@Override
  public void register(String source) {
    LOG.info("Registering source {} with in memory producer", source);
  }
Registers this producer to send messages from a specified Samza source, such as a StreamTask.
@param source String representing the source of the message.
samza-core/src/main/java/org/apache/samza/system/inmemory/InMemorySystemProducer.java