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rabbitmqmessagingamqp

RabbitMQ Prefetch

Up until now, my RabbitMQ consumer clients have used a prefetch value of 1. I'm looking to increase the value in order to gain performance. If I set the value to 2, will the RabbitMQ server send each consumer 2 messages at once such that I will need to parse the two messages and store the second one in a List until the first is processed and acknowledged? Or will the API handle this behind the scenes?

I'm using the Java AMQP client library:

ConnectionFactory factory = new ConnectionFactory();
...
Connection connection = factory.newConnection();
Channel channel = connection.createChannel();

channel.basicQos(2);

QueueingConsumer consumer = new QueueingConsumer(channel);
channel.basicConsume(CONSUME_QUEUE_NAME, false, consumer);

while (!Thread.currentThread().isInterrupted()) {
   try {
      QueueingConsumer.Delivery delivery = consumer.nextDelivery();
      String m = new String(delivery.getBody(), "UTF-8");

      // Will m contain two messages? Will I have to each message and keep track of them within a List?

      ...
}
Solution

  • The api handles this behind the scenes, so there are no worries there for you.

    Regarding which message gets where, RMQ will just deliver by using round robin, that is if you have the queue: 1 2 3 4 5 6 and consumer1 and consumer2.

    consumer1 will have 1 3 5
consumer2 will have 2 4 6

    Should the connection die to any of your consumers the prefetched messages will be redelivered to the active consumers using the same delivery method.

    This should be interesting reading and a good starting point to figure more exactly what happens: