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From jbert...@apache.org
Subject [42/51] [partial] activemq-6 git commit: ACTIVEMQ6-1 - Initial HornetQ Donation Commit
Date Mon, 10 Nov 2014 16:32:53 GMT
http://git-wip-us.apache.org/repos/asf/activemq-6/blob/8ecd255f/docs/user-manual/en/clusters.xml
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+<?xml version="1.0" encoding="UTF-8"?>
+<!-- ============================================================================= -->
+<!-- Copyright © 2009 Red Hat, Inc. and others.                                    -->
+<!--                                                                               -->
+<!-- The text of and illustrations in this document are licensed by Red Hat under  -->
+<!-- a Creative Commons Attribution–Share Alike 3.0 Unported license ("CC-BY-SA"). -->
+<!--                                                                               -->
+<!-- An explanation of CC-BY-SA is available at                                    -->
+<!--                                                                               -->
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+<!--                                                                               -->
+<!-- In accordance with CC-BY-SA, if you distribute this document or an adaptation -->
+<!-- of it, you must provide the URL for the original version.                     -->
+<!--                                                                               -->
+<!-- Red Hat, as the licensor of this document, waives the right to enforce,       -->
+<!-- and agrees not to assert, Section 4d of CC-BY-SA to the fullest extent        -->
+<!-- permitted by applicable law.                                                  -->
+<!-- ============================================================================= -->
+
+<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
+        <!ENTITY % BOOK_ENTITIES SYSTEM "HornetQ_User_Manual.ent">
+        %BOOK_ENTITIES;
+        ]>
+<chapter id="clusters">
+    <title>Clusters</title>
+    <section>
+        <title>Clusters Overview</title>
+        <para>HornetQ clusters allow groups of HornetQ servers to be grouped together in order to
+            share message processing load. Each active node in the cluster is an active HornetQ
+            server which manages its own messages and handles its own connections. </para>
+        <note id="clustered-deprecation">
+            <para>The <emphasis>clustered</emphasis> parameter is deprecated and no longer needed for
+            setting up a cluster. If your configuration contains this parameter it will be ignored and
+            a message with the ID <literal>HQ221038</literal> will be logged.</para>
+        </note>
+        <para>The cluster is formed by each node declaring <emphasis>cluster connections</emphasis>
+            to other nodes in the core configuration file <literal
+                    >hornetq-configuration.xml</literal>. When a node forms a cluster connection to
+            another node, internally it creates a <emphasis>core bridge</emphasis> (as described in
+            <xref linkend="core-bridges"/>) connection between it and the other node, this is
+            done transparently behind the scenes - you don't have to declare an explicit bridge for
+            each node. These cluster connections allow messages to flow between the nodes of the
+            cluster to balance load.</para>
+        <para>Nodes can be connected together to form a cluster in many different topologies, we
+            will discuss a couple of the more common topologies later in this chapter.</para>
+        <para>We'll also discuss client side load balancing, where we can balance client connections
+            across the nodes of the cluster, and we'll consider message redistribution where HornetQ
+            will redistribute messages between nodes to avoid starvation.</para>
+        <para>Another important part of clustering is <emphasis>server discovery</emphasis> where
+            servers can broadcast their connection details so clients or other servers can connect
+            to them with the minimum of configuration.</para>
+        <warning id="copy-warning">
+            <para>Once a cluster node has been configured it is common to simply copy that configuration
+            to other nodes to produce a symmetric cluster. However, care must be taken when copying the
+            HornetQ files. Do not copy the HornetQ <emphasis>data</emphasis> (i.e. the
+            <literal>bindings</literal>, <literal>journal</literal>, and <literal>large-messages</literal>
+            directories) from one node to another. When a node is started for the first time and initializes
+            its journal files it also persists a special identifier to the <literal>journal</literal>
+            directory. This id <emphasis>must</emphasis> be unique among nodes in the cluster or the
+            cluster will not form properly.</para>
+        </warning>
+    </section>
+    <section id="clusters.server-discovery">
+        <title>Server discovery</title>
+        <para>Server discovery is a mechanism by which servers can propagate their connection details to:</para>
+        <itemizedlist>
+            <listitem>
+                <para>
+                    Messaging clients. A messaging client wants to be able to connect
+                    to the servers of the cluster without having specific knowledge of which servers
+                    in the cluster are up at any one time.
+                </para>
+            </listitem>
+            <listitem>
+                <para>Other servers. Servers in a cluster want to be able to create
+                    cluster connections to each other without having prior knowledge of all the
+                    other servers in the cluster.</para>
+            </listitem>
+        </itemizedlist>
+        <para>
+            This information, let's call it the Cluster Topology, is actually sent around normal HornetQ
+            connections to clients and to other servers over cluster connections. This being the case we need a
+            way of establishing the initial first connection. This can be done using
+            dynamic discovery techniques like <ulink url="http://en.wikipedia.org/wiki/User_Datagram_Protocol" >UDP</ulink>
+            and <ulink url="http://www.jgroups.org/">JGroups</ulink>, or by
+            providing a list of initial connectors.
+        </para>
+        <section>
+            <title>Dynamic Discovery</title>
+            <para>
+                Server discovery uses <ulink url="http://en.wikipedia.org/wiki/User_Datagram_Protocol" >UDP</ulink>
+                multicast or <ulink url="http://www.jgroups.org/">JGroups</ulink> to broadcast server connection settings. 
+            </para>
+            <section id="clusters.broadcast-groups">
+                <title>Broadcast Groups</title>
+                <para>A broadcast group is the means by which a server broadcasts connectors over the
+                    network. A connector defines a way in which a client (or other server) can make
+                    connections to the server. For more information on what a connector is, please see
+                    <xref linkend="configuring-transports"/>.</para>
+                <para>The broadcast group takes a set of connector pairs, each connector pair contains
+                    connection settings for a live and backup server (if one exists) and broadcasts them on
+                    the network. Depending on which broadcasting technique you configure the cluster, it
+                    uses either UDP or JGroups to broadcast connector pairs information.</para>
+                <para>Broadcast groups are defined in the server configuration file <literal
+                        >hornetq-configuration.xml</literal>. There can be many broadcast groups per
+                    HornetQ server. All broadcast groups must be defined in a <literal
+                            >broadcast-groups</literal> element.</para>
+                <para>Let's take a look at an example broadcast group from <literal
+                        >hornetq-configuration.xml</literal> that defines a UDP broadcast group:</para>
+                <programlisting>
+&lt;broadcast-groups>
+   &lt;broadcast-group name="my-broadcast-group">
+      &lt;local-bind-address>172.16.9.3&lt;/local-bind-address>
+      &lt;local-bind-port>5432&lt;/local-bind-port>
+      &lt;group-address>231.7.7.7&lt;/group-address>
+      &lt;group-port>9876&lt;/group-port>
+      &lt;broadcast-period>2000&lt;/broadcast-period>
+      &lt;connector-ref connector-name="netty-connector"/>
+   &lt;/broadcast-group>
+&lt;/broadcast-groups></programlisting>
+                <para>Some of the broadcast group parameters are optional and you'll normally use the
+                    defaults, but we specify them all in the above example for clarity. Let's discuss
+                    each one in turn:</para>
+                <itemizedlist>
+                    <listitem>
+                        <para><literal>name</literal> attribute. Each broadcast group in the server must
+                            have a unique name. </para>
+                    </listitem>
+                    <listitem>
+                        <para><literal>local-bind-address</literal>. This is the local bind address that
+                            the datagram socket is bound to. If you have multiple network interfaces on
+                            your server, you would specify which one you wish to use for broadcasts by
+                            setting this property. If this property is not specified then the socket
+                            will be bound to the wildcard address, an IP address chosen by the
+                            kernel. This is a UDP specific attribute.</para>
+                    </listitem>
+                    <listitem>
+                        <para><literal>local-bind-port</literal>. If you want to specify a local port to
+                            which the datagram socket is bound you can specify it here. Normally you
+                            would just use the default value of <literal>-1</literal> which signifies
+                            that an anonymous port should be used. This parameter is always specified in conjunction with
+                            <literal>local-bind-address</literal>. This is a UDP specific attribute.</para>
+                    </listitem>
+                    <listitem>
+                        <para><literal>group-address</literal>. This is the multicast address to which
+                            the data will be broadcast. It is a class D IP address in the range <literal
+                                    >224.0.0.0</literal> to <literal>239.255.255.255</literal>, inclusive.
+                            The address <literal>224.0.0.0</literal> is reserved and is not available
+                            for use. This parameter is mandatory. This is a UDP specific attribute.</para>
+                    </listitem>
+                    <listitem>
+                        <para><literal>group-port</literal>. This is the UDP port number used for
+                            broadcasting. This parameter is mandatory. This is a UDP specific attribute.</para>
+                    </listitem>
+                    <listitem>
+                        <para><literal>broadcast-period</literal>. This is the period in milliseconds
+                            between consecutive broadcasts. This parameter is optional, the default
+                            value is <literal>2000</literal> milliseconds.</para>
+                    </listitem>
+                    <listitem>
+                        <para><literal>connector-ref</literal>. This specifies the connector and
+                            optional backup connector that will be broadcasted (see <xref
+                                    linkend="configuring-transports"/> for more information on connectors).
+                            The connector to be broadcasted is specified by the <literal
+                                    >connector-name</literal> attribute.</para>
+                    </listitem>
+                </itemizedlist>
+
+                <para id="clusters.jgroups-example">Here is another example broadcast group that defines a JGroups broadcast group:</para>
+                <programlisting>
+&lt;broadcast-groups>
+   &lt;broadcast-group name="my-broadcast-group">
+      &lt;jgroups-file>test-jgroups-file_ping.xml&lt;/jgroups-file>
+      &lt;jgroups-channel>hornetq_broadcast_channel&lt;/jgroups-channel>
+      &lt;broadcast-period>2000&lt;/broadcast-period>
+    &lt;connector-ref connector-name="netty-connector"/>
+   &lt;/broadcast-group>
+&lt;/broadcast-groups></programlisting>
+                <para>To be able to use JGroups to broadcast, one must specify two attributes, i.e. 
+                    <literal>jgroups-file</literal> and <literal>jgroups-channel</literal>, as discussed
+                    in details as following:</para>
+                <itemizedlist>
+                    <listitem>
+                        <para><literal>jgroups-file</literal> attribute. This is the name of JGroups configuration
+                            file. It will be used to initialize JGroups channels. Make sure the file is in the 
+                            java resource path so that HornetQ can load it. </para>
+                    </listitem>
+                    <listitem>
+                        <para><literal>jgroups-channel</literal> attribute. The name that JGroups channels connect
+                        to for broadcasting.</para>
+                    </listitem>
+                </itemizedlist>
+                <note>
+                    <para>The JGroups attributes (<literal>jgroups-file</literal> and <literal>jgroups-channel</literal>)
+                    and UDP specific attributes described above are exclusive of each other. Only one set can be
+                    specified in a broadcast group configuration. Don't mix them!</para>
+                </note>
+                <para id="clusters.jgroups-file">
+                   The following is an example of a JGroups file
+                   <programlisting>
+&lt;config xmlns="urn:org:jgroups"
+   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+   xsi:schemaLocation="urn:org:jgroups http://www.jgroups.org/schema/JGroups-3.0.xsd">
+   &lt;TCP loopback="true"
+      recv_buf_size="20000000"
+      send_buf_size="640000"
+      discard_incompatible_packets="true"
+      max_bundle_size="64000"
+      max_bundle_timeout="30"
+      enable_bundling="true"
+      use_send_queues="false"
+      sock_conn_timeout="300"
+
+      thread_pool.enabled="true"
+      thread_pool.min_threads="1"
+      thread_pool.max_threads="10"
+      thread_pool.keep_alive_time="5000"
+      thread_pool.queue_enabled="false"
+      thread_pool.queue_max_size="100"
+      thread_pool.rejection_policy="run"
+
+      oob_thread_pool.enabled="true"
+      oob_thread_pool.min_threads="1"
+      oob_thread_pool.max_threads="8"
+      oob_thread_pool.keep_alive_time="5000"
+      oob_thread_pool.queue_enabled="false"
+      oob_thread_pool.queue_max_size="100"
+      oob_thread_pool.rejection_policy="run"/>
+
+   &lt;FILE_PING location="../file.ping.dir"/>
+   &lt;MERGE2 max_interval="30000"
+      min_interval="10000"/>
+   &lt;FD_SOCK/>
+   &lt;FD timeout="10000" max_tries="5" />
+   &lt;VERIFY_SUSPECT timeout="1500"  />
+   &lt;BARRIER />
+   &lt;pbcast.NAKACK
+      use_mcast_xmit="false"
+      retransmit_timeout="300,600,1200,2400,4800"
+      discard_delivered_msgs="true"/>
+   &lt;UNICAST timeout="300,600,1200" />
+   &lt;pbcast.STABLE stability_delay="1000" desired_avg_gossip="50000"
+      max_bytes="400000"/>
+   &lt;pbcast.GMS print_local_addr="true" join_timeout="3000"
+      view_bundling="true"/>
+   &lt;FC max_credits="2000000"
+      min_threshold="0.10"/>
+   &lt;FRAG2 frag_size="60000"  />
+   &lt;pbcast.STATE_TRANSFER/>
+   &lt;pbcast.FLUSH timeout="0"/>
+&lt;/config></programlisting>
+                </para>
+                <para>
+                   As it shows, the file content defines a jgroups protocol stacks. If you want hornetq
+                   to use this stacks for channel creation, you have to make sure the value of 
+                   <literal>jgroups-file</literal> in your broadcast-group/discovery-group configuration
+                   to be the name of this jgroups configuration file. For example if the above stacks 
+                   configuration is stored in a file named "jgroups-stacks.xml" then your
+                   <literal>jgroups-file</literal> should be like
+                   <programlisting>
+&lt;jgroups-file>jgroups-stacks.xml&lt;/jgroups-file></programlisting>
+                </para>
+            </section>
+            <section id="clusters.discovery-groups">
+                <title>Discovery Groups</title>
+                <para>While the broadcast group defines how connector information is broadcasted from a
+                    server, a discovery group defines how connector information is received from a
+                    broadcast endpoint (a UDP multicast address or JGroup channel).</para>
+                <para>A discovery group maintains a list of connector pairs - one for each broadcast by
+                    a different server. As it receives broadcasts on the broadcast endpoint from a
+                    particular server it updates its entry in the list for that server.</para>
+                <para>If it has not received a broadcast from a particular server for a length of time
+                    it will remove that server's entry from its list.</para>
+                <para>Discovery groups are used in two places in HornetQ:</para>
+                <itemizedlist>
+                    <listitem>
+                        <para>By cluster connections so they know how to obtain an initial connection to download the topology</para>
+                    </listitem>
+                    <listitem>
+                        <para>By messaging clients so they know how to obtain an initial connection to download the topology</para>
+                    </listitem>
+                </itemizedlist>
+                <para>
+                    Although a discovery group will always accept broadcasts, its current list of available live and
+                    backup servers is only ever used when an initial connection is made, from then server discovery is
+                    done over the normal HornetQ connections.
+                </para>
+                <note>
+                    <para>
+                    Each discovery group must be configured with broadcast endpoint (UDP or JGroups) that matches its broadcast
+                    group counterpart. For example, if broadcast is configured using UDP, the discovery group must also use UDP, and the same
+                    multicast address.
+                    </para>
+                </note>
+            </section>
+            <section>
+                <title>Defining Discovery Groups on the Server</title>
+                <para>For cluster connections, discovery groups are defined in the server side
+                    configuration file <literal>hornetq-configuration.xml</literal>. All discovery
+                    groups must be defined inside a <literal>discovery-groups</literal> element. There
+                    can be many discovery groups defined by HornetQ server. Let's look at an
+                    example:</para>
+                <programlisting>
+&lt;discovery-groups>
+   &lt;discovery-group name="my-discovery-group">
+      &lt;local-bind-address>172.16.9.7&lt;/local-bind-address>
+      &lt;group-address>231.7.7.7&lt;/group-address>
+      &lt;group-port>9876&lt;/group-port>
+      &lt;refresh-timeout>10000&lt;/refresh-timeout>
+   &lt;/discovery-group>
+&lt;/discovery-groups></programlisting>
+                <para>We'll consider each parameter of the discovery group:</para>
+                <itemizedlist>
+                    <listitem>
+                        <para><literal>name</literal> attribute. Each discovery group must have a unique
+                            name per server.</para>
+                    </listitem>
+                    <listitem>
+                        <para><literal>local-bind-address</literal>. If you are running with multiple network interfaces on the same machine, you
+                            may want to specify that the discovery group listens only only a specific interface. To do this you can specify the interface
+                            address with this parameter. This parameter is optional. This is a UDP specific attribute.</para>
+                    </listitem>
+                    <listitem>
+                        <para><literal>group-address</literal>. This is the multicast IP address of the
+                            group to listen on. It should match the <literal>group-address</literal> in
+                            the broadcast group that you wish to listen from. This parameter is
+                            mandatory.  This is a UDP specific attribute.</para>
+                    </listitem>
+                    <listitem>
+                        <para><literal>group-port</literal>. This is the UDP port of the multicast
+                            group. It should match the <literal>group-port</literal> in the broadcast
+                            group that you wish to listen from. This parameter is mandatory. This is a UDP specific attribute.</para>
+                    </listitem>
+                    <listitem>
+                        <para><literal>refresh-timeout</literal>. This is the period the discovery group
+                            waits after receiving the last broadcast from a particular server before
+                            removing that servers connector pair entry from its list. You would normally
+                            set this to a value significantly higher than the <literal
+                                    >broadcast-period</literal> on the broadcast group otherwise servers
+                            might intermittently disappear from the list even though they are still
+                            broadcasting due to slight differences in timing. This parameter is
+                            optional, the default value is <literal>10000</literal> milliseconds (10
+                            seconds).</para>
+                    </listitem>
+                </itemizedlist>
+                <para>Here is another example that defines a JGroups discovery group:</para>
+                <programlisting>
+&lt;discovery-groups>
+   &lt;discovery-group name="my-broadcast-group">
+      &lt;jgroups-file>test-jgroups-file_ping.xml&lt;/jgroups-file>
+      &lt;jgroups-channel>hornetq_broadcast_channel&lt;/jgroups-channel>
+      &lt;refresh-timeout>10000&lt;/refresh-timeout>
+   &lt;/discovery-group>
+&lt;/discovery-groups></programlisting>
+                <para>To receive broadcast from JGroups channels, one must specify two attributes, 
+                    <literal>jgroups-file</literal> and <literal>jgroups-channel</literal>, as discussed
+                    in details as following:</para>
+                <itemizedlist>
+                    <listitem>
+                        <para><literal>jgroups-file</literal> attribute. This is the name of JGroups configuration
+                            file. It will be used to initialize JGroups channels. Make sure the file is in the 
+                            java resource path so that HornetQ can load it. </para>
+                    </listitem>
+                    <listitem>
+                        <para><literal>jgroups-channel</literal> attribute. The name that JGroups channels connect
+                        to for receiving broadcasts.</para>
+                    </listitem>
+                </itemizedlist>
+                <note>
+                    <para>The JGroups attributes (<literal>jgroups-file</literal> and <literal>jgroups-channel</literal>)
+                    and UDP specific attributes described above are exclusive of each other. Only one set can be
+                    specified in a discovery group configuration. Don't mix them!</para>
+                </note>
+            </section>
+            <section id="clusters-discovery.groups.clientside">
+                <title>Discovery Groups on the Client Side</title>
+                <para>Let's discuss how to configure a HornetQ client to use discovery to discover a
+                    list of servers to which it can connect. The way to do this differs depending on
+                    whether you're using JMS or the core API.</para>
+                <section>
+                    <title>Configuring client discovery using JMS</title>
+                    <para>If you're using JMS and you're also using the JMS Service on the server to
+                        load your JMS connection factory instances into JNDI, then you can specify which
+                        discovery group to use for your JMS connection factory in the server side xml
+                        configuration <literal>hornetq-jms.xml</literal>. Let's take a look at an
+                        example:</para>
+                    <programlisting>
+&lt;connection-factory name="ConnectionFactory">
+   &lt;discovery-group-ref discovery-group-name="my-discovery-group"/>
+   &lt;entries>
+      &lt;entry name="ConnectionFactory"/>
+   &lt;/entries>
+&lt;/connection-factory></programlisting>
+                    <para>The element <literal>discovery-group-ref</literal> specifies the name of a
+                        discovery group defined in <literal>hornetq-configuration.xml</literal>.</para>
+                    <para>When this connection factory is downloaded from JNDI by a client application
+                        and JMS connections are created from it, those connections will be load-balanced
+                        across the list of servers that the discovery group maintains by listening on
+                        the multicast address specified in the discovery group configuration.</para>
+                    <para>If you're using JMS, but you're not using JNDI to lookup a connection factory
+                        - you're instantiating the JMS connection factory directly then you can specify
+                        the discovery group parameters directly when creating the JMS connection
+                        factory. Here's an
+                        example:</para>
+                    <programlisting>
+final String groupAddress = "231.7.7.7";
+
+final int groupPort = 9876;
+
+ConnectionFactory jmsConnectionFactory =
+HornetQJMSClient.createConnectionFactory(new DiscoveryGroupConfiguration(groupAddress, groupPort,
+                       new UDPBroadcastGroupConfiguration(groupAddress, groupPort, null, -1)), JMSFactoryType.CF);
+
+Connection jmsConnection1 = jmsConnectionFactory.createConnection();
+
+Connection jmsConnection2 = jmsConnectionFactory.createConnection();</programlisting>
+                    <para>The <literal>refresh-timeout</literal> can be set directly on the DiscoveryGroupConfiguration
+                        by using the setter method <literal>setDiscoveryRefreshTimeout()</literal> if you
+                        want to change the default value.</para>
+                    <para>There is also a further parameter settable on the DiscoveryGroupConfiguration using the
+                        setter method <literal>setDiscoveryInitialWaitTimeout()</literal>. If the connection
+                        factory is used immediately after creation then it may not have had enough time
+                        to received broadcasts from all the nodes in the cluster. On first usage, the
+                        connection factory will make sure it waits this long since creation before
+                        creating the first connection. The default value for this parameter is <literal
+                                >10000</literal> milliseconds.</para>
+                </section>
+                <section>
+                    <title>Configuring client discovery using Core</title>
+                    <para>If you're using the core API to directly instantiate
+                        <literal>ClientSessionFactory</literal> instances, then you can specify the
+                        discovery group parameters directly when creating the session factory. Here's an
+                        example:</para>
+                        <programlisting>
+final String groupAddress = "231.7.7.7";
+final int groupPort = 9876;
+ServerLocator factory = HornetQClient.createServerLocatorWithHA(new DiscoveryGroupConfiguration(groupAddress, groupPort,
+                           new UDPBroadcastGroupConfiguration(groupAddress, groupPort, null, -1))));
+ClientSessionFactory factory = locator.createSessionFactory();
+ClientSession session1 = factory.createSession();
+ClientSession session2 = factory.createSession();</programlisting>
+                    <para>The <literal>refresh-timeout</literal> can be set directly on the DiscoveryGroupConfiguration
+                        by using the setter method <literal>setDiscoveryRefreshTimeout()</literal> if you
+                        want to change the default value.</para>
+                    <para>There is also a further parameter settable on the DiscoveryGroupConfiguration using the
+                        setter method <literal>setDiscoveryInitialWaitTimeout()</literal>. If the session factory
+                        is used immediately after creation then it may not have had enough time to
+                        received broadcasts from all the nodes in the cluster. On first usage, the
+                        session factory will make sure it waits this long since creation before creating
+                        the first session. The default value for this parameter is <literal
+                                >10000</literal> milliseconds.</para>
+                </section>
+            </section>
+        </section>
+        <section>
+            <title>Discovery using static Connectors</title>
+            <para>Sometimes it may be impossible to use UDP on the network you are using. In this case its
+                possible to configure a connection with an initial list if possible servers. This could be just
+                one server that you know will always be available or a list of servers where at least one will
+                be available.</para>
+            <para>This doesn't mean that you have to know where all your servers are going to be hosted, you
+                can configure these servers to use the reliable servers to connect to. Once they are connected
+                there connection details will be propagated via the server it connects to</para>
+            <section>
+                <title>Configuring a Cluster Connection</title>
+                <para>For cluster connections there is no extra configuration needed, you just need to make sure that any
+                    connectors are defined in the usual manner, (see <xref linkend="configuring-transports"/> for more
+                    information on connectors). These are then referenced by the cluster connection configuration.</para>
+            </section>
+            <section>
+                <title>Configuring a Client Connection</title>
+                <para>A static list of possible servers can also be used by a normal client.</para>
+                <section>
+                    <title>Configuring client discovery using JMS</title>
+                    <para>If you're using JMS and you're also using the JMS Service on the server to
+                        load your JMS connection factory instances into JNDI, then you can specify which
+                        connectors to use for your JMS connection factory in the server side xml
+                        configuration <literal>hornetq-jms.xml</literal>. Let's take a look at an
+                        example:</para>
+                    <programlisting>
+&lt;connection-factory name="ConnectionFactory">
+   &lt;connectors>
+      &lt;connector-ref connector-name="netty-connector"/>
+      &lt;connector-ref connector-name="netty-connector2"/>
+      &lt;connector-ref connector-name="netty-connector3"/>
+   &lt;/connectors>
+   &lt;entries>
+      &lt;entry name="ConnectionFactory"/>
+   &lt;/entries>
+&lt;/connection-factory></programlisting>
+                    <para>
+                        The element <literal>connectors</literal> contains a list of pre defined connectors in the
+                        <literal>hornetq-configuration.xml</literal> file. When this connection factory is downloaded
+                        from JNDI by a client application and JMS connections are created from it, those connections will
+                        be load-balanced across the list of servers defined by these connectors.
+                    </para>
+                    <para>
+                        If you're using JMS, but you're not using JNDI to lookup a connection factory - you're instantiating
+                        the JMS connection factory directly then you can specify the connector list directly when creating
+                        the JMS connection factory. Here's an example:
+                    </para>
+                    <programlisting>
+HashMap&lt;String, Object> map = new HashMap&lt;String, Object>();
+map.put("host", "myhost");
+map.put("port", "5445");
+TransportConfiguration server1 = new TransportConfiguration(NettyConnectorFactory.class.getName(), map);
+HashMap&lt;String, Object> map2 = new HashMap&lt;String, Object>();
+map2.put("host", "myhost2");
+map2.put("port", "5446");
+TransportConfiguration server2 = new TransportConfiguration(NettyConnectorFactory.class.getName(), map2);
+
+HornetQConnectionFactory cf = HornetQJMSClient.createConnectionFactoryWithHA(JMSFactoryType.CF, server1, server2);</programlisting>
+                </section>
+                <section>
+                    <title>Configuring client discovery using Core</title>
+                    <para>If you are using the core API then the same can be done as follows:</para>
+                    <programlisting>
+HashMap&lt;String, Object> map = new HashMap&lt;String, Object>();
+map.put("host", "myhost");
+map.put("port", "5445");
+TransportConfiguration server1 = new TransportConfiguration(NettyConnectorFactory.class.getName(), map);
+HashMap&lt;String, Object> map2 = new HashMap&lt;String, Object>();
+map2.put("host", "myhost2");
+map2.put("port", "5446");
+TransportConfiguration server2 = new TransportConfiguration(NettyConnectorFactory.class.getName(), map2);
+
+ServerLocator locator = HornetQClient.createServerLocatorWithHA(server1, server2);
+ClientSessionFactory factory = locator.createSessionFactory();
+ClientSession session = factory.createSession();</programlisting>
+                </section>
+            </section>
+        </section>
+    </section>
+    <section>
+        <title>Server-Side Message Load Balancing</title>
+        <para>If cluster connections are defined between nodes of a cluster, then HornetQ will load
+            balance messages arriving at a particular node from a client.</para>
+        <para>Let's take a simple example of a cluster of four nodes A, B, C, and D arranged in a
+            <emphasis>symmetric cluster</emphasis> (described in
+            <xref linkend="symmetric-cluster"/>). We have a queue called <literal>OrderQueue</literal>
+            deployed on each node of the cluster.</para>
+        <para>We have client Ca connected to node A, sending orders to the server. We have also have
+            order processor clients Pa, Pb, Pc, and Pd connected to each of the nodes A, B, C, D. If
+            no cluster connection was defined on node A, then as order messages arrive on node A
+            they will all end up in the <literal>OrderQueue</literal> on node A, so will only get
+            consumed by the order processor client attached to node A, Pa.</para>
+        <para>If we define a cluster connection on node A, then as ordered messages arrive on node A
+            instead of all of them going into the local <literal>OrderQueue</literal> instance, they
+            are distributed in a round-robin fashion between all the nodes of the cluster. The
+            messages are forwarded from the receiving node to other nodes of the cluster. This is
+            all done on the server side, the client maintains a single connection to node A.</para>
+        <para>For example, messages arriving on node A might be distributed in the following order
+            between the nodes: B, D, C, A, B, D, C, A, B, D. The exact order depends on the order
+            the nodes started up, but the algorithm used is round robin.</para>
+        <para>HornetQ cluster connections can be configured to always blindly load balance messages
+            in a round robin fashion irrespective of whether there are any matching consumers on
+            other nodes, but they can be a bit cleverer than that and also be configured to only
+            distribute to other nodes if they have matching consumers. We'll look at both these
+            cases in turn with some examples, but first we'll discuss configuring cluster
+            connections in general.</para>
+        <section id="clusters.cluster-connections">
+            <title>Configuring Cluster Connections</title>
+            <para>Cluster connections group servers into clusters so that messages can be load
+                balanced between the nodes of the cluster. Let's take a look at a typical cluster
+                connection. Cluster connections are always defined in <literal
+                        >hornetq-configuration.xml</literal> inside a <literal
+                        >cluster-connection</literal> element. There can be zero or more cluster
+                connections defined per HornetQ server.</para>
+            <programlisting>
+&lt;cluster-connections>
+   &lt;cluster-connection name="my-cluster">
+      &lt;address>jms&lt;/address>
+      &lt;connector-ref>netty-connector&lt;/connector-ref>
+      &lt;check-period>1000&lt;/check-period>
+      &lt;connection-ttl>5000&lt;/connection-ttl>
+      &lt;min-large-message-size>50000&lt;/min-large-message-size>
+      &lt;call-timeout>5000&lt;/call-timeout>
+      &lt;retry-interval>500&lt;/retry-interval>
+      &lt;retry-interval-multiplier>1.0&lt;/retry-interval-multiplier>
+      &lt;max-retry-interval>5000&lt;/max-retry-interval>
+      &lt;initial-connect-attempts>-1&lt;/initial-connect-attempts>
+      &lt;reconnect-attempts>-1&lt;/reconnect-attempts>
+      &lt;use-duplicate-detection>true&lt;/use-duplicate-detection>
+      &lt;forward-when-no-consumers>false&lt;/forward-when-no-consumers>
+      &lt;max-hops>1&lt;/max-hops>
+      &lt;confirmation-window-size>32000&lt;/confirmation-window-size>
+      &lt;call-failover-timeout>30000&lt;/call-failover-timeout>
+      &lt;notification-interval>1000&lt;/notification-interval>
+      &lt;notification-attempts>2&lt;/notification-attempts>
+      &lt;discovery-group-ref discovery-group-name="my-discovery-group"/>
+   &lt;/cluster-connection>
+&lt;/cluster-connections></programlisting>
+            <para>In the above cluster connection all parameters have been explicitly specified. The following
+               shows all the available configuration options</para>
+            <itemizedlist>
+                <listitem id="clusters.address">
+                    <para><literal>address</literal>. Each cluster connection only applies to
+                        messages sent to an address that starts with this value. Note: this does
+                        not use wild-card matching.</para>
+                    <para>In this case, this cluster connection will load balance messages sent to
+                        address that start with <literal>jms</literal>. This cluster connection,
+                        will, in effect apply to all JMS queues and topics since they map to core
+                        queues that start with the substring "jms".</para>
+                    <para>The address can be any value and you can have many cluster connections
+                        with different values of <literal>address</literal>, simultaneously
+                        balancing messages for those addresses, potentially to different clusters of
+                        servers. By having multiple cluster connections on different addresses a
+                        single HornetQ Server can effectively take part in multiple clusters
+                        simultaneously.</para>
+                    <para>Be careful not to have multiple cluster connections with overlapping
+                        values of <literal>address</literal>, e.g. "europe" and "europe.news" since
+                        this could result in the same messages being distributed between more than
+                        one cluster connection, possibly resulting in duplicate deliveries.</para>
+                    <para>This parameter is mandatory.</para>
+                </listitem>
+                <listitem>
+                    <para><literal>connector-ref</literal>. This is the connector which will be sent to other nodes in
+                    the cluster so they have the correct cluster topology.</para>
+                    <para>This parameter is mandatory.</para>
+                </listitem>
+                <listitem>
+                    <para><literal>check-period</literal>. The period (in milliseconds) used to check if the cluster connection
+                        has failed to receive pings from another server. Default is 30000.</para>
+                </listitem>
+                <listitem>
+                   <para><literal>connection-ttl</literal>. This is how long a cluster connection should stay alive if it
+                   stops receiving messages from a specific node in the cluster. Default is 60000.</para>
+                </listitem>
+                <listitem>
+                    <para><literal>min-large-message-size</literal>. If the message size (in bytes) is larger than this
+                    value then it will be split into multiple segments when sent over the network to other cluster
+                    members. Default is 102400.</para>
+                </listitem>
+                <listitem>
+                   <para><literal>call-timeout</literal>. When a packet is sent via a cluster connection and is a blocking
+                   call, i.e. for acknowledgements, this is how long it will wait (in milliseconds) for the reply before
+                   throwing an exception. Default is 30000.</para>
+                </listitem>
+                <listitem>
+                    <para><literal>retry-interval</literal>. We mentioned before that, internally,
+                        cluster connections cause bridges to be created between the nodes of the
+                        cluster. If the cluster connection is created and the target node has not
+                        been started, or say, is being rebooted, then the cluster connections from
+                        other nodes will retry connecting to the target until it comes back up, in
+                        the same way as a bridge does.</para>
+                    <para>This parameter determines the interval in milliseconds between retry
+                        attempts. It has the same meaning as the <literal>retry-interval</literal>
+                        on a bridge (as described in <xref linkend="core-bridges"/>).</para>
+                    <para>This parameter is optional and its default value is <literal>500</literal>
+                        milliseconds.</para>
+                </listitem>
+                <listitem>
+                   <para><literal>retry-interval-multiplier</literal>. This is a multiplier used to increase the
+                   <literal>retry-interval</literal> after each reconnect attempt, default is 1.</para>
+                </listitem>
+                <listitem>
+                   <para><literal>max-retry-interval</literal>. The maximum delay (in milliseconds) for retries.
+                   Default is 2000.</para>
+                </listitem>
+                <listitem>
+                    <para><literal>initial-connect-attempts</literal>. The number of times the system will
+                        try to connect a node in the cluster initially. If the max-retry is achieved this
+                        node will be considered permanently down and the system will not route messages
+                        to this node. Default is -1 (infinite retries).</para>
+                </listitem>
+                <listitem>
+                    <para><literal>reconnect-attempts</literal>. The number of times the system will
+                        try to reconnect to a node in the cluster. If the max-retry is achieved this node will
+                        be considered permanently down and the system will stop routing messages to this
+                        node. Default is -1 (infinite retries).</para>
+                </listitem>
+                <listitem>
+                    <para><literal>use-duplicate-detection</literal>. Internally cluster connections
+                        use bridges to link the nodes, and bridges can be configured to add a
+                        duplicate id property in each message that is forwarded. If the target node
+                        of the bridge crashes and then recovers, messages might be resent from the
+                        source node. By enabling duplicate detection any duplicate messages will be
+                        filtered out and ignored on receipt at the target node.</para>
+                    <para>This parameter has the same meaning as <literal>use-duplicate-detection</literal>
+                        on a bridge. For more information on duplicate detection, please see
+                        <xref linkend="duplicate-detection"/>. Default is true.</para>
+                </listitem>
+                <listitem>
+                    <para><literal>forward-when-no-consumers</literal>. This parameter determines
+                        whether messages will be distributed round robin between other nodes of the
+                        cluster <emphasis>regardless</emphasis> of whether or not there are matching or
+                        indeed any consumers on other nodes. </para>
+                    <para>If this is set to <literal>true</literal> then each incoming message will
+                        be round robin'd even though the same queues on the other nodes of the
+                        cluster may have no consumers at all, or they may have consumers that have
+                        non matching message filters (selectors). Note that HornetQ will
+                        <emphasis>not</emphasis> forward messages to other nodes if there are no
+                        <emphasis>queues</emphasis> of the same name on the other nodes, even if
+                        this parameter is set to <literal>true</literal>.</para>
+                    <para>If this is set to <literal>false</literal> then HornetQ will only forward
+                        messages to other nodes of the cluster if the address to which they are
+                        being forwarded has queues which have consumers, and if those consumers have
+                        message filters (selectors) at least one of those selectors must match the
+                        message.</para>
+                    <para>Default is false.</para>
+                </listitem>
+                <listitem>
+                    <para><literal>max-hops</literal>. When a cluster connection decides the set of
+                        nodes to which it might load balance a message, those nodes do not have to
+                        be directly connected to it via a cluster connection. HornetQ can be
+                        configured to also load balance messages to nodes which might be connected
+                        to it only indirectly with other HornetQ servers as intermediates in a
+                        chain.</para>
+                    <para>This allows HornetQ to be configured in more complex topologies and still
+                        provide message load balancing. We'll discuss this more later in this
+                        chapter.</para>
+                    <para>The default value for this parameter is <literal>1</literal>, which means
+                        messages are only load balanced to other HornetQ serves which are directly
+                        connected to this server. This parameter is optional.</para>
+                </listitem>
+                <listitem>
+                   <para><literal>confirmation-window-size</literal>. The size (in bytes) of the window
+                   used for sending confirmations from the server connected to. So once the server has
+                   received <literal>confirmation-window-size</literal> bytes it notifies its client,
+                   default is 1048576. A value of -1 means no window.</para>
+                </listitem>
+                <listitem>
+                   <para><literal>call-failover-timeout</literal>. Similar to <literal>call-timeout</literal> but used
+                   when a call is made during a failover attempt. Default is -1 (no timeout).</para>
+                </listitem>
+                <listitem>
+                   <para><literal>notification-interval</literal>. How often (in milliseconds) the cluster connection
+                   should broadcast itself when attaching to the cluster. Default is 1000.</para>
+                </listitem>
+                <listitem>
+                   <para><literal>notification-attempts</literal>. How many times the cluster connection should
+                   broadcast itself when connecting to the cluster. Default is 2.</para>
+                </listitem>
+                <listitem>
+                    <para><literal>discovery-group-ref</literal>. This parameter determines which
+                        discovery group is used to obtain the list of other servers in the cluster
+                        that this cluster connection will make connections to.</para>
+                </listitem>
+            </itemizedlist>
+            <para>
+                Alternatively if you would like your cluster connections to use a static list of
+                servers for discovery then you can do it like this.
+            </para>
+            <programlisting>
+&lt;cluster-connection name="my-cluster">
+   ...
+   &lt;static-connectors>
+      &lt;connector-ref>server0-connector&lt;/connector-ref>
+      &lt;connector-ref>server1-connector&lt;/connector-ref>
+   &lt;/static-connectors>
+&lt;/cluster-connection></programlisting>
+            <para>
+                Here we have defined 2 servers that we know for sure will that at least one will be available. There may
+                be many more servers in the cluster but these will; be discovered via one of these connectors once an
+                initial connection has been made.</para>
+        </section>
+        <section id="clusters.clusteruser">
+            <title>Cluster User Credentials</title>
+            <para>When creating connections between nodes of a cluster to form a cluster connection,
+                HornetQ uses a cluster user and cluster password which is defined in <literal
+                        >hornetq-configuration.xml</literal>:</para>
+            <programlisting>
+&lt;cluster-user>HORNETQ.CLUSTER.ADMIN.USER&lt;/cluster-user>
+&lt;cluster-password>CHANGE ME!!&lt;/cluster-password></programlisting>
+            <warning>
+                <para>It is imperative that these values are changed from their default, or remote
+                    clients will be able to make connections to the server using the default values.
+                    If they are not changed from the default, HornetQ will detect this and pester
+                    you with a warning on every start-up.</para>
+            </warning>
+        </section>
+    </section>
+    <section id="clusters.client.loadbalancing">
+        <title>Client-Side Load balancing</title>
+        <para>With HornetQ client-side load balancing, subsequent sessions created using a single
+            session factory can be connected to different nodes of the cluster. This allows sessions
+            to spread smoothly across the nodes of a cluster and not be "clumped" on any particular
+            node.</para>
+        <para>The load balancing policy to be used by the client factory is configurable. HornetQ
+            provides four out-of-the-box load balancing policies, and you can also implement your own
+            and use that.</para>
+        <para>The out-of-the-box policies are</para>
+        <itemizedlist>
+            <listitem>
+                <para>Round Robin. With this policy the first node is chosen randomly then each
+                    subsequent node is chosen sequentially in the same order.</para>
+                <para>For example nodes might be chosen in the order B, C, D, A, B, C, D, A, B or D,
+                    A, B, C, D, A, B, C, D or C, D, A, B, C, D, A, B, C.</para>
+                <para>Use <literal>org.hornetq.api.core.client.loadbalance.RoundRobinConnectionLoadBalancingPolicy</literal>
+                    as the <literal>&lt;connection-load-balancing-policy-class-name></literal>.</para>
+            </listitem>
+            <listitem>
+                <para>Random. With this policy each node is chosen randomly.</para>
+                <para>Use <literal>org.hornetq.api.core.client.loadbalance.RandomConnectionLoadBalancingPolicy</literal>
+                    as the <literal>&lt;connection-load-balancing-policy-class-name></literal>.</para>
+            </listitem>
+            <listitem>
+                <para>Random Sticky. With this policy the first node is chosen randomly and then re-used for subsequent
+                    connections.</para>
+                <para>Use <literal>org.hornetq.api.core.client.loadbalance.RandomStickyConnectionLoadBalancingPolicy</literal>
+                    as the <literal>&lt;connection-load-balancing-policy-class-name></literal>.</para>
+            </listitem>
+            <listitem>
+                <para>First Element. With this policy the "first" (i.e. 0th) node is always returned.</para>
+                <para>Use <literal>org.hornetq.api.core.client.loadbalance.FirstElementConnectionLoadBalancingPolicy</literal>
+                    as the <literal>&lt;connection-load-balancing-policy-class-name></literal>.</para>
+            </listitem>
+        </itemizedlist>
+        <para>You can also implement your own policy by implementing the interface <literal
+                >org.hornetq.api.core.client.loadbalance.ConnectionLoadBalancingPolicy</literal></para>
+        <para>Specifying which load balancing policy to use differs whether you are using JMS or the
+            core API. If you don't specify a policy then the default will be used which is <literal
+                    >org.hornetq.api.core.client.loadbalance.RoundRobinConnectionLoadBalancingPolicy</literal>.</para>
+        <para>If you're using JMS, and you're using JNDI on the server to put your JMS connection
+            factories into JNDI, then you can specify the load balancing policy directly in the
+            <literal>hornetq-jms.xml</literal> configuration file on the server as follows:</para>
+            <programlisting>
+&lt;connection-factory name="ConnectionFactory">
+   &lt;discovery-group-ref discovery-group-name="my-discovery-group"/>
+   &lt;entries>
+      &lt;entry name="ConnectionFactory"/>
+   &lt;/entries>
+   &lt;connection-load-balancing-policy-class-name>
+      org.hornetq.api.core.client.loadbalance.RandomConnectionLoadBalancingPolicy
+   &lt;/connection-load-balancing-policy-class-name>
+&lt;/connection-factory></programlisting>
+        <para>The above example would deploy a JMS connection factory that uses the random connection load
+            balancing policy. </para>
+        <para>If you're using JMS but you're instantiating your connection factory directly on the
+            client side then you can set the load balancing policy using the setter on the
+            <literal>HornetQConnectionFactory</literal> before using it:</para>
+            <programlisting>
+ConnectionFactory jmsConnectionFactory = HornetQJMSClient.createConnectionFactory(...);
+jmsConnectionFactory.setLoadBalancingPolicyClassName("com.acme.MyLoadBalancingPolicy");</programlisting>
+        <para>If you're using the core API, you can set the load balancing policy directly on the
+            <literal>ServerLocator</literal> instance you are using:</para>
+            <programlisting>
+ServerLocator locator = HornetQClient.createServerLocatorWithHA(server1, server2);
+locator.setLoadBalancingPolicyClassName("com.acme.MyLoadBalancingPolicy");</programlisting>
+        <para>The set of servers over which the factory load balances can be determined in one of
+            two ways:</para>
+        <itemizedlist>
+            <listitem>
+                <para>Specifying servers explicitly</para>
+            </listitem>
+            <listitem>
+                <para>Using discovery.</para>
+            </listitem>
+        </itemizedlist>
+    </section>
+    <section>
+        <title>Specifying Members of a Cluster Explicitly</title>
+        <para>
+            Sometimes you want to explicitly define a cluster more explicitly, that is control which
+            server connect to each other in the cluster. This is typically used to form non symmetrical clusters
+            such as chain cluster or ring clusters. This can only be done using a static list of connectors and is
+            configured as follows:
+        </para>
+        <programlisting>
+&lt;cluster-connection name="my-cluster">
+   &lt;address>jms&lt;/address>
+   &lt;connector-ref>netty-connector&lt;/connector-ref>
+   &lt;retry-interval>500&lt;/retry-interval>
+   &lt;use-duplicate-detection>true&lt;/use-duplicate-detection>
+   &lt;forward-when-no-consumers>true&lt;/forward-when-no-consumers>
+   &lt;max-hops>1&lt;/max-hops>
+   &lt;static-connectors allow-direct-connections-only="true">
+      &lt;connector-ref>server1-connector&lt;/connector-ref>
+   &lt;/static-connectors>
+&lt;/cluster-connection></programlisting>
+        <para>
+            In this example we have set the attribute <literal>allow-direct-connections-only</literal> which means that
+            the only server that this server can create a cluster connection to is server1-connector. This means you can
+            explicitly create any cluster topology you want.
+        </para>
+    </section>
+    <section id="clusters.message-redistribution">
+        <title>Message Redistribution</title>
+        <para>Another important part of clustering is message redistribution. Earlier we learned how
+            server side message load balancing round robins messages across the cluster. If <literal
+                    >forward-when-no-consumers</literal> is false, then messages won't be forwarded to
+            nodes which don't have matching consumers, this is great and ensures that messages don't
+            arrive on a queue which has no consumers to consume them, however there is a situation
+            it doesn't solve: What happens if the consumers on a queue close after the messages have
+            been sent to the node? If there are no consumers on the queue the message won't get
+            consumed and we have a <emphasis>starvation</emphasis> situation.</para>
+        <para>This is where message redistribution comes in. With message redistribution HornetQ can
+            be configured to automatically <emphasis>redistribute</emphasis> messages from queues
+            which have no consumers back to other nodes in the cluster which do have matching
+            consumers.</para>
+        <para>Message redistribution can be configured to kick in immediately after the last
+            consumer on a queue is closed, or to wait a configurable delay after the last consumer
+            on a queue is closed before redistributing. By default message redistribution is
+            disabled.</para>
+        <para>Message redistribution can be configured on a per address basis, by specifying the
+            redistribution delay in the address settings, for more information on configuring
+            address settings, please see <xref linkend="queue-attributes"/>.</para>
+        <para>Here's an address settings snippet from <literal>hornetq-configuration.xml</literal>
+            showing how message redistribution is enabled for a set of queues:</para>
+        <programlisting>
+&lt;address-settings>
+   &lt;address-setting match="jms.#">
+      &lt;redistribution-delay>0&lt;/redistribution-delay>
+   &lt;/address-setting>
+&lt;/address-settings></programlisting>
+        <para>The above <literal>address-settings</literal> block would set a <literal
+                >redistribution-delay</literal> of <literal>0</literal> for any queue which is bound
+            to an address that starts with "jms.". All JMS queues and topic subscriptions are bound
+            to addresses that start with "jms.", so the above would enable instant (no delay)
+            redistribution for all JMS queues and topic subscriptions.</para>
+        <para>The attribute <literal>match</literal> can be an exact match or it can be a string
+            that conforms to the HornetQ wildcard syntax (described in <xref
+                    linkend="wildcard-syntax"/>).</para>
+        <para>The element <literal>redistribution-delay</literal> defines the delay in milliseconds
+            after the last consumer is closed on a queue before redistributing messages from that
+            queue to other nodes of the cluster which do have matching consumers. A delay of zero
+            means the messages will be immediately redistributed. A value of <literal>-1</literal>
+            signifies that messages will never be redistributed. The default value is <literal
+                    >-1</literal>.</para>
+        <para>It often makes sense to introduce a delay before redistributing as it's a common case
+            that a consumer closes but another one quickly is created on the same queue, in such a
+            case you probably don't want to redistribute immediately since the new consumer will
+            arrive shortly.</para>
+    </section>
+    <section>
+        <title>Cluster topologies</title>
+        <para>HornetQ clusters can be connected together in many different topologies, let's
+            consider the two most common ones here</para>
+        <section id="symmetric-cluster">
+            <title>Symmetric cluster</title>
+            <para>A symmetric cluster is probably the most common cluster topology, and you'll be
+                familiar with if you've had experience of JBoss Application Server
+                clustering.</para>
+            <para>With a symmetric cluster every node in the cluster is connected to every other
+                node in the cluster. In other words every node in the cluster is no more than one
+                hop away from every other node.</para>
+            <para>To form a symmetric cluster every node in the cluster defines a cluster connection
+                with the attribute <literal>max-hops</literal> set to <literal>1</literal>.
+                Typically the cluster connection will use server discovery in order to know what
+                other servers in the cluster it should connect to, although it is possible to
+                explicitly define each target server too in the cluster connection if, for example,
+                UDP is not available on your network.</para>
+            <para>With a symmetric cluster each node knows about all the queues that exist on all
+                the other nodes and what consumers they have. With this knowledge it can determine
+                how to load balance and redistribute messages around the nodes.</para>
+            <para>Don't forget <link linkend="copy-warning">this warning</link> when creating a
+                symmetric cluster.</para>
+        </section>
+        <section>
+            <title>Chain cluster</title>
+            <para>With a chain cluster, each node in the cluster is not connected to every node in
+                the cluster directly, instead the nodes form a chain with a node on each end of the
+                chain and all other nodes just connecting to the previous and next nodes in the
+                chain.</para>
+            <para>An example of this would be a three node chain consisting of nodes A, B and C.
+                Node A is hosted in one network and has many producer clients connected to it
+                sending order messages. Due to corporate policy, the order consumer clients need to
+                be hosted in a different network, and that network is only accessible via a third
+                network. In this setup node B acts as a mediator with no producers or consumers on
+                it. Any messages arriving on node A will be forwarded to node B, which will in turn
+                forward them to node C where they can get consumed. Node A does not need to directly
+                connect to C, but all the nodes can still act as a part of the cluster.</para>
+            <para>To set up a cluster in this way, node A would define a cluster connection that
+                connects to node B, and node B would define a cluster connection that connects to
+                node C. In this case we only want cluster connections in one direction since we're
+                only moving messages from node A->B->C and never from C->B->A.</para>
+            <para>For this topology we would set <literal>max-hops</literal> to <literal
+                    >2</literal>. With a value of <literal>2</literal> the knowledge of what queues and
+                consumers that exist on node C would be propagated from node C to node B to node A.
+                Node A would then know to distribute messages to node B when they arrive, even
+                though node B has no consumers itself, it would know that a further hop away is node
+                C which does have consumers.</para>
+        </section>
+    </section>
+   <section>
+      <title>Scaling Down</title>
+      <para>HornetQ supports scaling down a cluster with no message loss (even for non-durable messages). This is especially
+         useful in certain environments (e.g. the cloud) where the size of a cluster may change relatively frequently.
+         When scaling up a cluster (i.e. adding nodes) there is no risk of message loss, but when scaling down a cluster
+         (i.e. removing nodes) the messages on those nodes would be lost unless the broker sent them to another node in
+         the cluster. HornetQ can be configured to do just that.</para>
+      <para>The simplest way to enable this behavior is to set <literal>scale-down</literal> to
+         <literal>true</literal>. If the server is clustered and <literal>scale-down</literal> is
+         <literal>true</literal> then when the server is shutdown gracefully (i.e. stopped without crashing) it will find
+         another node in the cluster and send <emphasis>all</emphasis> of its messages (both durable and non-durable)
+         to that node. The messages are processed in order and go to the <emphasis>back</emphasis> of the respective
+         queues on the other node (just as if the messages were sent from an external client for the first time).</para>
+      <para>If more control over where the messages go is required then specify <literal>scale-down-group-name</literal>.
+         Messages will only be sent to another node in the cluster that uses the same <literal>scale-down-group-name</literal>
+         as the server being shutdown.</para>
+      <warning>
+         <para>If cluster nodes are grouped together with different <literal>scale-down-group-name</literal> values beware.
+            If all the nodes in a single group are shut down then the messages from that node/group will be lost.</para>
+      </warning>
+      <para>If the server is using multiple <literal>cluster-connection</literal> then use <literal>scale-down-clustername</literal>
+         to identify the name of the <literal>cluster-connection</literal> which should be used for scaling down.</para>
+   </section>
+</chapter>


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