How To Prevent Cache Stampedes

Think cache invalidation and naming things are hard? Cute.

One of the biggest issues about ledgers, and money software in general, is performance. Numbers have to add up with total precision, and that often requires the kind of complex logic that makes operations slow down.

So you build a cache right?

In fact the CQRS pattern we talked about a couple of weeks ago is just that, to some degree: the write team continually builds materialized views of the state, which is just a fancy way of saying “pre-calculated results”. A cache.

Performant ledgers always involve some form of cache.

But here’s the problem: not everyone understands that caching has a subtle form of race condition built in. And, if you’re not careful, you might think you’re releasing your database from unnecessary read load, when in reality you’re not.

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In today’s article, I’m going to describe a common issue with cache layers called cache stampede. This article covers:

• What cache stampede is and why it happens

• Working code you can run on your laptop to reproduce it

• Common approaches to avoid them

Let’s jump in.