Precise scheduling with RabbitMQ

The things we'll do to avoid dependencies.

On a project recently, we needed to be able to process jobs asynchronously, but we also needed to be able to specify that they should be run at a certain point in the future. We also needed to be able to implement exponential backoff on failure. We initially tried to integrate Sidekiq, but unfortunately it turned out to not be a good fit for the way we structured the code base.

Now, RabbitMQ is very commonly used as a work queue. Sadly for us, because AMQP was originally designed as a protocol for integrating various systems, scheduling wasn’t baked into the core protocol. Thankfully, it’s extensible enough that this turns out not to be a problem.

One suggestion that often gets suggested is to use a dead letter exchange. In this case, it’s not possible to specify per message when we want the job to be processed. It may well be able to implement this using a series of cascading work / retry queues, but I can see the potential for it becoming quite complex quite quickly.

So, alternatively, we decided to make use of the fact that when you use message acknowledgements, the client can delay sending an acknowledgement to a message, as long as you can still send the response over the same channel (AMQP sub-divides a TCP connection into channels) the message was delivered on. This usually means that you can hold a claim on a job for as long as the TCP connection to the broker stays up.

So, the very simplest approach one can take here is to add a message header when sending the message that states when we want the job to be processed, and look for that on the receiving side. If we take a very simple example, where we publish via the default exchange to a queue named in the variable a_queue_name:

And in our worker, we can pull it out like so:

However, this is of course, only half of the story. We know when we want to process the job, but we don’t yet have a method of delaying it. A degenerate approach would be call Kernel#sleep in the subscribe block until the received item is due, and then process it, but you would then quite clearly experience Head of line blocking, so a job to be processed very soon, but that arrives after a job that is to be processed far in the future, would be delayed until the first job had been processed.

So, to make this work reasonably, we need to use a priority queue that supports access from multiple threads, i.e. the thread in which the Queue#subscribe block runs in, and the thread that actually does the work.

We ended up writing our own variant of ruby’s Queue class that wraps a priority queue for safe concurrent access, as well as blocking any consumer until a job is due.

So, you end up with a consumer implementation that looks roughly like this:

Now, as it stands, we don’t yet support backoff for failed jobs. That will require a few extra moving parts, which we’ll introduce another time.

Edit: Thanks to a twitter exchange with Sean T Allen I was reminded that this really is only suitable for systems with low throughput. In our case, we have very few jobs outstanding at any one time (the most we’ve seen is six), and so performance isn’t a huge problem for us. In different circumstances we’d have maybe implemented a database-backed service which fed jobs into RabbitMQ when they become due, but in this case, would been too costly in terms of developer time.