I can see that. One can argue that the “difficulty” is front loaded. If or not you can identify the list of “context” is local. I guess you can probably come up with complicated dependencies and argue it’s hard to capture the dependencies and I would agree with you.

Now getting back to the “what’s really hard about cache invalidation” part. Even with a much simpler model. Say you just have a k/v store, no joins, nothing. Is cache invalidation simple in that case?

I went into details about why it’s still insanely hard. And the big example at the end might help make that point.

And this is one level beneath challenges from tracking dependencies, and I argue that’s what makes cache invalidation hard.

Now going back to your example with complicated dependencies. Maybe TTL is a better solution. With many dependencies, any changes from the dependency list might trigger invalidation. At some point, just doing TTL, would be simpler.