docker-compose

Plum

WARNING: This is a work in progress and probably won't work yet. Feedback welcome.

Plum is tool for defining and running apps with Docker. It uses a simple, version-controllable YAML configuration file that looks something like this:

db:
  image: orchardup/postgresql

web:
  build: web/
  link: db

Installing

$ sudo pip install plum

Defining your app

Put a plum.yml in your app's directory. Each top-level key defines a "service", such as a web app, database or cache. For each service, Plum will start a Docker container, so at minimum it needs to know what image to use.

The way to get started is to just give it an image name:

db:
  image: orchardup/postgresql

Alternatively, you can give it the location of a directory with a Dockerfile (or a Git URL, as per the docker build command), and it'll automatically build the image for you:

db:
  build: /path/to/postgresql/build/directory

You've now given Plum the minimal amount of configuration it needs to run:

$ plum start
Building db... done
db is running at 127.0.0.1:45678
<...output from postgresql server...>

For each service you've defined, Plum will start a Docker container with the specified image, building or pulling it if necessary. You now have a PostgreSQL server running at 127.0.0.1:45678.

By default, plum start will run until each container has shut down, and relay their output to the terminal. To run in the background instead, pass the -d flag:

$ plum start -d
Building db... done
db is running at 127.0.0.1:45678

$ plum ps
SERVICE  STATE  PORT
db       up     45678

Getting your code in

Some services may include your own code. To get that code into the container, ADD it in a Dockerfile.

plum.yml:

web:
  build: web/

web/Dockerfile:

FROM orchardup/rails
ADD . /code
CMD bundle exec rackup

Communicating between containers

Your web app will probably need to talk to your database. You can use [Docker links] to enable containers to communicate, pass in the right IP address and port via environment variables:

db:
  image: orchardup/postgresql

web:
  build: web/
  link: db

This will pass an environment variable called DB_PORT into the web container, whose value will look like tcp://172.17.0.4:45678. Your web app's code can then use that to connect to the database.

You can pass in multiple links, too:

link:
 - db
 - memcached
 - redis

In each case, the resulting environment variable will begin with the uppercased name of the linked service (DB_PORT, MEMCACHED_PORT, REDIS_PORT).

Container configuration options

You can pass extra configuration options to a container, much like with docker run:

web:
  build: web/

  -- override the default run command
  run: bundle exec thin -p 3000

  -- expose ports - can also be an array
  ports: 3000

  -- map volumes - can also be an array
  volumes: /tmp/cache

  -- add environment variables - can also be a dictionary
  environment:
   RACK_ENV: development

Running a one-off command

If you want to run a management command, use plum run to start a one-off container:

$ plum run db createdb myapp_development
$ plum run web rake db:migrate
$ plum run web bash

Running more than one container for a service

You can set the number of containers to run for each service with plum scale:

$ plum start -d
db is running at 127.0.0.1:45678
web is running at 127.0.0.1:45679

$ plum scale db=0,web=3
Stopped db (127.0.0.1:45678)
Started web (127.0.0.1:45680)
Started web (127.0.0.1:45681)