IIRC, if you use docker swarm it wil handle the routing for you. So if an app is running on port 8080 on two swarm nodes, if you access port 8080 on each node, you might actually be accessing another node.

Since docker swarm knows if an instance is down, it will know to not use that instance.

See https://docs.docker.com/engine/swarm/ingress/

I run on Docker Swarm and there are hiccups as one container goes up and another goes down during deployment.

I think you can try a tool like Envoy for doing routing between versions plus some other custom stuff.

At my job, the devops team had to write some kind of special program that maps whether a Marathon/Mesos instance is up when dealing with Envoy. Not sure if the same is required for just plain Docker?