@Override
  protected Collection<Void> handleEndOfStream(MessageCollector collector, TaskCoordinator coordinator) {
    sendControlMessage(new EndOfStreamMessage(taskName), collector);
    return Collections.emptyList();
  }
An operator that sends sends messages to an output {@link SystemStream} for repartitioning them.
samza-core/src/main/java/org/apache/samza/operators/impl/PartitionByOperatorImpl.java