rabbitmq amqp - listening to ack messages from consumer

Question

I have a producer and broker on the same machine. The producer sends messages like so:

channel = connection.createChannel();

//Create a durable queue (if not already present)
channel.queueDeclare(merchantId, true, false, false, null);

//Publish message onto the queue
channel.basicPublish("", consumerId, true, false,
    MessageProperties.MINIMAL_PERSISTENT_BASIC, "myMessage");

The consumer sits on another machine and listens to messages. It uses explicit acknowledgement like so:

while (true) {
    QueueingConsumer.Delivery delivery = consumer.nextDelivery();
    //Handle message here  
    channel.basicAck(delivery.getEnvelope().getDeliveryTag(), false);
}   

From what I understand, the ack is meant for the broker to dequeue the message.

But how can my producer come to know about the ack that the consumer sent?

Answer 1

Producers and consumers normally don't interact. This is by AMQP protocol design. For example, consuming a specific message may be done a long time after it was published, and there is no sense in leaving the producer up and running for a long time. Another example is when a publisher sends one message to a broker, and due to routing logic that message gets duplicated to more than one queue, leading to ambiguity (because multiple consumers can acknowledge the same message). AMQP protocol is asynchronous (mostly), and letting the publisher know about its message being consumed just doesn't fit the AMQP async model.

There are exceptions from that, notably, RPC calls. Then the producer becomes a producer-consumer. It sends a message and then immediately waits for a reply (there is a good RabbitMQ manual - Direct reply-to related to RPC with RabbtiMQ).

In general, you can ensure that a message is delivered to a broker with Confirms (aka Publisher Acknowledgements) alongside with Dead Letter Exchanges and Alternate Exchanges. Those cover most cases under which a message can be lost from its normal flow.