How to setup a cluster of XWiki instance based on distributed events

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.

Install two instances of XWiki on the same MySQL database

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:

• modify the hibernate.cfg.xml file to change hsqldb configuration by a proper MySQL configuration
• copy your database driver JAR in WEB-INF/lib or in some shared lib directory

See http://www.xwiki.org/xwiki/bin/view/AdminGuide/Installation for more details on how to install XWiki.

At this point you almost have a XWiki cluster: you have to instance of XWiki which are using the same datas. But there is a remaining "detail": the problem is that for performence reason XWiki is using lots of differents caches which mean that even if one instance of XWiki modified a document it's possible the other XWiki instance will not see it and keep showing the document in its cache.

Configure event distribution

To handle this we will use the network event distribution system. When anything append 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.

Enable remote observation

First you need to enable remote observation system.

For this go to xwiki.properties file and set the property observation.remote.enabled to true.

Set the channels

Then you need to indicate to remote observation manager which communications channels it should start when XWiki starts.

For this you need to list in property observation.remote.channels the names of the channels.

Here we set tcp1 in one instance and tcp2 in the other XWiki installation.

Put the JGroups channels xml configuration files

The names of the channels are used to match the configuration files names in located in WEB-INF/observation/remote/jgroups/ configuration folder. For example in this case we will add a tcp1.xml} file in one XWiki instance and {tcp2.xml file in the other XWiki instance.

Start XWiki instance on different ports

For this tutorial we run two instance of XWiki in the same server. So we need each instance to use different ports.

To run a XWik instance if a custom port you can provide it in parameter of the script start_xwiki.sh.

Here we start the first instance with:

and the second one with:

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