@Override
  public void stop() {
    for (Thread thread : threads) {
      thread.interrupt();
    }

    try {
      for (Thread thread : threads) {
        thread.join();
      }
    } catch (InterruptedException e) {
    }
  }
Kill all the threads, and shutdown.
samza-test/src/main/java/org/apache/samza/system/mock/MockSystemConsumer.java