Identify different replicas in docker swarm

TL;DR

If the number of clients is much smaller than the number of replicas, consider using a look-aside server. Otherwise, there's no problem leaving the job to docker and just increase the replica count like specified above.

Long description

I ended up using a look-aside server for my specific use case. However, what I described on the post above, wasn't such a big issue after all. Let me explain:

Let's say that there's service A, which talks to service B via gRPC. The issue would only exist if the number of clients of A is smaller than the number of available B replicas. For example: 2 replicas of A and 4 of B. It this were the case, A1 would connect (and keep an open connection) to B1, A2 would connect to B2, leaving B3 and B4 on idle.

On the other hand, if the number of clients A is greater or equal than the number of available B replicas, the services would distribute the load according to the load balancing strategy efficiently. For example 4 A replicas, and 2 B replicas. In this case, B1 would have open connections with A1 and A2; while leaving the rest to B2.

An argument could be made about dropped connections leaving uneven distribution on B services, however, that greatly depends on the load balancing strategy.