docker-compose links between app and db

Question

i have a problem regrading docker-compose links. I am trying to links between a ruby rack app and mysql, and resolve the db host name.

docker-compose.yml:

version: '2'
services:
  db:
    image: library/mysql:5.6.22
    environment:
      MYSQL_ROOT_PASSWORD: xxx
      MYSQL_DATABASE: xxx
      MYSQL_USER: xxx
      MYSQL_PASSWORD: xxx
    ports:
      - "3306:3306"
    restart: always
  snow:
    build: .
    links:
      - db
    depends_on:
      - db
    ports:
      - "9292:9292"
    restart: always

Dockerfile of snow:

# Docker file for CentOS with RVM
FROM centos:centos7

MAINTAINER "XXXXX"

RUN useradd -ms /bin/bash t_user
RUN echo "t_user ALL=(ALL) NOPASSWD: ALL" >> /etc/sudoers

WORKDIR "/home/t_user"

RUN yum -y install curl which tar sudo wget git vim mysql-devel

USER t_user

## Install RVM + ruby 2.1.2
RUN gpg2 --keyserver hkp://keys.gnupg.net --recv-keys 409B6B1796C275462A1703113804BB82D39DC0E3
RUN curl -sSL https://get.rvm.io | bash -s stable

ENV PATH /usr/local/rvm/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

RUN /bin/bash -l -c "rvm requirements"
RUN /bin/bash -l -c "rvm install ruby-2.1.2"
RUN /bin/bash -l -c "gem install bundler --no-ri --no-rdoc"

# Copy snow
USER t_user

WORKDIR /tmp

ADD Gemfile Gemfile
ADD Gemfile.lock Gemfile.lock
RUN /bin/bash -l -c "bundle"

WORKDIR "/home/t_user"

RUN mkdir -p snow/current/ && mkdir -p snow/shared/config/
COPY . /home/t_user/snow/current/
RUN rm /home/t_user/snow/current/Dockerfile

USER root
RUN chown -R t_user.t_user snow/

USER trusteer

WORKDIR /home/t_user/snow/current/
RUN /bin/bash -l -c "rake db:migrate"
RUN chmod +x ./bin/runner
ENTRYPOINT ["./bin/snow"]

EXPOSE 9292

the error i get after i try to run docker-compose up -d is:

rake aborted!
Mysql2::Error: Unknown MySQL server host 'db' (2)
/home/trusteer/.rvm/gems/ruby-2.1.2/gems/mysql2-0.4.4/lib/mysql2/client.rb:87:in `connect'
/home/trusteer/.rvm/gems/ruby-2.1.2/gems/mysql2-0.4.4/lib/mysql2/client.rb:87:in `initialize'
/home/trusteer/.rvm/gems/ruby-2.1.2/gems/activerecord-4.2.6/lib/active_record/connection_adapters/mysql2_adapter.rb:18:in `new'
/home/trusteer/.rvm/gems/ruby-2.1.2/gems/activerecord-4.2.6/lib/active_record/connection_adapters/mysql2_adapter.rb:18:in `mysql2_connection'
/home/trusteer/.rvm/gems/ruby-2.1.2/gems/activerecord-4.2.6/lib/active_record/connection_adapters/abstract/connection_pool.rb:438:in `new_connection'
/home/trusteer/.rvm/gems/ruby-2.1.2/gems/activerecord-4.2.6/lib/active_record/connection_adapters/abstract/connection_pool.rb:448:in `checkout_new_connection'
/home/trusteer/.rvm/gems/ruby-2.1.2/gems/activerecord-4.2.6/lib/active_record/connection_adapters/abstract/connection_pool.rb:422:in `acquire_connection'
/home/trusteer/.rvm/gems/ruby-2.1.2/gems/activerecord-4.2.6/lib/active_record/connection_adapters/abstract/connection_pool.rb:349:in `block in checkout'
/home/trusteer/.rvm/gems/ruby-2.1.2/gems/activerecord-4.2.6/lib/active_record/connection_adapters/abstract/connection_pool.rb:348:in `checkout'
/home/trusteer/.rvm/gems/ruby-2.1.2/gems/activerecord-4.2.6/lib/active_record/connection_adapters/abstract/connection_pool.rb:263:in `block in connection'
/home/trusteer/.rvm/gems/ruby-2.1.2/gems/activerecord-4.2.6/lib/active_record/connection_adapters/abstract/connection_pool.rb:262:in `connection'
/home/trusteer/.rvm/gems/ruby-2.1.2/gems/activerecord-4.2.6/lib/active_record/connection_adapters/abstract/connection_pool.rb:571:in `retrieve_connection'
/home/trusteer/.rvm/gems/ruby-2.1.2/gems/activerecord-4.2.6/lib/active_record/connection_handling.rb:113:in `retrieve_connection'
/home/trusteer/.rvm/gems/ruby-2.1.2/gems/activerecord-4.2.6/lib/active_record/connection_handling.rb:87:in `connection'
/home/trusteer/.rvm/gems/ruby-2.1.2/gems/activerecord-4.2.6/lib/active_record/migration.rb:916:in `initialize'
/home/trusteer/.rvm/gems/ruby-2.1.2/gems/activerecord-4.2.6/lib/active_record/migration.rb:823:in `new'
/home/trusteer/.rvm/gems/ruby-2.1.2/gems/activerecord-4.2.6/lib/active_record/migration.rb:823:in `up'
/home/trusteer/.rvm/gems/ruby-2.1.2/gems/activerecord-4.2.6/lib/active_record/migration.rb:801:in `migrate'
/home/trusteer/snow/current/Rakefile:10:in `block (2 levels) in <top (required)>'
/home/trusteer/.rvm/gems/ruby-2.1.2/bin/ruby_executable_hooks:15:in `eval'
/home/trusteer/.rvm/gems/ruby-2.1.2/bin/ruby_executable_hooks:15:in `<main>'

docker info:

 Containers: 2
 Running: 1
 Paused: 0
 Stopped: 1
 Images: 72
 Server Version: 1.11.0
 Storage Driver: aufs
 Root Dir: /var/lib/docker/aufs
 Backing Filesystem: extfs
 Dirs: 110
 Dirperm1 Supported: false
 Logging Driver: json-file
 Cgroup Driver: cgroupfs
 Plugins: 
  Volume: local
  Network: bridge null host
 Kernel Version: 3.13.0-58-generic
 Operating System: Ubuntu 14.04.4 LTS
 OSType: linux
 Architecture: x86_64
 CPUs: 4
 Total Memory: 7.507 GiB
 Name: galt-tp
 ID: DKCR:ZB6N:PW4O:76PF:UZPU:HVZO:XHL4:SRPM:6ACQ:M6T5:AZXN:KVPQ
 Docker Root Dir: /var/lib/docker
 Debug mode (client): false
 Debug mode (server): false
 Registry: https://index.docker.io/v1/
 WARNING: No swap limit support

I've docker's documentation about links, and it says that the link directive add the DNS record of the relevant container, but it doesn't work for me.

any idea ?

Answer 1

In docker-compose v2 you don't need links. All containers will be created in one common network ( read more ). And with next configuration you can resolve services by their names:

version: '2'
services:
  db:
    image: library/mysql:5.6.22
    environment:
      MYSQL_ROOT_PASSWORD: xxx
      MYSQL_DATABASE: xxx
      MYSQL_USER: xxx
      MYSQL_PASSWORD: xxx
    ports:
      - "3306:3306"
    restart: always
  snow:
    build: .
    depends_on:
      - db
    ports:
      - "9292:9292"
    restart: always

For example you can ping db container from snow:

ping db

Updated :

You have problem with your snow dockerfile. Сoncretely with next line:

RUN /bin/bash -l -c "rake db:migrate"

You should understand that this command is executed on image build stage. And on this stage you have no service with db name. db service appears only after image snow was build. depends_on option doc says only about order of services start and nothing about build order.

As solution you can create bash script with this command and execute it on you dockerfile entrypoint.