Messages with expiration are not removed from RabbitMQ

Question

I am sending a normal message through a producer to RabbitMQ and then I send a second message with the expiration attribute assigned to a value. Then using the rabbitmqctl list_queues command I monitor the status of the messages.

I found that if I send a normal message first and then a message with expiration, the rabbitmqctl list_queues is always showing me 2 messages pending on the queue. When I consume them, I get only one.

On the other hand if I send just 1 message with expiration, in the beginning I see the message and then after the correct expiration time, I find it deleted.

My question is, on the first situation is actually the message taking space? Or it is an interface bug?

My rabbitMQ version is: rabbitmq-server.noarch -> 3.1.5-1.el6

Solution

Looks like you missed some of the documentation on this feature. If you read the RabbitMQ documentation on per-message TTL (expiration), you will notice the following warning for exactly the behavior you are seeing (emphasis added):

Caveats

While consumers never see expired messages, only when expired messages reach the head of a queue will they actually be discarded (or dead-lettered). When setting a per-queue TTL this is not a problem, since expired messages are always at the head of the queue. When setting per-message TTL however, expired messages can queue up behind non-expired ones until the latter are consumed or expired. Hence resources used by such expired messages will not be freed, and they will be counted in queue statistics (e.g. the number of messages in the queue).