Long gone are the days when systems needed to do only one thing at a time.

Concurrency is the word but it often leads to complex code, dealing with locks, mutexes etc…

There are several different abstractions which allows us to both model and think about asynchronous code in a more sane fashion: futures, promises and events/callbacks are but a few of them.

I won’t get into the merits - or lack thereof - of these alternatives in this post but rather focus on a different alternative: Communicating Sequential Processes - CSP.

So I had just finished writing this feature. I was confident it’d work - after all I had a good level of tests around it. It sounded like yet another successful deployment.

One week in and something starts breaking. It was hard to track down but at the end I realised it was caused by having concurrent processes running in parallel.

That’s what happened in a recent production release at our current client. It was a very interesting problem to track down and solve and I’ll do my best to explain and walk you through it here.