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2010-06-03T17:36:07.297
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2010-06-04T14:04:06.313
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Jgroups is a Java network stack which includes <code>DistributedLockManager</code> type of support and cluster voting capabilities. These allow any number of unix daemons to agree on who should be active. All of the nodes could be trying to obtain a lock (for example) and only one will succeed until the application or the node fails. Jgroups also have the concept of the coordinator of a specific communication channel. Only one node can be coordinator at one time and when a node fails, another node becomes coordinator. It is simple to test to see if you are the coordinator in which case you would be active. See: <a href="http://www.jgroups.org/javadoc/org/jgroups/blocks/DistributedLockManager.html" rel="nofollow noreferrer">http://www.jgroups.org/javadoc/org/jgroups/blocks/DistributedLockManager.html</a> If you are going to implement this yourself there is a bunch of stuff to keep in mind: <ul> <li>Each node needs to have a consistent view of the cluster.</li> <li>All nodes will need to inform all of the rest of the nodes that they are online -- maybe with multicast.</li> <li>Nodes that go offline (because of ap or node failure) will need to be removed from all other nodes' "view".</li> <li>You can then have the node with the lowest IP or something be the active node.</li> <li>If this isn't appropriate then you will need to have some sort of voting exchange so the nodes can agree who is active. Something like: <a href="http://en.wikipedia.org/wiki/Two-phase_commit_protocol" rel="nofollow noreferrer">http://en.wikipedia.org/wiki/Two-phase_commit_protocol</a></li> </ul>
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