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Version 19.1 by Paul Libbrecht on 2012/10/05
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1 {{warning}}
2 This tutorial is a simple way to setup a XWiki clustering for test purpose.
3 {{/warning}}
4
5 The goal of a cluster of XWiki instance is to provide several XWiki in different servers accessing the same database server to be able to do load balancing.
6
7 = Install two instances of XWiki on the same MySQL database =
8
9 Here we will use the XWiki standard distribution but instead of using the embedded hsqldb database we will use a MySql database. For this you need to:
10
11 * modify the ##hibernate.cfg.xml## file to change hsqldb configuration by a proper MySQL configuration
12 * copy your database driver JAR in WEB-INF/lib or in some shared lib directory
13
14 See http://www.xwiki.org/xwiki/bin/view/AdminGuide/Installation for more details on how to install XWiki.
15
16 At this point you almost have a XWiki cluster: you have two instances of XWiki which are using the same datas. But there is a remaining "detail": the problem is that for performance reason XWiki is using lots of different caches which mean that even if one instance of XWiki modify a document it's possible the other XWiki instance will not see it and keep showing the document from its cache.
17
18 = Configure event distribution =
19
20 To handle this we will use the network event distribution system. When anything happens in a XWiki instances it generate a local event which is used to update caches. Here we will make theses events send to other XWiki instances as well and emulate local events. This way all the code updating its cache each time something changed in a particular document for example will also be notified if it has been done by another instance of XWiki.
21
22 == Enable remote observation ==
23
24 First you need to enable remote observation system.
25
26 For this go to xwiki.properties file and set the property ##observation.remote.enabled## to ##true##.
27
28 == Set the channels ==
29
30 Then you need to indicate to remote observation manager which communications channels it should start when XWiki starts.
31
32 For this you need to list in property ##observation.remote.channels## the names of the channels.
33
34 Here we set ##udp## in both instances to use embedded JGroups udp.xml configuration file which auto discover cluster members.
35
36 = Start XWiki instance on different ports =
37
38 For this tutorial we run two instance of XWiki in the same server. So we need each instance to use different ports.
39
40 To run a XWik instance if a custom port you can provide it in parameter of the script start_xwiki.sh.
41
42 Here we start the first instance with:
43
44 {{code}}
45 sh start_xwiki.sh 8080 8070
46 {{/code}}
47
48 and the second one with:
49
50 {{code}}
51 sh start_xwiki.sh 8081 8071
52 {{/code}}
53
54 The second port is the port used to stop jetty server, it's not mandatory to run XWiki but it's better to have differents ports if you don't want to have to kill the java process instead of using the stop_xwiki.sh script ;)
55
56 Sometimes you have to force the bind address, you can do it using ##-Djgroups.bind_addr=localhost##
57
58 = Check the clustering setup =
59
60 * Load a page in both instances
61 * Modify a page in one instance
62 * Reload it in the other instance
63 * It should have taken into account the modification made on the other instance
64
65 = Add load balancing with apache2 ##mod_proxy_balancer## =
66
67 You probably want to do load-balancing in front of the clustered XWiki instances. This section provides sample configurations to do that.
68
69 Replace if you have one your apache proxy configuration by the following one, or set it as proxy of your apache site configuration if you don't have one yet :
70
71 {{code}}
72 ProxyRequests Off
73 ProxyPreserveHost On
74
75 <Proxy balancer://mycluster>
76 BalancerMember ajp://127.0.0.1:8009 route=jvm1
77 BalancerMember ajp://127.0.0.1:9009 route=jvm2
78 ProxySet stickysession=JSESSIONID
79 </Proxy>
80 ProxyPass /xwiki balancer://mycluster/xwiki
81 {{/code}}
82
83 You need also to precise the name of the JVM route for each member of the balancer. On Tomcat, in ##conf/server.xml##, edit the ##Engine## node as follow :
84 {{code}} <Engine name="Catalina" defaultHost="localhost" jvmRoute="jvm1">{{/code}}
85
86 On GlassFish and Tomcat and probably others, you can set the system property jvmRoute to the same value.
87
88 Where you replace ##jvm1## by the name of the route you have precised in the Apache configuration.
89
90 This setting will complement the JSESSIONID with a suffix made by this jvmRoute: thus any session will contain the node name and this will instruct Apache to choose that route the next time a request comes. Thus, the browser will remain hooked to that server as long as the session remains.
91
92 == Testing the clustered setup ==
93 To test the load-balancing setup, you can edit the ##version.properties## file of your wiki to add a suffix to be able to recognize each member of the cluster. For example ##version=2.6.33077-node1## for the first node of a cluster of XWiki 2.6 nodes. Then, connect to the wiki via the URL that points to the load-balancer with a first browser, and look at which node is being used. Just after connect with a second browser (to have a different session), and make sure the node used is the other one (since mod_proxy_rewrite affects balancing members sequentially.
94
95 You can also verify the JSESSIONID cookie value which will end with the route name.
96
97 Finally, to test that clustering works you can either:
98
99 * set-up a dedicated virtual host to each cluster node so that you can check that, for example, a changed document, is changed on every node
100 * force the route by adding the URL parameter JSESSIOND=jvm1: this will make sure that this route is used which, in turn, will set the cookie to a session-identifier suffixed with jvm1.
101
102 = Monitoring and Troubleshooting =
103
104 The error log of Apache reports an amount of details about the choice of load-balancing if the log-level is sufficient.
105
106 The balancer-manager is also precious:
107
108 {{code}}
109 <Location /balancer-manager>
110 SetHandler balancer-manager
111 AuthType digest
112 AuthName "XWiki Cluster Management"
113 AuthType Digest
114 AuthDigestDomain http://example.com
115 AuthDigestFile /path/to/file.htdigest
116 Require valid-user
117 </Location>
118 {{/code}}
119
120 Using this configuration, properly specialized, provides a small interface at /balancer-manager where you can see the cluster nodes which are active, their load, and where you can disable them to take them offline for a bit.
121
122 == Debugging ==
123
124 See [[Debugging Section>>extensions:Extension.Observation Module Remote#HDebugging]] in the Observation Reference documentation.
125
126 You can also easily include the following HTML to force your browser to change from one node of the cluster to the other. This is extremely important to verify that clustering works:
127
128
129 {{code}}
130 <a href="$doc.name"
131 onclick="d = new Date(); d.setDate(d.getDate()+7); document.cookie='JSESSIONID=xxx.$hostname; ' + d.toUTCString()" ##
132 >$hostname</a>
133 {{/code}}
134
135 (where you replace $hostname (twice) and $doc.name appropriately.

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