RabbitMQ message acknowledgment

To understand better how it works, could be useful compare the behavior of a very simple rabbit consumer when the acknowledgment flag is set to true or false in the Channel.basicConsume() call.

I have written a stripped down example where the producer puts a message in a queue and then quits. The consumer works with automatic or explicit acknowledgment accordingly to an input value is provided to it. Looking at the code, we can see how the changes are very localized. We specify that the QueueingConsumer for the channel/queue is in one mode or the other, and then, if an explicit ack is required, we give it when the job is done:

// ...
QueueingConsumer consumer = new QueueingConsumer(channel);
channel.basicConsume(QUEUE_NAME, autoAck, consumer); // 1

QueueingConsumer.Delivery delivery = consumer.nextDelivery();
// ...

if(!autoAck) { // 2
    channel.basicAck(delivery.getEnvelope().getDeliveryTag(), false);
    // ...
}

1. Here we specify if an handshake is required (autoAck sets to false) or if we rely on the standard auto-acknowledging RabbitMQ policy.
2. When we are done with the message, and if we are not in auto-acknowledgment mode, we give our explicit acknowledgment to the rabbit broker.

The complete Java code is on github.