In Django code, looking at a class for caching REST calls. The cache is using Redis via Django's cache layer. In order to store different sets of parameters, each endpoint gets a "master" cache, that lists the other Redis keys, so they can be deleted when evicting the cache. Something isn't right, though. The cache has steadily increased in size and slowed down since 2014 even though many events clear the whole thing!

... And then it hit me. Nothing empties the list of cache keys. Nothing. So it has been growing endlessly since 2014. And everytime it grows, cache eviction gets a little more expensive, network traffic increases a little more, and cache evictions get a little slower.

Fixing this bug took things that were taking routinely an entire minute to complete and made them take a couple seconds.