@Override
  public void start() {
    for (int i = 0; i < threadCount; ++i) {
      Set<SystemStreamPartition> threadSsps = new HashSet<SystemStreamPartition>();

      // Assign SystemStreamPartitions for this thread.
      for (SystemStreamPartition ssp : ssps) {
        if (Math.abs(ssp.hashCode()) % threadCount == i) {
          threadSsps.add(ssp);
        }
      }

      // Start thread.
      Thread thread = new Thread(new MockSystemConsumerRunnable(threadSsps), "MockSystemConsumer-" + i);
      thread.setDaemon(true);
      threads.add(thread);
      thread.start();
    }
  }
Assign SystemStreamPartitions to all of the threads, and start them up to
begin simulating consuming messages.
samza-test/src/main/java/org/apache/samza/system/mock/MockSystemConsumer.java