service B doesn't actually know that - it happens at the level of the service mesh (you have to have a service mesh installed, right now just supporting istio but we plan on making that configurable)
we use trace headers to keep track of where the request originally came from to route it to the right database. it's transparent to service B as long as service B is properly propagating the request context from incoming request to outgoing requests
we use trace headers to keep track of where the request originally came from to route it to the right database. it's transparent to service B as long as service B is properly propagating the request context from incoming request to outgoing requests