RabbitMQ + Reactor, sending ACK after passing on message?

Gelunox :

I'm trying to figure out if Project-Reactor is a viable option for my use case.

I need high consistency in my application, I want to:

1. Receive a message from RabbitMQ
2. Do some manipulation/transformation
3. Put the message on a (different) RabbitMQ queue
4. Send an ACK for the original message

With the normal RabbitMQ Java library it would roughly look something like this:

var connectionFactory = new ConnectionFactory();
var connection = connectionFactory.newConnection();
var channel = connection.createChannel();
channel.txSelect();
channel.queueDeclare("queue.A", true, false, false, null);
channel.queueDeclare("queue.B", true, false, false, null);

channel.basicConsume("queue.A", false, (tag, delivery) ->
{
    String data = new String(delivery.getBody(), StandardCharsets.UTF_8);
    System.out.println("Received: " + data);
    channel.basicPublish("", "queue.B", null, data.getBytes(StandardCharsets.UTF_8));
    channel.txCommit();
    System.out.println("Sending ACK");
    channel.basicAck(delivery.getEnvelope().getDeliveryTag(), false);
    channel.txCommit();

}, tag -> System.err.println("Tag " + tag + " failed"));

This nicely prints

Received: DATA
Sending ACK

Using Reactor-RabbitMQ I've come up with the following:

var options = new ReceiverOptions();
var receiver = RabbitFlux.createReceiver(options);
var sender = RabbitFlux.createSender();
sender.declare(QueueSpecification.queue("queue.A")).block();
sender.declare(QueueSpecification.queue("queue.B")).block();
sender
    .send(receiver.consumeManualAck("queue.A")
        .doOnError(ex -> ex.printStackTrace())
        .doOnEach(s ->
        {
            s.get().ack();
            print("ACK");
        })
        .map(ad ->
        {
            print("MAP");
            return new OutboundMessage("", "queue.B", ad.getBody());
        }))
    .doOnEach(v ->
    {
        print("SUB");
    })
    .subscribe();

However, this prints

ACK
MAP

Which is not the right order, obviously, because I'm doing the doOnEach before map. But the AcknowledgableDelivery is no longer available after I've mapped it to OutboundMessage for the sender.

note that SUB is never printed. Probably because the send subscription is continuous and never reaches the Mono<Void> terminated state.

My knowledge of Reactor is not that extensive, so I feel like I'm missing something I could use here. Is it possible to do this neatly with Reactor or should I forget about it?

---

_{Footnote: I'm using var because I'm in Java 11, and the print statements as a way to quantify the order in which things are executed.}

pmackowski :

I do not think it is possible to acknowledge inside send method. Instead you can try:

receiver.consumeManualAck("queue.A")
        .flatMap(ad -> {
           Mono<OutboundMessage> outboundMessageMono =
                Mono.just(new OutboundMessage("", "queue.B", ad.getBody()));
           return sender.send(outboundMessageMono)
                        .doFinally(signalType -> {
                            if (signalType == SignalType.ON_COMPLETE) {
                               ad.ack();
                            } else {
                               ad.nack(false);
                           }
                         });
         });