@Override
  public MessageStream<M> sendTo(OutputStream<M> outputStream) {
    String opId = this.streamAppDesc.getNextOpId(OpCode.SEND_TO);
    OutputOperatorSpec<M> op = OperatorSpecs.createSendToOperatorSpec(
        (OutputStreamImpl<M>) outputStream, 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