Try Docker Compose

This tutorial is designed to introduce the key concepts of Docker Compose whilst building a simple Python web application. The application uses the Flask framework and maintains a hit counter in Redis.

The concepts demonstrated here should be understandable even if you're not familiar with Python.


You need to have Docker Engine and Docker Compose on your machine. You can either:

You don't need to install Python or Redis, as both are provided by Docker images.

Step 1: Define the application dependencies

  1. Create a directory for the project:

    $ mkdir composetest
    $ cd composetest
  2. Create a file called in your project directory and paste the following code in:

    import time
    import redis
    from flask import Flask
    app = Flask(__name__)
    cache = redis.Redis(host='redis', port=6379)
    def get_hit_count():
        retries = 5
        while True:
                return cache.incr('hits')
            except redis.exceptions.ConnectionError as exc:
                if retries == 0:
                    raise exc
                retries -= 1
    def hello():
        count = get_hit_count()
        return 'Hello World! I have been seen {} times.\n'.format(count)

    In this example, redis is the hostname of the redis container on the application's network. We use the default port for Redis, 6379.

    Handling transient errors

    Note the way the get_hit_count function is written. This basic retry loop lets us attempt our request multiple times if the redis service is not available. This is useful at startup while the application comes online, but also makes the application more resilient if the Redis service needs to be restarted anytime during the app's lifetime. In a cluster, this also helps handling momentary connection drops between nodes.

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


Step 2: Create a Dockerfile

The Dockerfile is used to build 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 code in:

# syntax=docker/dockerfile:1
FROM python:3.10-alpine
RUN apk add --no-cache gcc musl-dev linux-headers
COPY requirements.txt requirements.txt
RUN pip install -r requirements.txt
COPY . .
CMD ["flask", "run"]

This tells Docker to:

  • Build an image starting with the Python 3.10 image.
  • Set the working directory to /code.
  • Set environment variables used by the flask command.
  • Install gcc and other dependencies
  • Copy requirements.txt and install the Python dependencies.
  • Add metadata to the image to describe that the container is listening on port 5000
  • Copy the current directory . in the project to the workdir . in the image.
  • Set the default command for the container to flask run.


Check that the Dockerfile has no file extension like .txt. Some editors may append this file extension automatically which results in an error when you run the application.

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 compose.yaml in your project directory and paste the following:

    build: .
      - "8000:5000"
    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. It then binds the container and the host machine to the exposed port, 8000. This example service uses the default port for the Flask web server, 5000.

The redis service uses a 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 by running docker compose up.

    $ docker compose up
    Creating network "composetest_default" with the default driver
    Creating composetest_web_1 ...
    Creating composetest_redis_1 ...
    Creating composetest_web_1
    Creating composetest_redis_1 ... done
    Attaching to composetest_web_1, composetest_redis_1
    web_1    |  * Running on (Press CTRL+C to quit)
    redis_1  | 1:C 17 Aug 22:11:10.480 # oO0OoO0OoO0Oo Redis is starting oO0OoO0OoO0Oo
    redis_1  | 1:C 17 Aug 22:11:10.480 # Redis version=4.0.1, bits=64, commit=00000000, modified=0, pid=1, just started
    redis_1  | 1:C 17 Aug 22:11:10.480 # Warning: no config file specified, using the default config. In order to specify a config file use redis-server /path/to/redis.conf
    web_1    |  * Restarting with stat
    redis_1  | 1:M 17 Aug 22:11:10.483 * Running mode=standalone, port=6379.
    redis_1  | 1:M 17 Aug 22:11:10.483 # WARNING: The TCP backlog setting of 511 cannot be enforced because /proc/sys/net/core/somaxconn is set to the lower value of 128.
    web_1    |  * Debugger is active!
    redis_1  | 1:M 17 Aug 22:11:10.483 # Server initialized
    redis_1  | 1:M 17 Aug 22:11:10.483 # WARNING you have Transparent Huge Pages (THP) support enabled in your kernel. This will create latency and memory usage issues with Redis. To fix this issue run the command 'echo never > /sys/kernel/mm/transparent_hugepage/enabled' as root, and add it to your /etc/rc.local in order to retain the setting after a reboot. Redis must be restarted after THP is disabled.
    web_1    |  * Debugger PIN: 330-787-903
    redis_1  | 1:M 17 Aug 22:11:10.483 * Ready to accept connections

    Compose pulls a Redis image, builds an image for your code, and starts the services you defined. In this case, the code is statically copied into the image at build time.

  2. Enter http://localhost:8000/ in a browser to see the application running.

    If this doesn't resolve, you can also try

    You should see a message in your browser saying:

    Hello World! I have been seen 1 times.
    hello world in browser
  3. Refresh the page.

    The number should increment.

    Hello World! I have been seen 2 times.
    hello world in browser
  4. Switch to another terminal window, and type docker image ls to list local images.

    Listing images at this point should return redis and web.

    $ docker image ls
    REPOSITORY        TAG           IMAGE ID      CREATED        SIZE
    composetest_web   latest        e2c21aa48cc1  4 minutes ago  93.8MB
    python            3.4-alpine    84e6077c7ab6  7 days ago     82.5MB
    redis             alpine        9d8fa9aa0e5b  3 weeks ago    27.5MB

    You can inspect images with docker inspect <tag or id>.

  5. Stop the application, either by running docker compose down from within your project directory in the second terminal, or by hitting CTRL+C in the original terminal where you started the app.

Step 5: Edit the Compose file to add a bind mount

Edit the compose.yaml file in your project directory to add a bind mount for the web service:

    build: .
      - "8000:5000"
      - .:/code
      FLASK_DEBUG: "true"
    image: "redis:alpine"

The new volumes key mounts the project directory (current directory) on the host to /code inside the container, allowing you to modify the code on the fly, without having to rebuild the image. The environment key sets the FLASK_DEBUG environment variable, which tells flask run to run in development mode and reload the code on change. This mode should only be used in development.

Step 6: Re-build and run the app with Compose

From your project directory, type docker compose up to build the app with the updated Compose file, and run it.

$ docker compose up

Creating network "composetest_default" with the default driver
Creating composetest_web_1 ...
Creating composetest_redis_1 ...
Creating composetest_web_1
Creating composetest_redis_1 ... done
Attaching to composetest_web_1, composetest_redis_1
web_1    |  * Running on (Press CTRL+C to quit)

Check the Hello World message in a web browser again, and refresh to see the count increment.

Shared folders, volumes, and bind mounts

  • 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 Desktop for Mac and Docker Desktop for Windows that uses Linux containers. For more information, see File sharing on Docker for Mac, File sharing on Docker for Windows, File sharing on Docker for Linux. and the general examples on how to Manage data in containers.

  • If you are using Oracle VirtualBox on an older Windows OS, you might encounter an issue with shared folders as described in this VB trouble ticket. Newer Windows systems meet the requirements for Docker Desktop for Windows and do not need VirtualBox.

Step 7: Update the application

As the application code is now 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.

Change the greeting in and save it. For example, change the Hello World! message to Hello from Docker!:

return 'Hello from Docker! I have been seen {} times.\n'.format(count)

Refresh the app in your browser. The greeting should be updated, and the counter should still be incrementing.

hello world in browser

Step 8: 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 redis ...   Up      6379/tcp              
composetest_web_1     flask run                        Up>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.

If you started Compose with docker compose up -d, 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

Where to go next