Mastering Apache Storm

Direct grouping

In direct grouping, the emitter decides where each tuple will go for processing. For example, say we have a log stream and we want to process each log entry to be processed by a specific bolt task on the basis of the type of resource. In this case, we can use direct grouping.

Direct grouping can only be used with direct streams. To declare a stream as a direct stream, use the backtype.storm.topology.OutputFieldsDeclarer.declareStream method, which takes a boolean parameter. Once you have a direct stream to emit to, use backtype.storm.task.OutputCollector.emitDirect instead of emit methods to emit it. The emitDirect method takes a taskId parameter to specify the task. You can get the number of tasks for a component using the ...