In Docker Swarm how does the services route when there's an update

Question

If I do docker stack deploy I was wondering what the behaviour is as it's kind of hard to test on a single node system.

Say I have it set to order: start-first and say I have 10 replicas with parallelism: 2. What I want is something like this.

1. So I would have 10 tasks running V1

2. I deploy the manifest so that the service should start using V2

3. It starts a two new V2 replicas

4. VIP still routes to the V1 replicas while it's still starting up

5. The following then occur when the the 2 V2 replicas have started:

   a. Two of the V1 replicas start a shutdown

   b. Two more V2 replicas have start

6. Not sure about this one and I just want to confirm this the VIP now routes new requests to the V2 replicas and stops routing new requests to the V1 replicas

The information does not seem to be documented in update_config