Most people know that “cache invalidation” is one of two hard things in computer science (the other one is “naming”) and a strong cache invalidation strategy is as important as your caching strategy. This is because you need to be able to free memory from your storage to make room for new cached content (purging) or you may ban content that should not be cached any more because it is outdated or because it was cached in error and never should have been in cache at all (mistakes happen sometimes).