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https://issues.apache.org/jira/browse/ACTIVEMQ6-3 We are renaming packages from activemq6 to activemq as that's more generic and version independent The previous commit renamed the directories. On this commit now I'm changing the code. If we changed the code and the directories on the same commit git would remove and add a lot of files without recognizing the renames.
1019 lines
68 KiB
XML
1019 lines
68 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!-- ============================================================================= -->
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<!-- Copyright © 2009 Red Hat, Inc. and others. -->
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<!-- -->
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<!-- The text of and illustrations in this document are licensed by Red Hat under -->
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<!-- a Creative Commons Attribution–Share Alike 3.0 Unported license ("CC-BY-SA"). -->
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<!-- -->
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<!-- An explanation of CC-BY-SA is available at -->
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<!-- -->
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<!-- http://creativecommons.org/licenses/by-sa/3.0/. -->
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<!-- -->
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<!-- In accordance with CC-BY-SA, if you distribute this document or an adaptation -->
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<!-- of it, you must provide the URL for the original version. -->
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<!-- -->
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<!-- Red Hat, as the licensor of this document, waives the right to enforce, -->
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<!-- and agrees not to assert, Section 4d of CC-BY-SA to the fullest extent -->
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<!-- permitted by applicable law. -->
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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 "HornetQ_User_Manual.ent">
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%BOOK_ENTITIES;
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]>
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<chapter id="clusters">
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<title>Clusters</title>
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<section>
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<title>Clusters Overview</title>
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<para>HornetQ clusters allow groups of HornetQ servers to be grouped together in order to
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share message processing load. Each active node in the cluster is an active HornetQ
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server which manages its own messages and handles its own connections. </para>
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<note id="clustered-deprecation">
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||
<para>The <emphasis>clustered</emphasis> parameter is deprecated and no longer needed for
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||
setting up a cluster. If your configuration contains this parameter it will be ignored and
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a message with the ID <literal>HQ221038</literal> will be logged.</para>
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</note>
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||
<para>The cluster is formed by each node declaring <emphasis>cluster connections</emphasis>
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to other nodes in the core configuration file <literal
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>hornetq-configuration.xml</literal>. When a node forms a cluster connection to
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another node, internally it creates a <emphasis>core bridge</emphasis> (as described in
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<xref linkend="core-bridges"/>) connection between it and the other node, this is
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done transparently behind the scenes - you don't have to declare an explicit bridge for
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each node. These cluster connections allow messages to flow between the nodes of the
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cluster to balance load.</para>
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<para>Nodes can be connected together to form a cluster in many different topologies, we
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will discuss a couple of the more common topologies later in this chapter.</para>
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<para>We'll also discuss client side load balancing, where we can balance client connections
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across the nodes of the cluster, and we'll consider message redistribution where HornetQ
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||
will redistribute messages between nodes to avoid starvation.</para>
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||
<para>Another important part of clustering is <emphasis>server discovery</emphasis> where
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servers can broadcast their connection details so clients or other servers can connect
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to them with the minimum of configuration.</para>
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<warning id="copy-warning">
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||
<para>Once a cluster node has been configured it is common to simply copy that configuration
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to other nodes to produce a symmetric cluster. However, care must be taken when copying the
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||
HornetQ files. Do not copy the HornetQ <emphasis>data</emphasis> (i.e. the
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<literal>bindings</literal>, <literal>journal</literal>, and <literal>large-messages</literal>
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directories) from one node to another. When a node is started for the first time and initializes
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||
its journal files it also persists a special identifier to the <literal>journal</literal>
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directory. This id <emphasis>must</emphasis> be unique among nodes in the cluster or the
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cluster will not form properly.</para>
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</warning>
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</section>
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<section id="clusters.server-discovery">
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<title>Server discovery</title>
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<para>Server discovery is a mechanism by which servers can propagate their connection details to:</para>
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<itemizedlist>
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<listitem>
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<para>
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Messaging clients. A messaging client wants to be able to connect
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to the servers of the cluster without having specific knowledge of which servers
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in the cluster are up at any one time.
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</para>
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</listitem>
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<listitem>
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<para>Other servers. Servers in a cluster want to be able to create
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cluster connections to each other without having prior knowledge of all the
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other servers in the cluster.</para>
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</listitem>
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||
</itemizedlist>
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<para>
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This information, let's call it the Cluster Topology, is actually sent around normal HornetQ
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connections to clients and to other servers over cluster connections. This being the case we need a
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way of establishing the initial first connection. This can be done using
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dynamic discovery techniques like <ulink url="http://en.wikipedia.org/wiki/User_Datagram_Protocol" >UDP</ulink>
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and <ulink url="http://www.jgroups.org/">JGroups</ulink>, or by
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providing a list of initial connectors.
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</para>
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<section>
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<title>Dynamic Discovery</title>
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<para>
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Server discovery uses <ulink url="http://en.wikipedia.org/wiki/User_Datagram_Protocol" >UDP</ulink>
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multicast or <ulink url="http://www.jgroups.org/">JGroups</ulink> to broadcast server connection settings.
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</para>
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<section id="clusters.broadcast-groups">
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<title>Broadcast Groups</title>
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<para>A broadcast group is the means by which a server broadcasts connectors over the
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network. A connector defines a way in which a client (or other server) can make
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connections to the server. For more information on what a connector is, please see
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<xref linkend="configuring-transports"/>.</para>
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<para>The broadcast group takes a set of connector pairs, each connector pair contains
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connection settings for a live and backup server (if one exists) and broadcasts them on
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the network. Depending on which broadcasting technique you configure the cluster, it
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||
uses either UDP or JGroups to broadcast connector pairs information.</para>
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||
<para>Broadcast groups are defined in the server configuration file <literal
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||
>hornetq-configuration.xml</literal>. There can be many broadcast groups per
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HornetQ server. All broadcast groups must be defined in a <literal
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||
>broadcast-groups</literal> element.</para>
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||
<para>Let's take a look at an example broadcast group from <literal
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>hornetq-configuration.xml</literal> that defines a UDP broadcast group:</para>
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<programlisting>
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<broadcast-groups>
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<broadcast-group name="my-broadcast-group">
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<local-bind-address>172.16.9.3</local-bind-address>
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<local-bind-port>5432</local-bind-port>
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<group-address>231.7.7.7</group-address>
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<group-port>9876</group-port>
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<broadcast-period>2000</broadcast-period>
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<connector-ref connector-name="netty-connector"/>
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</broadcast-group>
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</broadcast-groups></programlisting>
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<para>Some of the broadcast group parameters are optional and you'll normally use the
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defaults, but we specify them all in the above example for clarity. Let's discuss
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each one in turn:</para>
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<itemizedlist>
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<listitem>
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<para><literal>name</literal> attribute. Each broadcast group in the server must
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have a unique name. </para>
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</listitem>
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<listitem>
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<para><literal>local-bind-address</literal>. This is the local bind address that
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the datagram socket is bound to. If you have multiple network interfaces on
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your server, you would specify which one you wish to use for broadcasts by
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setting this property. If this property is not specified then the socket
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will be bound to the wildcard address, an IP address chosen by the
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kernel. This is a UDP specific attribute.</para>
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</listitem>
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<listitem>
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<para><literal>local-bind-port</literal>. If you want to specify a local port to
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which the datagram socket is bound you can specify it here. Normally you
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would just use the default value of <literal>-1</literal> which signifies
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that an anonymous port should be used. This parameter is always specified in conjunction with
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<literal>local-bind-address</literal>. This is a UDP specific attribute.</para>
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</listitem>
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<listitem>
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<para><literal>group-address</literal>. This is the multicast address to which
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the data will be broadcast. It is a class D IP address in the range <literal
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>224.0.0.0</literal> to <literal>239.255.255.255</literal>, inclusive.
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The address <literal>224.0.0.0</literal> is reserved and is not available
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for use. This parameter is mandatory. This is a UDP specific attribute.</para>
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</listitem>
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<listitem>
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<para><literal>group-port</literal>. This is the UDP port number used for
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broadcasting. This parameter is mandatory. This is a UDP specific attribute.</para>
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</listitem>
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<listitem>
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<para><literal>broadcast-period</literal>. This is the period in milliseconds
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between consecutive broadcasts. This parameter is optional, the default
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value is <literal>2000</literal> milliseconds.</para>
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</listitem>
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<listitem>
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<para><literal>connector-ref</literal>. This specifies the connector and
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optional backup connector that will be broadcasted (see <xref
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linkend="configuring-transports"/> for more information on connectors).
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The connector to be broadcasted is specified by the <literal
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>connector-name</literal> attribute.</para>
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</listitem>
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</itemizedlist>
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<para id="clusters.jgroups-example">Here is another example broadcast group that defines a JGroups broadcast group:</para>
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<programlisting>
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<broadcast-groups>
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<broadcast-group name="my-broadcast-group">
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<jgroups-file>test-jgroups-file_ping.xml</jgroups-file>
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<jgroups-channel>hornetq_broadcast_channel</jgroups-channel>
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<broadcast-period>2000</broadcast-period>
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<connector-ref connector-name="netty-connector"/>
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</broadcast-group>
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</broadcast-groups></programlisting>
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<para>To be able to use JGroups to broadcast, one must specify two attributes, i.e.
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<literal>jgroups-file</literal> and <literal>jgroups-channel</literal>, as discussed
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||
in details as following:</para>
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||
<itemizedlist>
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||
<listitem>
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||
<para><literal>jgroups-file</literal> attribute. This is the name of JGroups configuration
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||
file. It will be used to initialize JGroups channels. Make sure the file is in the
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||
java resource path so that HornetQ can load it. </para>
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||
</listitem>
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||
<listitem>
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<para><literal>jgroups-channel</literal> attribute. The name that JGroups channels connect
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to for broadcasting.</para>
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</listitem>
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</itemizedlist>
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||
<note>
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||
<para>The JGroups attributes (<literal>jgroups-file</literal> and <literal>jgroups-channel</literal>)
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||
and UDP specific attributes described above are exclusive of each other. Only one set can be
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||
specified in a broadcast group configuration. Don't mix them!</para>
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||
</note>
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||
<para id="clusters.jgroups-file">
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||
The following is an example of a JGroups file
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<programlisting>
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||
<config xmlns="urn:org:jgroups"
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xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
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||
xsi:schemaLocation="urn:org:jgroups http://www.jgroups.org/schema/JGroups-3.0.xsd">
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||
<TCP loopback="true"
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recv_buf_size="20000000"
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send_buf_size="640000"
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||
discard_incompatible_packets="true"
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||
max_bundle_size="64000"
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||
max_bundle_timeout="30"
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||
enable_bundling="true"
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||
use_send_queues="false"
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||
sock_conn_timeout="300"
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||
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||
thread_pool.enabled="true"
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||
thread_pool.min_threads="1"
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||
thread_pool.max_threads="10"
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||
thread_pool.keep_alive_time="5000"
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||
thread_pool.queue_enabled="false"
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||
thread_pool.queue_max_size="100"
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||
thread_pool.rejection_policy="run"
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||
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||
oob_thread_pool.enabled="true"
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||
oob_thread_pool.min_threads="1"
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||
oob_thread_pool.max_threads="8"
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||
oob_thread_pool.keep_alive_time="5000"
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||
oob_thread_pool.queue_enabled="false"
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||
oob_thread_pool.queue_max_size="100"
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oob_thread_pool.rejection_policy="run"/>
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||
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<FILE_PING location="../file.ping.dir"/>
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||
<MERGE2 max_interval="30000"
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min_interval="10000"/>
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<FD_SOCK/>
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<FD timeout="10000" max_tries="5" />
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<VERIFY_SUSPECT timeout="1500" />
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<BARRIER />
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<pbcast.NAKACK
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||
use_mcast_xmit="false"
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||
retransmit_timeout="300,600,1200,2400,4800"
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||
discard_delivered_msgs="true"/>
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||
<UNICAST timeout="300,600,1200" />
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||
<pbcast.STABLE stability_delay="1000" desired_avg_gossip="50000"
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max_bytes="400000"/>
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<pbcast.GMS print_local_addr="true" join_timeout="3000"
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view_bundling="true"/>
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<FC max_credits="2000000"
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min_threshold="0.10"/>
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<FRAG2 frag_size="60000" />
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<pbcast.STATE_TRANSFER/>
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<pbcast.FLUSH timeout="0"/>
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</config></programlisting>
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</para>
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||
<para>
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||
As it shows, the file content defines a jgroups protocol stacks. If you want hornetq
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to use this stacks for channel creation, you have to make sure the value of
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<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
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||
configuration is stored in a file named "jgroups-stacks.xml" then your
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||
<literal>jgroups-file</literal> should be like
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<programlisting>
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<jgroups-file>jgroups-stacks.xml</jgroups-file></programlisting>
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</para>
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</section>
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||
<section id="clusters.discovery-groups">
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<title>Discovery Groups</title>
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||
<para>While the broadcast group defines how connector information is broadcasted from a
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server, a discovery group defines how connector information is received from a
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broadcast endpoint (a UDP multicast address or JGroup channel).</para>
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<para>A discovery group maintains a list of connector pairs - one for each broadcast by
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a different server. As it receives broadcasts on the broadcast endpoint from a
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||
particular server it updates its entry in the list for that server.</para>
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<para>If it has not received a broadcast from a particular server for a length of time
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it will remove that server's entry from its list.</para>
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<para>Discovery groups are used in two places in HornetQ:</para>
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||
<itemizedlist>
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||
<listitem>
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<para>By cluster connections so they know how to obtain an initial connection to download the topology</para>
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||
</listitem>
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||
<listitem>
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<para>By messaging clients so they know how to obtain an initial connection to download the topology</para>
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</listitem>
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||
</itemizedlist>
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||
<para>
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Although a discovery group will always accept broadcasts, its current list of available live and
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backup servers is only ever used when an initial connection is made, from then server discovery is
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||
done over the normal HornetQ connections.
|
||
</para>
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||
<note>
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||
<para>
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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>
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||
</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>
|
||
<discovery-groups>
|
||
<discovery-group name="my-discovery-group">
|
||
<local-bind-address>172.16.9.7</local-bind-address>
|
||
<group-address>231.7.7.7</group-address>
|
||
<group-port>9876</group-port>
|
||
<refresh-timeout>10000</refresh-timeout>
|
||
</discovery-group>
|
||
</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>
|
||
<discovery-groups>
|
||
<discovery-group name="my-broadcast-group">
|
||
<jgroups-file>test-jgroups-file_ping.xml</jgroups-file>
|
||
<jgroups-channel>hornetq_broadcast_channel</jgroups-channel>
|
||
<refresh-timeout>10000</refresh-timeout>
|
||
</discovery-group>
|
||
</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>
|
||
<connection-factory name="ConnectionFactory">
|
||
<discovery-group-ref discovery-group-name="my-discovery-group"/>
|
||
<entries>
|
||
<entry name="ConnectionFactory"/>
|
||
</entries>
|
||
</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>
|
||
<connection-factory name="ConnectionFactory">
|
||
<connectors>
|
||
<connector-ref connector-name="netty-connector"/>
|
||
<connector-ref connector-name="netty-connector2"/>
|
||
<connector-ref connector-name="netty-connector3"/>
|
||
</connectors>
|
||
<entries>
|
||
<entry name="ConnectionFactory"/>
|
||
</entries>
|
||
</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<String, Object> map = new HashMap<String, Object>();
|
||
map.put("host", "myhost");
|
||
map.put("port", "5445");
|
||
TransportConfiguration server1 = new TransportConfiguration(NettyConnectorFactory.class.getName(), map);
|
||
HashMap<String, Object> map2 = new HashMap<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<String, Object> map = new HashMap<String, Object>();
|
||
map.put("host", "myhost");
|
||
map.put("port", "5445");
|
||
TransportConfiguration server1 = new TransportConfiguration(NettyConnectorFactory.class.getName(), map);
|
||
HashMap<String, Object> map2 = new HashMap<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>
|
||
<cluster-connections>
|
||
<cluster-connection name="my-cluster">
|
||
<address>jms</address>
|
||
<connector-ref>netty-connector</connector-ref>
|
||
<check-period>1000</check-period>
|
||
<connection-ttl>5000</connection-ttl>
|
||
<min-large-message-size>50000</min-large-message-size>
|
||
<call-timeout>5000</call-timeout>
|
||
<retry-interval>500</retry-interval>
|
||
<retry-interval-multiplier>1.0</retry-interval-multiplier>
|
||
<max-retry-interval>5000</max-retry-interval>
|
||
<initial-connect-attempts>-1</initial-connect-attempts>
|
||
<reconnect-attempts>-1</reconnect-attempts>
|
||
<use-duplicate-detection>true</use-duplicate-detection>
|
||
<forward-when-no-consumers>false</forward-when-no-consumers>
|
||
<max-hops>1</max-hops>
|
||
<confirmation-window-size>32000</confirmation-window-size>
|
||
<call-failover-timeout>30000</call-failover-timeout>
|
||
<notification-interval>1000</notification-interval>
|
||
<notification-attempts>2</notification-attempts>
|
||
<discovery-group-ref discovery-group-name="my-discovery-group"/>
|
||
</cluster-connection>
|
||
</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 addresses that match the
|
||
specified address field. An address is matched on the cluster connection when it begins with the
|
||
string specified in this field. The address field on a cluster connection also supports comma
|
||
separated lists and an exclude syntax '!'. To prevent an address from being matched on this
|
||
cluster connection, prepend a cluster connection address string with '!'.</para>
|
||
<para>In the case shown above the cluster connection will load balance messages sent to
|
||
addresses 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>
|
||
Examples:
|
||
<itemizedlist>
|
||
<listitem><literal>'jms.eu'</literal> matches all addresses starting with 'jms.eu'</listitem>
|
||
<listitem><literal>'!jms.eu'</literal> matches all address except for those starting with
|
||
'jms.eu'</listitem>
|
||
<listitem><literal>'jms.eu.uk,jms.eu.de'</literal> matches all addresses starting with either
|
||
'jms.eu.uk' or 'jms.eu.de'</listitem>
|
||
<listitem><literal>'jms.eu,!jms.eu.uk'</literal> matches all addresses starting with 'jms.eu'
|
||
but not those starting with 'jms.eu.uk'</listitem>
|
||
</itemizedlist>
|
||
Notes:
|
||
<itemizedlist>
|
||
<listitem>Address exclusion will always takes precedence over address inclusion.</listitem>
|
||
<listitem>Address matching on cluster connections does not support wild-card matching.
|
||
</listitem>
|
||
</itemizedlist>
|
||
</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>
|
||
<cluster-connection name="my-cluster">
|
||
...
|
||
<static-connectors>
|
||
<connector-ref>server0-connector</connector-ref>
|
||
<connector-ref>server1-connector</connector-ref>
|
||
</static-connectors>
|
||
</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>
|
||
<cluster-user>HORNETQ.CLUSTER.ADMIN.USER</cluster-user>
|
||
<cluster-password>CHANGE ME!!</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.apache.activemq.api.core.client.loadbalance.RoundRobinConnectionLoadBalancingPolicy</literal>
|
||
as the <literal><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.apache.activemq.api.core.client.loadbalance.RandomConnectionLoadBalancingPolicy</literal>
|
||
as the <literal><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.apache.activemq.api.core.client.loadbalance.RandomStickyConnectionLoadBalancingPolicy</literal>
|
||
as the <literal><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.apache.activemq.api.core.client.loadbalance.FirstElementConnectionLoadBalancingPolicy</literal>
|
||
as the <literal><connection-load-balancing-policy-class-name></literal>.</para>
|
||
</listitem>
|
||
</itemizedlist>
|
||
<para>You can also implement your own policy by implementing the interface <literal
|
||
>org.apache.activemq.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.apache.activemq.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>
|
||
<connection-factory name="ConnectionFactory">
|
||
<discovery-group-ref discovery-group-name="my-discovery-group"/>
|
||
<entries>
|
||
<entry name="ConnectionFactory"/>
|
||
</entries>
|
||
<connection-load-balancing-policy-class-name>
|
||
org.apache.activemq.api.core.client.loadbalance.RandomConnectionLoadBalancingPolicy
|
||
</connection-load-balancing-policy-class-name>
|
||
</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>
|
||
<cluster-connection name="my-cluster">
|
||
<address>jms</address>
|
||
<connector-ref>netty-connector</connector-ref>
|
||
<retry-interval>500</retry-interval>
|
||
<use-duplicate-detection>true</use-duplicate-detection>
|
||
<forward-when-no-consumers>true</forward-when-no-consumers>
|
||
<max-hops>1</max-hops>
|
||
<static-connectors allow-direct-connections-only="true">
|
||
<connector-ref>server1-connector</connector-ref>
|
||
</static-connectors>
|
||
</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>
|
||
<address-settings>
|
||
<address-setting match="jms.#">
|
||
<redistribution-delay>0</redistribution-delay>
|
||
</address-setting>
|
||
</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>
|