Question: How does port pairing work?

[tnolet]

Was looking around how to map multiple exposed ports to marathon/mesos ports using deimos. I was surprised by the following:

Starting a container that exposes ports 8080 and 8081, I (naively) assigned two ports in my json file:

{
    "container": {
    "image": "docker:///tnolet/hello:1.0",
    "options" : ["-P"]
  },
  "id": "hello2",
  "instances": "1",
  "cpus": ".5",
  "mem": "512",
  "uris": [],
  "cmd": "",
  "ports" : [40195,40196]
}

However, this resulted in deimos picking and auto incrementing two ports, 31267 and 31268:

Jul 31 17:17:17 ec2-54-84-57-179 deimos[5707]: deimos.docker.run() Port pairings (Mesos, Docker) // [(31627, 8080), (31628, 8081)]
Jul 31 17:17:17 ec2-54-84-57-179 deimos[5707]: deimos.containerizer.docker.launch() call // docker run --sig-proxy --rm --cidfile /tmp/deimos/mesos/54f421fd-0935-4f34-8be9-33a17862024e/cid -w /tmp/mesos-sandbox -v /tmp/deimos/mesos/54f421fd-0935-4f34-8be9-33a17862024e/fs:/tmp/mesos-sandbox -P -p 31627:8080 -p 31628:8081 -c 512 -m 512m -e PORT=31627 -e PORT0=31627 -e PORTS=31627,31628 -e PORT1=31628 tnolet/hello:1.0

If I leave the "port" key out of my json file, deimos only maps the lowest numbered port.
I love the auto incremented ports, but I'm not sure how this is triggered.

[solidsnack]

These auto-incremented ports are actually provided by Mesos, at Marathon's request. Deimos just maps the ports it sees in the TaskInfo -- it doesn't have any port management logic. That is why the port pairings are listed as (Mesos, Docker).

Jul 31 17:17:17 ec2-54-84-57-179 deimos[5707]: deimos.docker.run() Port pairings (Mesos, Docker) // [(31627, 8080), (31628, 8081)]

The idea is to access the endpoints resource on Marathon and then construct a proxy configuration (using HAProxy, for example) to map between the ports allocated for the container and the "service ports" requested in the Marathon task JSON.