@Override
  public byte[] toBytes(String key) {
    Object[] keyArray = new Object[]{CoordinatorStreamMessage.VERSION, type, key};
    return keySerde.toBytes(Arrays.asList(keyArray));
  }
Serializer for keys written into coordinator stream.
samza-core/src/main/java/org/apache/samza/coordinator/stream/CoordinatorStreamKeySerde.java