Getting Started

On this page you build a simple Python web application running on Docker Compose. The application uses the Flask framework and increments a value 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, it is provided by a Docker image.

Step 1: Setup

  1. Create a directory for the project:

    $ mkdir composetest
    $ cd composetest
    
  2. With your favorite text editor create a file called app.py in your project directory.

    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)
    
  3. Create another file called requirements.txt in your project directory and add the following:

    flask
    redis
    

These define the applications dependencies.

Step 2: Create a Docker image

In this step, you build a new Docker image. The image contains all the dependencies the Python application requires, including Python itself.

  1. In your project directory create a file named Dockerfile and add the following:

    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 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.

  1. Build the image.

    $ docker build -t web .
    

This command builds an image named web from the contents of the current directory. The command automatically locates the Dockerfile, app.py, and requirements.txt files.

Step 3: Define services

Define a set of services using docker-compose.yml:

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

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

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

  • Builds 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.
  • Links the web service to the Redis service.

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

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.

  1. 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.

  1. Refresh the page.

The number should increment.

Step 5: 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

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

Where to go next