@Override
  public <OM> MessageStream<OM> flatMapAsync(AsyncFlatMapFunction<? super M, ? extends OM> flatMapFn) {
    String opId = this.streamAppDesc.getNextOpId(OpCode.ASYNC_FLAT_MAP);
    AsyncFlatMapOperatorSpec<M, OM> op = OperatorSpecs.createAsyncOperatorSpec(flatMapFn, opId);
    this.operatorSpec.registerNextOperatorSpec(op);
    return new MessageStreamImpl<>(this.streamAppDesc, op);
  }
The {@link OperatorSpec} associated with this {@link MessageStreamImpl}
samza-core/src/main/java/org/apache/samza/operators/MessageStreamImpl.java