A way make cache service to update subscribers

[motorro]

Consider a cache service with complex internal structure that is updated by some internal logic.
For example a database that saves entities and something that updates records' internal structure.
In case of Room you may observe a query and get updates if something changes underneath. But sometimes you have a complex entity with relations that are not so easy to fetch as they need conditional processing in synchronous way.
In this case you may write an SQL delegate for sync-delegate service to implement reactive cache with updates.
When you get/put the whole entity the solution works. But as soon as you start to update entity parts you need some way to notify subscribers of data change.