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<p>For 1.1 onwards we're implementing a range of <em>Group Organisation</em> protocols to help arrange nodes into groups. To get an idea of the progress we're making, check out the <a href="http://activecluster.codehaus.org/maven/apidocs/org/codehaus/activecluster/group/package-summary.html">javadocs</a>.</p>

<h2>Organising Groups</h2>

<p>There are a few different ways in which we want to arrange nodes into groups...</p>

<h3>Groups</h3>

<p>The basic idea is we want to arrange nodes into logical groups. Each group acts as a single logical entity. </p>

<p>For example to implement the master/slave type High Availability (HA) model, we may wish to have just 1 single group with a <em>master</em> node and a collection of one or more <em>slave nodes</em>. If the master goes down or becomes unavailable or overworked; we can failover to a slave.</p>

<p>The default GroupModel supports this behaviour and can handle one or more groups. So there could be several groups (with a single master and multiple slaves) for different instances of a service. You can configure the GroupModel with a maximum number of groups - so if you wish you can restrict the number of groups.</p>

<h3>Buddy Groups</h3>

<p>The organisation policy for buddy groups is often a little different. Generally we want each node in a cluster to be a master node, in its own group - but for other nodes in the cluster to be buddies to act as backups.</p>

<p>(We may wish to restrict some nodes from being master nodes, so there is an optional masterNodeFilter we can use to restrict those nodes on the BuddyGroupModel being master nodes).</p>

<p>So the BuddyGroupModel will create a group for every possble node joining and then assign buddy groups from other members in the cluster.</p>

<h2>Using Groups</h2>

<p>To use groups, the leader is elected by the cluster then it decides, based on the current node state, what groups to create / destroy and how to arrange the cluster. As nodes come and go the leader decides what to do.</p>

<p>If the leader fails, a new leader is elected and the process starts again.</p>

<p>When the leader decides that a node must join or leave a particular group, it communicates with the node in question. Depending on the application; some kind of <em>node synchronization</em> is typically required - such as state transfer from buddy nodes. When that is complete (or if it fails) then the node should announce its membership status to other nodes - then the leader can decide what to do next.</p>