> Suppose you have source-of-truth A (doesn't really matter if it's a key value store or whatever, it could be a function for all intents) and a few clients B1, B2, B3, ... that rely on the data from A. You have to keep them in sync. When should B* check if A has changed? Every time they need it? Every minute? Every hour? Every day? That is the cache invalidation problem.

A few things to clarify here what you are referring to as clients are cache hosts (as they keep data). You seem to imply that the cache is running on the client? I was referring to cache servers (think memcache, Redis, etc.), for which the membership can be determined. So on update (e.g. when you mutate A, you know all the Bs to invalidate).

Now continuing with your example, with cache running on the client. Assuming we are talking about same concept when we say "client", the membership is non deterministic. Clients can come and go (connect and disconnect as they wish). There are some attempts to do invalidation-based cache on clients, but they are hard because of the reason I just mentioned. So usually client cache is TTL'ed. E.g. the very browser you are using to see this comment has a lot of things cached. DNS is not going to send an invalidate event to your browser. It't TTL based.

I guess what I am saying is that cache invalidation rarely applies to cache side cache as far as I know. Maybe you have a different example, which we can discuss.