Mastering Apache Storm

Global grouping

Global grouping does not partition the stream but sends the complete stream to the bolt's task, the smallest ID. A general use case of this is when there needs to be a reduce phase in your topology where you want to combine the results from previous steps in the topology into a single bolt.

Global grouping might seem redundant at first, as you can achieve the same results by defining the parallelism for the bolt as one if you only have one input stream. However, when you have multiple streams of data coming through a different path, you might want only one of the streams to be reduced and others to be parallel processes.

For example, consider the following topology. In this, you might want to combine all the tuples coming ...