Gets the SINGLE downstream processor task index to send message to.
Gets the SINGLE downstream processor task index to send message to.
Message you want to send
How many tasks does the downstream processor have.
Upstream task's task index who trigger the getPartition() call.
ONE task index of downstream processor.
For processor chain: A (3 tasks)
->
B (3 tasks), UnicastPartitioner does ONE-task->
ONE-task mapping.