CODEWITHSUNDEEP
dockerdocker-compose
Sergey Melekhin
Sergey Melekhin

Reputation: 109

Host names are not set in docker compose

I created simple compose config to try Postgres BDR replication. I expect containers to have host names as service names I defined and I expect one container to be able to resolve and reach another with this hostname. I expect it to be true because of that: https://docs.docker.com/compose/networking/

My config:

version: '2'

services:
  bdr1:
    image: bdr
    volumes: 
      - /var/lib/postgresql/data1:/var/lib/postgresql/data
    ports:
      - "5001:5432"
  bdr2:
    image: bdr
    volumes: 
      - /var/lib/postgresql/data2:/var/lib/postgresql/data    
    ports:  
      - "5002:5432"

But in reality both containers get rubbish hostnames and are not reachable by container names:

Creating network "bdr_default" with the default driver
Creating bdr_bdr1_1
Creating bdr_bdr2_1
Attaching to bdr_bdr1_1, bdr_bdr2_1
bdr1_1 | Hostname: 938e0585fee2
bdr2_1 | Hostname: 7153165f4d5b

Is it a bug, or I did something wrong? I use Ubuntu 14.04.4 LTS, Docker version 1.10.1, build 9e83765, docker-compose version 1.6.0, build d99cad6

Upvotes: 2

Views: 3245

Answers (2)

dnephin
dnephin

Reputation: 28060

The hostname inside the container should be the short container id, so this is correct (note there was a bug with Compose 1.6.0 and the short container id, so you should use at least version 1.6.2). Also /etc/hosts is not longer used, there is now an embedded dns server that handles resolving names to container ip addresses.

The container is discoverable by other containers with 3 names: the container name, the container short id, and the service name.

However, the other container may not be available immediately when the first one starts. You can use depends_on to set the order.

If you are testing the discovery, try using ping, and make sure to retry , because the name may not resolve immediately.

Upvotes: 0

Heikki Toivonen
Heikki Toivonen

Reputation: 31130

docker-compose gives you the option of scaling services up or down, meaning you can launch multiple instances of the same service. That is at least one reason why the hostnames are not just service names. You will notice that if you scale bdr1 to 2 instance, you will then have bdr_bdr1_1 and bdr_bdr1_2 containers.

You can work around this inside the containers that were started up by docker-compose in at least two ways:

  • If a service refers to other service, you can use links section, for example make bdr1 link to bdr2. In this case when you are inside bdr1 you can call host bdr2 by name. I have not tried what happens when you scale up bdr2 in this case.
  • You can force the hostname of a container internally to the name you want by using the hostname section. For example if you add hostname: bdr1 to bdr1, then you can internally connect to bdr1, which is itself.

You can possibly achieve similar result with the networks section, but I have not yet used it myself so I don't know for sure.

Upvotes: 2

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