Cache Invalidation Strategies: When Does It Get Tricky?

hey folks, i've been thinking a lot about caching lately, specifically around invalidation. we all know `ttl` is the simplest, but what happens when you need more granular control? like, when a single piece of data can be updated in multiple places or derived from several sources, keeping the cache consistent becomes a real headache. what are your go-to strategies or patterns when simple TTL isn't cutting it. any war stories or clever solutions out there