160 lines
10 KiB
XML
160 lines
10 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!-- ============================================================================= -->
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<!-- Licensed to the Apache Software Foundation (ASF) under one or more -->
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<!-- contributor license agreements. See the NOTICE file distributed with -->
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<!-- this work for additional information regarding copyright ownership. -->
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<!-- The ASF licenses this file to You under the Apache License, Version 2.0 -->
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<!-- (the "License"); you may not use this file except in compliance with -->
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<!-- the License. You may obtain a copy of the License at -->
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<!-- -->
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<!-- http://www.apache.org/licenses/LICENSE-2.0 -->
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<!-- -->
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<!-- Unless required by applicable law or agreed to in writing, software -->
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<!-- distributed under the License is distributed on an "AS IS" BASIS, -->
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<!-- WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. -->
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<!-- See the License for the specific language governing permissions and -->
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<!-- limitations under the License. -->
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<!-- ============================================================================= -->
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<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
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<!ENTITY % BOOK_ENTITIES SYSTEM "ActiveMQ_User_Manual.ent">
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%BOOK_ENTITIES;
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]>
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<chapter id="undelivered-messages">
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<title>Message Redelivery and Undelivered Messages</title>
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<para>Messages can be delivered unsuccessfully (e.g. if the transacted session used to consume
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them is rolled back). Such a message goes back to its queue ready to be redelivered. However,
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this means it is possible for a message to be delivered again and again without any success
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and remain in the queue, clogging the system.</para>
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<para>There are 2 ways to deal with these undelivered messages:</para>
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<itemizedlist>
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<listitem>
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<para>Delayed redelivery.</para>
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<para>It is possible to delay messages redelivery to let the client some time to recover
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from transient failures and not overload its network or CPU resources</para>
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</listitem>
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<listitem>
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<para>Dead Letter Address.</para>
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<para>It is also possible to configure a dead letter address so that after a specified
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number of unsuccessful deliveries, messages are removed from the queue and will not be
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delivered again</para>
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</listitem>
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</itemizedlist>
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<para>Both options can be combined for maximum flexibility.</para>
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<section>
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<title>Delayed Redelivery</title>
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<para>Delaying redelivery can often be useful in the case that clients regularly fail or
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rollback. Without a delayed redelivery, the system can get into a "thrashing" state, with
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delivery being attempted, the client rolling back, and delivery being re-attempted ad
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infinitum in quick succession, consuming valuable CPU and network resources.</para>
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<section id="undelivered-messages.delay">
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<title>Configuring Delayed Redelivery</title>
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<para>Delayed redelivery is defined in the address-setting configuration:</para>
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<programlisting>
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<!-- delay redelivery of messages for 5s -->
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<address-setting match="jms.queue.exampleQueue">
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<!-- default is 1.0 -->
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<redelivery-delay-multiplier>1.5</redelivery-delay-multiplier>
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<!-- default is 0 (no delay) -->
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<redelivery-delay>5000</redelivery-delay>
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<!-- default is redelivery-delay * 10 -->
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<max-redelivery-delay>50000</max-redelivery-delay>
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</address-setting></programlisting>
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<para>If a <literal>redelivery-delay</literal> is specified, ActiveMQ will wait this delay
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before redelivering the messages.</para>
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<para>By default, there is no redelivery delay (<literal>redelivery-delay</literal>is set
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to 0).</para>
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<para>Other subsequent messages will be delivery regularly, only the cancelled message
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will be sent asynchronously back to the queue after the delay.</para>
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<para>You can specify a multiplier that will take effect on top of the redelivery-delay
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with a max-redelivery-delay to be taken into account.</para>
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<para>The max-redelivery-delay is defaulted to redelivery-delay * 10</para>
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<para>Address wildcards can be used to configure redelivery delay for a set of addresses
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(see <xref linkend="wildcard-syntax"/>), so you don't have to specify redelivery delay
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individually for each address.</para>
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</section>
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<section>
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<title>Example</title>
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<para>See <xref linkend="examples.delayed-redelivery"/> for an example which shows how
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delayed redelivery is configured and used with JMS.</para>
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</section>
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</section>
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<section>
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<title>Dead Letter Addresses</title>
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<para>To prevent a client infinitely receiving the same undelivered message (regardless of
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what is causing the unsuccessful deliveries), messaging systems define <emphasis
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role="italic">dead letter addresses</emphasis>: after a specified unsuccessful delivery
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attempts, the message is removed from the queue and send instead to a dead letter address. </para>
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<para>Any such messages can then be diverted to queue(s) where they can later be perused by
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the system administrator for action to be taken.</para>
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<para>ActiveMQ's addresses can be assigned a dead letter address. Once the messages have been
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unsuccessfully delivered for a given number of attempts, they are removed from the queue
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and sent to the dead letter address. These <emphasis>dead letter</emphasis> messages can
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later be consumed for further inspection.</para>
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<section id="undelivered-messages.configuring">
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<title>Configuring Dead Letter Addresses</title>
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<para>Dead letter address is defined in the address-setting configuration:</para>
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<programlisting>
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<!-- undelivered messages in exampleQueue will be sent to the dead letter address
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deadLetterQueue after 3 unsuccessful delivery attempts -->
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<address-setting match="jms.queue.exampleQueue">
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<dead-letter-address>jms.queue.deadLetterQueue</dead-letter-address>
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<max-delivery-attempts>3</max-delivery-attempts>
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</address-setting></programlisting>
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<para>If a <literal>dead-letter-address</literal> is not specified, messages will removed
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after <literal>max-delivery-attempts</literal> unsuccessful attempts.</para>
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<para>By default, messages are redelivered 10 times at the maximum. Set <literal
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>max-delivery-attempts</literal> to -1 for infinite redeliveries.</para>
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<para>For example, a dead letter can be set globally for a set of matching addresses and
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you can set <literal>max-delivery-attempts</literal> to -1 for a specific address
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setting to allow infinite redeliveries only for this address.</para>
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<para>Address wildcards can be used to configure dead letter settings for a set of
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addresses (see <xref linkend="wildcard-syntax"/>).</para>
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</section>
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<section>
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<title>Dead Letter Properties</title>
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<para>Dead letter messages which are consumed from a dead letter address have the following
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properties:</para>
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<itemizedlist>
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<listitem>
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<para><literal>_HQ_ORIG_ADDRESS</literal></para>
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<para>a String property containing the <emphasis>original address</emphasis> of
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the dead letter message</para>
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</listitem>
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<listitem>
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<para><literal>_HQ_ORIG_QUEUE</literal></para>
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<para>a String property containing the <emphasis>original queue</emphasis> of
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the dead letter message</para>
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</listitem>
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</itemizedlist>
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</section>
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<section>
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<title>Example</title>
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<para>See <xref linkend="examples.dead-letter"/> for an example which shows how dead letter
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is configured and used with JMS.</para>
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</section>
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</section>
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<section id="configuring.delivery.count.persistence">
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<title>Delivery Count Persistence</title>
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<para>In normal use, ActiveMQ does not update delivery count <emphasis>persistently</emphasis>
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until a message is rolled back (i.e. the delivery count is not updated
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<emphasis>before</emphasis> the message is delivered to the consumer). In most messaging
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use cases, the messages are consumed, acknowledged and forgotten as soon as they are
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consumed. In these cases, updating the delivery count persistently before delivering the
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message would add an extra persistent step <emphasis>for each message delivered</emphasis>,
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implying a significant performance penalty.</para>
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<para>However, if the delivery count is not updated persistently before the message delivery
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happens, in the event of a server crash, messages might have been delivered but that will
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not have been reflected in the delivery count. During the recovery phase, the server will
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not have knowledge of that and will deliver the message with <literal>redelivered</literal>
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set to <literal>false</literal> while it should be <literal>true</literal>. </para>
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<para>As this behavior breaks strict JMS semantics, ActiveMQ allows to persist delivery count
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before message delivery but disabled it by default for performance implications.</para>
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<para>To enable it, set <literal>persist-delivery-count-before-delivery</literal> to <literal
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>true</literal> in <literal>activemq-configuration.xml</literal>:</para>
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<programlisting>
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<persist-delivery-count-before-delivery>true</persist-delivery-count-before-delivery></programlisting>
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</section>
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</chapter>
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