Mastering Apache Storm

Field grouping

This grouping enables you to partition a stream on the basis of some of the fields in the tuples. For example, if you want all the tweets from a particular user to go to a single task, then you can partition the tweet stream using field grouping by username in the following manner:

builder.setSpout("1", new TweetSpout()); 
builder.setBolt("2", new TweetCounter()).fieldsGrouping("1", new Fields("username")) 

As a result of the field grouping being hash (fields) % (no. of tasks), it does not guarantee that each of the tasks will get tuples to process. For example, if you have applied a field grouping on a field, say X, with only two possible values, A and B, and created two tasks for the bolt, then it might be possible that both ...