If you have a replicated array, you can “ask the server” to update it with a “server method”, UFUNCTION(Server,…), the server queries the DB in a background thread (extending FRunnable for example), modifies the array and then the client can be notified of the elements of the array that have changed if you configure it using UPROPERTY(ReplicatedUsing=…). If the client needs to act on the world, you’ll probably need then to perform the updates in the Game thread (using TGraphTask<FMyUpdateTask>::CreateTask(…) for example).
It’s not simple and straight, but it can be done ;). We do similar things to notify the DB* of events that have to be persisted and/or query the DB* for information that we need (persistent data for the current user etc.)
HIH, cheers
D.
PD: Not exactly the DB but the backend server.