Clustered Singletons

Fabric3 makes implementing highly-available clustered singletons trivial by using the @Scope("DOMAIN") annotation:

@Scope("DOMAIN")
public class TheComponent ... {
 
   public void onEvent(Event event) {
      //...
   }
}

As explained in Java Components, a clustered singleton instance is created on the zone leader. If the leader fails or otherwise becomes inactive, a new instance is created on the newly elected leader.

There are a number of common use cases for clustered singletons:

• Timers that can only have a single emitter but must be highly-available (see Timer Components)
• Consumers that listen on a remote channel bound to a transport such as JMS which can only have one instance active at a time