a durable subscription to this topic, no queue deletion would be applied on disconnect + - which is likely to be undesirable (see section above). </para> + +<programlisting> +<topics> + <topic> + <name>stocks.us.*</name> + <slow-consumer-detection> + <!-- The maximum depth before which --> + <!-- the policy will be applied--> + <depth>4235264</depth> + <!-- The maximum message age before which --> + <!-- the policy will be applied--> + <messageAge>600000</messageAge> + <!-- The maximum number of message before --> + <!-- which the policy will be applied--> + <messageCount>50</messageCount> + <!-- Policy Selection --> + <policy name="TopicDelete"/> + </slow-consumer-detection> + </topic> +</topics> +</programlisting> + + </section> + + </section> + + <section> +<title>Important Points To Note</title> + <para> Client application developers should be educated about how to correctly handle being + disconnected with a 506 error code, to avoid them getting into a thrashing state where they + continually attempt to connect, fail to consume fast enough and are disconnected again. </para> + + <para>Clients affected by slow consumer disconnect configuration should always use transactions + where duplicate processing of an incoming message would have adverse affects, since they may + receive a message more than once if disconnected before acknowledging a message in flight. </para> + + </section> + + </section> + |