That's exactly what I'm advocating, it's not unnecessary, and should be trivial. If you are afraid of overhead, you can have a caching layer ala Redis or Memcac

That's exactly what I'm advocating, it's not unnecessary, and should be trivial. If you are afraid of overhead, you can have a caching layer ala Redis or Memcache or even local process memory (but don't eagerly optimize, the costs are trivial to make this call). Either way, you need to communicate with an updated cache or a user record of store. Don't trust information sent to you from the client because it may not be accurate with updates (there are exceptions to this when you can tolerate staleness, but for the general approach this is what you want to do).