.net - How do I set a number of retry attempts in RabbitMQ?

Question

I am using RabbitMQ and I have a queue that holds email messages. My consumer service de-queues messages and attempts to send them. If, for any reason, my consumer cannot send the message, I would like to re-queue the message to send again. I realize I can do a basicNack and set the requeue flag to be true, however, I don't want to requeue the message indefinitely (say, if our email system goes down, I don't want to continuously requeue unsent messages). I would like to define a finite number of times that I can requeue the message to be sent again. I can't set a field on the email message object, however, when I dequeue it and send a nack. The updated field is not present on the message in the queue. Is there any other way in which I can approach this? Thanks in advance.

Answer 1

There are no such feature like retry attempts in RabbitMQ (as well as in AMQP protocol).

Possible solution to implement retry attempts limit behavior:

1. Redeliver message if it was not previously redelivered (check redelivered parameter on basic.deliver method - your library should have some interface for this) and drop it and then catch in dead letter exchange, then process somehow.

2. Each time message cannot be processed publish it again but set or increment/decrement header field, say x-redelivered-count (you can chose any name you like, though). To get control over redeliveries in this case you have to check the field you set whether it reaches some limit (top or bottom - 0 is my choise, a-la ttl in ip header from tcp/ip).

3. Store message unique key (say uuid, but you have to set it manually when you publish message) in Redis, memcache or other storage, even in mysql alongside with redeliveries count and then on each redelivery increment/decrement this value until it reach the limit.

4. (for real geeks) write plugin that will implement such behavior like you want.

The pro of #3 is that redelivered message stay in queue head. This is important if you have long queue or if message order is important for you (note, that redeliveries will break strict messages order, see official docs for details or this question on SO).

P.S.:

There is similar answer in this topic, but in php. Look through it, maybe it helps you a bit (start reading it from words "There are multiple techniques to deal with cycle redeliver problem".