Get started with Docker Compose

Estimated reading time: 6 minutes

On this page you build a simple Python web application running on Docker Compose. The application uses the Flask framework and maintains a hit counter in Redis. While the sample uses Python, the concepts demonstrated here should be understandable even if you’re not familiar with it.

Prerequisites

Make sure you have already installed both Docker Engine and Docker Compose. You don’t need to install Python or Redis, as both are provided by Docker images.

Step 1: Setup

  1. Create a directory for the project:

    $ mkdir composetest
    $ cd composetest
    
  2. Create a file called app.py in your project directory and paste this in:

    from flask import Flask
    from redis import Redis
    
    app = Flask(__name__)
    redis = Redis(host='redis', port=6379)
    
    @app.route('/')
    def hello():
        count = redis.incr('hits')
        return 'Hello World! I have been seen {} times.\n'.format(count)
    
    if __name__ == "__main__":
        app.run(host="0.0.0.0", debug=True)
    
  3. Create another file called requirements.txt in your project directory and paste this in:

    flask
    redis
    

These define the application’s dependencies.

Step 2: Create a Dockerfile

In this step, you write a Dockerfile that builds a Docker image. The image contains all the dependencies the Python application requires, including Python itself.

In your project directory, create a file named Dockerfile and paste the following:

FROM python:3.4-alpine
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 3.4 image.
  • Add the current directory . into the path /code in the image.
  • Set the working directory to /code.
  • Install the 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.

Step 3: Define services in a Compose file

Create a file called docker-compose.yml in your project directory and paste the following:

version: '2'
services:
  web:
    build: .
    ports:
     - "5000:5000"
    volumes:
     - .:/code
  redis:
    image: "redis:alpine"

This Compose file defines two services, web and redis. The web service:

  • Uses an image that’s built from the Dockerfile in the current directory.
  • Forwards the exposed port 5000 on the container to port 5000 on the host machine.
  • Mounts the project directory on the host to /code inside the container, allowing you to modify the code without having to rebuild the image.

The redis service uses a public Redis image pulled from the Docker Hub registry.

Tip: If your project is outside of the Users directory (cd ~), then you need to share the drive or location of the Dockerfile and volume you are using. If you get runtime errors indicating an application file is not found, a volume mount is denied, or a service cannot start, try enabling file or drive sharing. Volume mounting requires shared drives for projects that live outside of C:\Users (Windows) or /Users (Mac), and is required for any project on Docker for Windows that uses Linux containers. For more information, see Shared Drives on Docker for Windows, File sharing on Docker for Mac, and the general examples on how to Manage data in containers.

Step 4: Build and run your app with Compose

  1. From your project directory, start up your application.

     $ 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
    

    Compose pulls a Redis image, builds an image for your code, and start the services you defined.

  2. Enter http://0.0.0.0:5000/ in a browser to see the application running.

    If you’re using Docker on Linux natively, 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 http://localhost:5000.

    If you’re using Docker Machine on a Mac, use docker-machine ip MACHINE_VM to get the IP address of your Docker host. Then, open http://MACHINE_VM_IP:5000 in a browser.

    You should see a message in your browser saying:

    Hello World! I have been seen 1 times.

  3. Refresh the page.

    The number should increment.

Tip: You can list local images with docker image ls and inspect them with docker inspect <tag or id>. Listing images at this point should return redis and web.

Step 5: Update the application

Because the application code is mounted into the container using a volume, you can make changes to its code and see the changes instantly, without having to rebuild the image.

  1. Change the greeting in app.py and save it. For example:

    return 'Hello from Docker! I have been seen {} times.\n'.format(count)
    
  2. Refresh the app in your browser. The greeting should be updated, and the counter should still be incrementing.

Step 6: Experiment with some other commands

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

You can bring everything down, removing the containers entirely, with the down command. Pass --volumes to also remove the data volume used by the Redis container:

$ docker-compose down --volumes

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

Where to go next

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