Why can't you look at messages in the Rabbit Queue

Question

If my understanding is correct, you can't actually look at messages in the rabbit queue without taking them out and putting them back in. There's no way to use rabbitmqctl to inspect a queue.

In some debugging contexts, knowing what is currently in the queue is very useful. Is there a way to get at the messages? Also, what is it about the design of Rabbit that makes this process cumbersome?

Sheffy

Answer 1

I have not used this personally yet but I saw RabbitMQ's management plugin that I thought allowed you to monitor the queue.

http://www.rabbitmq.com/management.html

Converter

Yes, and it's quite useful! Unfortunately, that tool mostly provides aggregate statistics, and no mechanism to see what's inside the queue (unless I'm mistaken). – Sheffy

@Sheffy now you got me interested even more because I had assumed the plugin did this. I looked at the mailing list and some people were building plugins for something similar lists.rabbitmq.com/pipermail/rabbitmq-discuss/2011-March/… I hope this exists but if the features does not, I might just have my program write this data as a quick query to mysql or a file so I can monitor. I hope something exists, sorry I couldn't be of more help. – Converter

According to Brian Kelly, it appears you can in fact look at message contents from the web interface. I'll go look at it again. – Sheffy

@Sheffy I thought so too but didn't get around to testing it yet. Glad to see you got the answer. I learned something too, thanks! – Converter

Answer 2

There is a "Get Messages" section for each queue in the management API. However this causes the message to be consumed and hence is a destructive action. We can re-queue this message to the queue only at the expense of sacrificing the ordering of messages [for rabbitmq versions < 2.7.0].

A more viable alternative would be to use the firehose tracer, http://www.rabbitmq.com/firehose.html [for rabbitmq versions> 2.5]. This essentially publishes the message to a different exchange (amq.rabbitmq.trace) just for debugging purposes.

Here is another GUI written on top of firehose for better visibility, http://www.rabbitmq.com/blog/2011/09/09/rabbitmq-tracing-a-ui-for-the-firehose/

Henigman

If it's something you're going to do quite often, then you can sort of do this yourself - have your Exchange post to your normal queue and a secondary 'monitoring' queue, which either consumes all messages and provides some way to see them, or else expires them when the queue gets too long or too old. – Sesquicentennial

Answer 3

This is old, but just for anyone interested in this. By visiting the Queues you have a list for all the queues of the broker.

Press any queue you are interested and scroll down to find this section

The really important option to set here is the Requeue option. If is set to Yes, this operation will consume the message, so you can read it, but it will requeue it, so it won't be lost.

Very Important: Original order of the message will be lost when the message is re-queued

Ecliptic

it won't be lost, but unfortunately it will be requeued so the original order (and possibly timestamp) of the messages will be lost – Chard

Not sure that's how it works -- it seems to leave the top message at the top after the requeue -- I use this interface to first view the top message, and then set requeue to no to get rid of the first message. So perhaps 'Requeue' really means get it without acking it, and put it back to ready by destroying the consumer. – Lees

Official documentation says "When a message is requeued, it will be placed to its original position in its queue, if possible. If not (due to concurrent deliveries and acknowledgements from other consumers when multiple consumers share a queue), the message will be requeued to a position closer to queue head." rabbitmq.com/nack.html – Hairdresser

Answer 4

You can certainly look at the contents of a queue in RabbitMQ, or any AMQP broker, for that matter. Just consume messages but don't acknowledge them. Once you close the channel the messages will be available for consumption by your 'real' consumers. Keep in mind that doing so might affect the ordering of messages in the queue which you inspect.

Also, the web management plugin offered by RabbitMQ allows you to view the contents of messages from the web interface. If you're trying to debug your system, it's a very helpful tool.

Cum

Ah, so the web interface does do that. I guess I'll have to look at it again. Thank you. – Sheffy

It's in "Queues", pick a queue, scroll down to "Get messages". – Antiquary

Sure, but doing that eats the message in question. Not the intended result. – Meeker

@Matthias not if you don't ack them. – Cum

@BrianKelly So you get them again if nobody else picks them up. Or you miss some if somebody else picks them up first. Both don't match my definition of "monitor". – Meeker

@MatthiasUrlichs neither the question nor this answer mentioned "monitor". – Ppm

Answer 5

You can click the Queue name first in the Web Management and click to the GetMessages to get your message. Now it will show your messages here

Jewelljewelle

Answer 6

There's no sane way to look at a queue, but maybe monitoring what goes in is a sufficient substitute. To do this, you need to implement a man-in-the-middle monitor. This requires cooperating clients: you need to teach either all senders or all receivers to use a different exchange.

Suppose you want to monitor messages to exchange "foo". You create a (direct) exchange named "foo-in" (or whatever), set up "foo" as an alternate exchange for "foo-in", and teach all your senders to send their messages to the "foo-in" exchange instead of "foo".

Your queue monitor then needs to listen to "foo-in", and to re-publish all messages to "foo". Whenever the monitor is not running, rabbitmq will route them to "foo" by itself; the performance penalty for this is negligible.

This is a rabbitmq extension. See http://www.rabbitmq.com/ae.html for details on how alternate exchanges work. Of course you can use "foo" and "foo-out", respectively, if that's easier to do in your setup.

Monitoring a specific queue (again: queue input, not output) is easier but again requires changing the client (or the code which creates your queues, if they're persistent). Set up a fan-out exchange, bind the client's queue to that, and then bind the exchange to the original messages source. This is another rabbitmq extension; see http://www.rabbitmq.com/e2e.html. Your monitor simply needs to bind to that exchange and will get copies of all messages sent to the client's queue.

Meeker

Good solution. I believe you can also simply have your exchange fan out to each queue it knows, and your logger can just attach (bind) a queue when it needs. rabbitmq.com/tutorials/tutorial-three-python.html – Lees

Answer 7

It's possible to get a message without acknowledging and then reject it, that wouldn't get the message out of the queue. But this is not implemented in the management tool.

And also the message is locked until released meaning that no other consumer can consume it before it gets rejected.

Noisy

Answer 8

You could stuff them into something else first before sending them to RabbitMQ. I wrote message queuing software to do this. Check out http://qdb.io/

Cyd

Answer 9

You can use Queue Viewer (https://www.queueviewer.com). It requires that RabbitMQ's management plugin is enabled.

Githens

Not sure if Queue Viewer still exist but, I found that QueueExplorer works perfectly with RabbitMQ, exactly what I needed. – Smalls