Understanding overlay networks in docker

Question

Please help me get this right,

An overlay network in swarm mode allows cross-node containers to be able see each other as on the same network.
Is that right?

So if I had the following setup:

swarm-mode cluster of two similar nodes M (manager) and W (worker)
two containers CM and CW running on nodes M and W respectively
CM depends_on: - CM:target
both containers are connected the an overlay network OVNET

Would I be able to ping target from CM? If not, why?

Demo: docker stack deploy -c test.yml test where test.yml is as follows:

version: "3"
services:
  CM:
    image: alpine
    command: sh -c 'ping CW'
    depends_on:
      - CW
    networks:
      - OVNET
    deploy:
      placement:
        constraints:
          - node.role == manager
  CW:
    image: alpine
    command: sh -c 'ping localhost'
    networks:
      - OVNET
    deploy:
      placement:
        constraints:
          - node.role == worker
networks:
  OVNET:
    driver: overlay

Thnaks in advance!

score 3 · Answer 1 · answered Apr 09 '17 at 15:14

3

Yes, it works as you would expect. The network is created first, then the containers are placed according to their constraints. I would expect that the first instance of CM might fail as CW would need time to pull the image but after that would work fine. Containers on the same overlay network can talk to each other via their service names over the in-built DNS.

answered Apr 09 '17 at 15:14

johnharris85

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That's what I thought as well. But running that example – Ayman Nedjmeddine Apr 09 '17 at 18:49
Running that example works fine for me. Does it not for you? – johnharris85 Apr 09 '17 at 19:00

Understanding overlay networks in docker

1 Answers1