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Overview of Docker Compose

Compose is a tool for defining and running multi-container applications with Docker. With Compose, you define a multi-container application in a single file, then spin your application up in a single command which does everything that needs to be done to get it running.

Compose is great for development environments, staging servers, and CI. We don't recommend that you use it in production yet.

Using Compose is basically a three-step process.

  1. Define your app's environment with a Dockerfile so it can be reproduced anywhere.
  2. Define the services that make up your app in docker-compose.yml so they can be run together in an isolated environment:
  3. Lastly, run docker-compose up and Compose will start and run your entire app.

A docker-compose.yml looks like this:

web:
  build: .
  ports:
   - "5000:5000"
  volumes:
   - .:/code
  links:
   - redis
redis:
  image: redis

Compose has commands for managing the whole lifecycle of your application:

  • Start, stop and rebuild services
  • View the status of running services
  • Stream the log output of running services
  • Run a one-off command on a service

Compose documentation

Quick start

Let's get started with a walkthrough of getting a simple Python web app running on Compose. It assumes a little knowledge of Python, but the concepts demonstrated here should be understandable even if you're not familiar with Python.

Installation and set-up

First, install Docker and Compose.

Next, you'll want to make a directory for the project:

$ mkdir composetest
$ cd composetest

Inside this directory, create app.py, a simple web app that uses the Flask framework and increments a value in Redis. Don't worry if you don't have Redis installed, docker is going to take care of that for you when we define services:

from flask import Flask
from redis import Redis

app = Flask(__name__)
redis = Redis(host='redis', port=6379)

@app.route('/')
def hello():
    redis.incr('hits')
    return 'Hello World! I have been seen %s times.' % redis.get('hits')

if __name__ == "__main__":
    app.run(host="0.0.0.0", debug=True)

Next, define the Python dependencies in a file called requirements.txt:

flask
redis

Create a Docker image

Now, create a Docker image containing all of your app's dependencies. You specify how to build the image using a file called Dockerfile:

FROM python:2.7
ADD . /code
WORKDIR /code
RUN pip install -r requirements.txt
CMD python app.py

This tells Docker to:

  • Build an image starting with the Python 2.7 image.
  • Add the current directory . into the path /code in the image.
  • Set the working directory to /code.
  • Install your Python dependencies.
  • Set the default command for the container to python app.py

For more information on how to write Dockerfiles, see the Docker user guide and the Dockerfile reference.

You can test that this builds by running docker build -t web ..

Define services

Next, define a set of services using docker-compose.yml:

web:
  build: .
  ports:
   - "5000:5000"
  volumes:
   - .:/code
  links:
   - redis
redis:
  image: redis

This defines two services:

web

  • Builds from the Dockerfile in the current directory.
  • Forwards the exposed port 5000 on the container to port 5000 on the host machine.
  • Connects the web container to the Redis service via a link.
  • Mounts the current directory on the host to /code inside the container allowing you to modify the code without having to rebuild the image.

redis

  • Uses the public Redis image which gets pulled from the Docker Hub registry.

Build and run your app with Compose

Now, when you run docker-compose up, Compose will pull a Redis image, build an image for your code, and start everything up:

$ docker-compose up
Pulling image redis...
Building web...
Starting composetest_redis_1...
Starting composetest_web_1...
redis_1 | [8] 02 Jan 18:43:35.576 # Server started, Redis version 2.8.3
web_1   |  * Running on http://0.0.0.0:5000/
web_1   |  * Restarting with stat

If you're using Docker Machine, then docker-machine ip MACHINE_VM will tell you its address and you can open http://MACHINE_VM_IP:5000 in a browser.

If you're not using Boot2docker and are on linux, then the web app should now be listening on port 5000 on your Docker daemon host. If http://0.0.0.0:5000 doesn't resolve, you can also try localhost:5000.

You should get a message in your browser saying:

Hello World! I have been seen 1 times.

Refreshing the page will increment the number.

If you want to run your services in the background, you can pass the -d flag (for "detached" mode) to docker-compose up and use docker-compose ps to see what is currently running:

$ docker-compose up -d
Starting composetest_redis_1...
Starting composetest_web_1...
$ docker-compose ps
    Name                 Command            State       Ports
-------------------------------------------------------------------
composetest_redis_1   /usr/local/bin/run         Up
composetest_web_1     /bin/sh -c python app.py   Up      5000->5000/tcp

The docker-compose run command allows you to run one-off commands for your services. For example, to see what environment variables are available to the web service:

$ docker-compose run web env

See docker-compose --help to see other available commands. You can also install command completion for the bash and zsh shell, which will also show you available commands.

If you started Compose with docker-compose up -d, you'll probably want to stop your services once you've finished with them:

$ docker-compose stop

At this point, you have seen the basics of how Compose works.

Release Notes

To see a detailed list of changes for past and current releases of Docker Compose, please refer to the CHANGELOG.

Getting help

Docker Compose is still in its infancy and under active development. If you need help, would like to contribute, or simply want to talk about the project with like-minded individuals, we have a number of open channels for communication.

  • To report bugs or file feature requests: please use the issue tracker on Github.

  • To talk about the project with people in real time: please join the #docker-compose channel on IRC.

  • To contribute code or documentation changes: please submit a pull request on Github.

For more information and resources, please visit the Getting Help project page.