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

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 instead of %s, and the alias in /etc/hosts on the linking container is the same.

Why is the alias of the redis container test_redis_1 instead of redis?

Here are the relevant files and output:

# docker-compose.yaml
monkey:
    build: ../../monkey
    dockerfile: test.Dockerfile
    links:
      - redis:redis
    ports:
      - "9006:9006"

redis:
    build: ../storage/redis
    ports:
      - "6379:6379"

After running docker-compose build && docker-compose up:

$ docker-compose ps
Name           Command                        State  Ports
---------------------------------------------------------------------------
test_redis_1   redis-server /usr/local/et ...   Up   0.0.0.0:6379->6379/tcp
test_monkey_1  python -m SimpleHTTPServer ...   Up   0.0.0.0:9006->9006/tcp

Output from the test_monkey_1 container:

Step 14 : RUN cat /etc/hosts
 ---> Running in 1c104e3d9bf5
172.17.1.26     75a485df1325
127.0.0.1       localhost
::1     localhost ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
172.17.1.26     thirsty_lumiere.bridge
172.17.0.220    test_redis_1
172.17.0.220    test_redis_1.bridge
172.17.1.26     thirsty_lumiere
 ---> 3a06bac9b3ca
Removing intermediate container 1c104e3d9bf5
Step 15 : RUN echo
 ---> Running in a1c5b5f8ae0f

 ---> 385c9ee44332
Removing intermediate container a1c5b5f8ae0f
Step 16 : RUN ifconfig eth0 | grep inet addr
 ---> Running in 17cd638c6473
          inet addr:172.17.1.28  Bcast:0.0.0.0  Mask:255.255.0.0
 ---> 21235e29abc1
Removing intermediate container 17cd638c6473
Step 17 : RUN echo
 ---> Running in 8c0b1db2a69b

 ---> e2dd190eb4d1
Removing intermediate container 8c0b1db2a69b
Step 18 : RUN env
 ---> Running in 50cd1b6bf9da
HOSTNAME=75a485df1325
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
PWD=/data/web/tunnelbear.com/play/tPWD=/data/web/tunnelbear.com/root
_=/usr/bin/env

According to docker compose issue #745:

> By default, Compose bases the project name on basename of the > directory compose commands are run from. The project name can be > overridden either by passing a -p / --project-name option for each > command or setting the COMPOSE_PROJECT_NAME environment variable.

You can set the prefix of your choice to your container names by writing something like this:

$ docker-compose -p MY_PROJECT_NAME

which is the equivalent to (the more readable):

$ docker-compose --project-name MY_PROJECT_NAME

You could just set container name to what you want via container_name:

redis:
    build: ../storage/redis
    container_name: redis
    ports:
      - "6379:6379"

And container hostname could also be set via hostname.

Docker compose will use the name of the directory with your compose file as the name of the project unless you set if via the -p option

-p, --project-name NAME     Specify an alternate project name (default: directory name)

Compose supports declaring default environment variables in an environment file named .env placed in the folder where the docker-compose command is executed (current working directory).

Where you'll want to set COMPOSE_PROJECT_NAME

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.