How to do error handling with EasyNetQ / RabbitMQ

Question

I'm using RabbitMQ in C# with the EasyNetQ library. I'm using a pub/sub pattern here. I still have a few issues that I hope anyone can help me with:

1. When there's an error while consuming a message, it's automatically moved to an error queue. How can I implement retries (so that it's placed back on the originating queue, and when it fails to process X times, it's moved to a dead letter queue)?
2. As far as I can see there's always 1 error queue that's used to dump messages from all other queues. How can I have 1 error queue per type, so that each queue has its own associated error queue?
3. How can I easily retry messages that are in an error queue? I tried Hosepipe, but it justs republishes the messages to the error queue instead of the originating queue. I don't really like this option either because I don't want to be fiddling around in a console. Preferably I'd just program against the error queue.

Anyone?

Answer 1

The problem you are running into with EasyNetQ/RabbitMQ is that it's much more "raw" when compared to other messaging services like SQS or Azure Service Bus/Queues, but I'll do my best to point you in the right direction.

Question 1.

This will be on you to do. The simplest way is that you can No-Ack a message in RabbitMQ/EasyNetQ, and it will be placed at the head of the queue for you to retry. This is not really advisable because it will be retried almost immediately (With no time delay), and will also block other messages from being processed (If you have a single subscriber with a prefetch count of 1).

I've seen other implementations of using a "MessageEnvelope". So a wrapper class that when a message fails, you increment a retry variable on the MessageEnvelope and redeliver the message back onto the queue. YOU would have to do this and write the wrapping code around your message handlers, it would not be a function of EasyNetQ.

Using the above, I've also seen people use envelopes, but allow the message to be dead lettered. Once it's on the dead letter queue, there is another application/worker reading items from the dead letter queue.

All of these approaches above have a small issue in that there isn't really any nice way to have a logarithmic/exponential/any sort of increasing delay in processing the message. You can "hold" the message in code for some time before returning it to the queue, but it's not a nice way around.

Out of all of these options, your own custom application reading the dead letter queue and deciding whether to reroute the message based on an envelope that contains the retry count is probably the best way.

Question 2.

You can specify a dead letter exchange per queue using the advanced API. (https://github.com/EasyNetQ/EasyNetQ/wiki/The-Advanced-API#declaring-queues). However this means you will have to use the advanced API pretty much everywhere as using the simple IBus implementation of subscribe/publish looks for queues that are named based on both the message type and subscriber name. Using a custom declare of queue means you are going to be handling the naming of your queues yourself, which means when you subscribe, you will need to know the name of what you want etc. No more auto subscribing for you!

Question 3

An Error Queue/Dead Letter Queue is just another queue. You can listen to this queue and do what you need to do with it. But there is not really any out of the box solution that sounds like it would fit your needs.