Hazelcast (hazelcast.com) provides an easy way to implement distributed locking to allow your applications to run multiple, fault-tolerant instances without worrying about issues related concurrent access to shared resources (like files, databases or whatever).

[sourcecode language=”java”]
try {
java.util.concurrent.locks.Lock lock = Hazelcast.getLock(“mylock”);
while (true) {
lock.lock();
try {
// do some work involving access to shared resources
} finally {
lock.unlock();
}
}
} finally {
Hazelcast.shutdown();
}
[/sourcecode]

We have an SMS server which retrieved mail messages from a POP3 mailbox, entered them to a database and then delivered via the Clickatell messaging gateway. Making it run multiple instances concurrently would have been a headache since it would involve various issues relating to transactions across the POP3 and the Clickatell interfaces. It was a whole lot easier to wrap the business logic with a hazelcast lock and let it run on two servers. The beauty of hazelcast is that it just works – since the default configuration uses multicast to detect other members of the cluster, there’s no additional infrastructure – just add the jar to your application and off you go. We might still implement Zookeeper or Terracotta in future, but both of them require more infrastructure (i.e. dedicated (virtual) servers) and configuration.
Postscript: I had occasional hangs with Hazelcast 1.8.4 which caused me to switch to Zookeeper. As expected, Zookeeper was a lot harder to use than Hazelcast – you need Zookeeper installed on 3 servers. There’s no official java client, just some recipes and I found an implementation of Zookeeper locks called Cages on google code. For a java developer, Hazelcast is obviously way easier to use. Anyway, after upgrading to Hazelcast version 1.8.5, the hang problem went away so I happily went back to using Hazelcast. We’re also now using the distributed queues – works great so far.