RabbitMQ - How to check if queue is empty?

Question

I have a web service interface that abstracts a RabbitMQ server (don't ask me why, I know it's an unnecessary step, but I have to). That is, I poll messages from the queue through a web service call, not directly over amqp.

Consuming via basic.consumer blocks the execution thread till there are messages in the queue. This makes the web service not return.

Code for illustration:

    $connection = new AMQPStreamConnection('localhost', 5672, 'guest', 'guest');
    $channel = $connection->channel();

    $channel->queue_declare(QUEUE_NAME, false, true, false, false);
    $ret = array('body' => '');

    $callback = function($msg) use ($channel, &$ret) {
        $ret['body'] = $msg->body;
        /*
        Here I would basic.cancel the consumer if there were no messages in the queue
        */
    };

    $channel->basic_consume(QUEUE_NAME, 'tag', false, true, false, false, $callback);

    if (count($channel->callbacks)) {
        $channel->wait(); // blocks here...
    }

    return $ret;

possible duplicate of [RabbitMQ - Get total count of messages enqueued](http://stackoverflow.com/questions/17684848/rabbitmq-get-total-count-of-messages-enqueued) — pinepain, Sep 09 '15 at 06:19

score 6 · Answer 1 · answered Sep 06 '17 at 15:59

If you want to get the size of queue, you can call queue_declare with php-amqlib, the second argument of return is the number of messages in the queue.

  list($queue, $messageCount, $consumerCount) = $channel->queue_declare(QUEUE_NAME, true);

It is important to give the $passive argument to true when you call the queue_declare() method

score 3 · Answer 2 · edited Jun 20 '20 at 09:12

3

What I wanted to do is achieved by basic.get.

In php-amqlib:

$channel->basic_get(QUEUE_NAME, true); // the second arg is no_ack.

The second argument marks that no acknowledgment is expected for that message. That is, you don't have to "flag" the message as read for RabbitMQ to confidently dequeue it. Excluding it (having it = false) results in not popping the top message.

Why all this hassle?

I was wrapping the RabbitMQ code inside an http web service. And this is a not good idea (at least for my use case). As when the web service returns, and thus the rabbitmq connection terminates, the non (yet) acknowledged message is requeued back to the queue. So, if you have to adopt an http wrapper, make sure you isolate the lifetime of the rabbitmq connection from the http request lieftime. However, I didn't try this.

edited Jun 20 '20 at 09:12

Community

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answered Sep 08 '15 at 22:51

Ahmed Khalaf

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in case you were still wanting to get the size of the queue, when you declare exchange, it returns a data structure that has the current queue size – Derick Bailey Sep 09 '15 at 00:22
AMQP reference provides: "If this field is set the server does not expect acknowledgements for messages. That is, when a message is delivered to the client the server assumes the delivery will succeed and immediately dequeues it. This functionality may increase performance but at the cost of reliability. Messages can get lost if a client dies before they are delivered to the application.". If you don't set it, you have to ack the message you receive. – Dereckson Dec 12 '15 at 13:50
I don't understand the second half of this answer (beneath the heading). Recommend edit or deletion if it is irrelevant to the answer. – theMayer Feb 21 '19 at 16:18

RabbitMQ - How to check if queue is empty?

2 Answers2

Why all this hassle?

Linked