Each Flow and Cascade has the ability to execute callbacks via an event listener. This ability is useful when an external application needs to be notified that either a Flow or Cascade has started, halted, completed, or either has thrown an exception.
For instance, at the completion of a flow that runs on an Amazon EC2 Hadoop cluster, an Amazon SQS message can be sent to notify another application to fetch the job results from S3 or begin the shutdown of the cluster.
Flows support event listeners through the
cascading.flow.FlowListener interface and
Cascades support event listeners through the
cascading.cascade.CascadeListener, which supports
four events:
The onStarting event is fired when a Flow or Cascade
instance receives the start() message.
The onStopping event is fired when a Flow or Cascade
instance receives the stop() message.
The onCompleted event is fired when a Flow or Cascade instance has completed all work, regardless of success or failure. If an exception was thrown, onThrowable will be fired before this event.
The onThrowable event is fired if any internal job client
throws a Throwable type. This throwable is passed as an argument
to the event. onThrowable should return true if the given
throwable was handled, and should not be rethrown from the
Flow.complete() or Cascade.complete()
methods.
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