Docker compose linking appears to not work

Question

I'm using Docker Compose to run an Elixir/Phoenix app in development. The setup is pretty standard, with a postgres container and a web container.

However, I'm having a hard time getting the web container to talk to the database container.

Here is my web container Dockerfile:

FROM ubuntu:14.04
MAINTAINER [email protected]

RUN locale-gen en_US.UTF-8  
ENV LANG en_US.UTF-8  
ENV LANGUAGE en_US:en  
ENV LC_ALL en_US.UTF-8 

ENV DEBIAN_FRONTEND noninteractive
RUN apt-get update
RUN apt-get install -y wget
RUN apt-get install -y curl
RUN apt-get install -y inotify-tools
RUN apt-get install -y postgresql-client
RUN wget https://packages.erlang-solutions.com/erlang-solutions_1.0_all.deb \
    && dpkg -i erlang-solutions_1.0_all.deb
RUN curl -sL https://deb.nodesource.com/setup_5.x | sudo -E bash -
RUN apt-get install -y nodejs
RUN apt-get update
RUN apt-get install -y esl-erlang 
RUN apt-get install -y elixir
RUN mix local.rebar
RUN mix local.hex --force

ADD . src/blog/
WORKDIR src/blog/

RUN mix deps.get
RUN mix deps.compile

Here's my docker-compose.yml:

db:
  image: postgres
web:
  build: .
  command: mix phoenix.server
  volumes:
    - .:/src/blog
  ports:
    - "4000:4000"
  links:
    - db

When I run docker-compose up, things appear to work okay. However, when I try to run (to create the database):

$ docker run blogphoenix_web mix ecto.create

I get the following error:

** (Mix) The database for Blog.Repo couldn't be created, reason given: psql: could not translate host name "db" to address: Name or service not known

Then, if I inspect the hosts file of the web container with:

$ docker run blogphoenix_web cat /etc/hosts

... I get this output:

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
172.17.0.4      a86e4f02ea56

Isn't Docker Compose supposed to create a hostname entry for the db container?

Here are some relevant version numbers for my Docker tooling:

$ docker-machine --version
#=> docker-machine version 0.6.0, build e27fb87
$ docker-compose --version
#=> docker-compose version 1.6.0, build unknown
$ docker --version
#=> Docker version 1.10.0, build 590d510

EDIT

Okay, I just noticed something that may help someone else reading this. This command, docker run blogphoenix_web cat /etc/hosts enters a new container, whereas this command docker exec 845f9d69cb1e cat /etc/hosts enters a running container. 845f9d69cb1e is the container ID for the running version of the blogphoenix_web image.

$ docker ps

CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS              PORTS                    NAMES
845f9d69cb1e        blogphoenix_web     "mix phoenix.server"     About an hour ago   Up 2 minutes        0.0.0.0:4000->4000/tcp   blogphoenix_web_1
21a6f48dfc3b        postgres            "/docker-entrypoint.s"   About an hour ago   Up 2 minutes        5432/tcp                 blogphoenix_db_1

Running the exec command I get the expected output from the hosts file, showing the appropriate hostname linkage for the db container:

$ docker exec 845f9d69cb1e cat /etc/hosts

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
172.17.0.2      db_1 21a6f48dfc3b blogphoenix_db_1
172.17.0.2      blogphoenix_db_1 21a6f48dfc3b
172.17.0.2      db 21a6f48dfc3b blogphoenix_db_1
172.17.0.3      845f9d69cb1e

In other words, when I ran the docker run blogphoenix_web mix ecto.create command, I was executing mix ecto.create in a new container based on the blogphoenix_web image. This new container wasn't started with docker-compose and thus did not have the appropriate host file linkage to the db container setup.

Try running "docker-compose run web mix ecto.create", and you should have linked containers working — TopperH
Wow, okay, that worked. Thank you. But, I thought that Docker Compose created a hosts entry for the various containers so they could talk to each other. If not, then where does this linkage exist? — Elliot Larson
The linkage exists between containers, not between images. I explained it by example in the answer below. — TopperH

TopperH TopperH · Accepted Answer · 2016-02-08T20:25:34

You need to run it using docker-compose:

docker-compose run web mix ecto.create

Docker compose creates linked containers, but the images themselves are not linked. This means that blogphoenix_web is not linked to blogphoenix_db, but when you will run

docker-compose up

the newly created containers "blogphoenix_web_1" and "blogphoenix_db_1" will be linked together.

Docker compose linking appears to not work

EDIT

1 Answers