[necovek]

I wasn't going to say any of that ;) Yes, Postgres does have WAL log you can subscribe to, but you are creating a replicated slave really. There are many nuances to doing that right and you are downplaying how hard that is (it's not about API, it's about ACID).

More importantly, you are pushing all the complex logic to this imaginary DB replication client: your "caching system" is still nothing more than a KV store.

[EGreg]

Well what are the “real” caching systems, if my system can be used for caching but isnt “real”? This feels like a no true scotsman argument.

[necovek]

Caching system would be what you described as a response to my example case: your key-value store combined together with the entire mechanism to watch for DB writes (eg with triggers or the replication log) and tie them back to cached values which need to have their lifetime managed.