A store where you do a query, get a result set, but also a callback (reactivity) in the future if that query would return a different result set if you were to

A store where you do a query, get a result set, but also a callback (reactivity) in the future if that query would return a different result set if you were to run it again. Imagine a "row count" HTML element which is automatically updated (reactive) when a different part of the web app inserts a row in the data store.