docker-compose image named: “prefix_%s_1” instead of “%s”

Question

When I set up a couple of Docker containers in docker-compose.yaml file with links, the name of the containers ends up being of the format prefix_%s_1

Answer 1

That's just how docker-compose names containers so that it can manage them.

The basename is the name of the directory containing the docker-compose.yaml file. This is followed by the name of the container as specified in your docker-compose.yaml file, and finally that is followed by an instance number which increases if you bring up multiple instances of a container using something like docker-compose scale.

This naming scheme is how docker-compose is able to identify your containers when you attempt to operate on them using something like docker-compose stop.

I don't think this conflicts with the documentation in any way. That is, if I start with, say, this docker-compose.yaml in a directory named sotest:

irc:
  image: docker.io/xena/elemental-ircd
  links:
    - web

web:
  image: larsks/thttpd

And then bring up the compose:

$ docker-compose up

I get two containers:

CONTAINER ID        IMAGE                           ...NAMES
960c1491c03e        docker.io/xena/elemental-ircd   ...sotest_irc_1
422bba313e71        larsks/thttpd                   ...sotest_web_1

If I look at the /etc/hosts file inside of sotest_irc_1, I see:

172.17.0.28 web 422bba313e71 sotest_web_1

In addition to a number of other names. So the linked host is available by name as described in the docs.