Get started with Docker Compose

Estimated reading time: 11 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

Define the application dependencies.

Create a directory for the project:
```
$ mkdir composetest
$ cd composetest
```

Create a file called app.py in your project directory and paste this 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:
        try:
            return cache.incr('hits')
        except redis.exceptions.ConnectionError as exc:
            if retries == 0:
                raise exc
            retries -= 1
            time.sleep(0.5)

@app.route('/')
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 our 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.

Create another file called requirements.txt in your project directory and paste this in:
```
flask
redis
```

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:3.7-alpine
WORKDIR /code
ENV FLASK_APP=app.py
ENV FLASK_RUN_HOST=0.0.0.0
RUN apk add --no-cache gcc musl-dev linux-headers
COPY requirements.txt requirements.txt
RUN pip install -r requirements.txt
EXPOSE 5000
COPY . .
CMD ["flask", "run"]

This tells Docker to:

Build an image starting with the Python 3.7 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.

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: "3.9"
services:
  web:
    build: .
    ports:
      - "5000:5000"
  redis:
    image: "redis:alpine"

This Compose file defines two services: web and redis.

Web service

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, 5000. This example service uses the default port for the Flask web server, 5000.

Redis service

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

Step 4: Build and run your app with Compose

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 http://0.0.0.0:5000/ (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.

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

If you’re using Docker natively on Linux, Docker Desktop for Mac, or Docker Desktop for Windows, then the web app should now be listening on port 5000 on your Docker daemon host. Point your web browser to http://localhost:5000 to find the Hello World message. If this doesn’t resolve, you can also try http://127.0.0.1:5000.

If you’re using Docker Machine on a Mac or Windows, 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.
```
Refresh the page.

The number should increment.
```
Hello World! I have been seen 2 times.
```

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>.

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 docker-compose.yml in your project directory to add a bind mount for the web service:

version: "3.9"
services:
  web:
    build: .
    ports:
      - "5000:5000"
    volumes:
      - .:/code
    environment:
      FLASK_ENV: development
  redis:
    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_ENV 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 http://0.0.0.0:5000/ (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 Windows that uses Linux containers. For more information, see File sharing on Docker for Mac, 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

Because 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 app.py 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   docker-entrypoint.sh redis ...   Up      6379/tcp              
composetest_web_1     flask run                        Up      0.0.0.0:5000->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 also shows you 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

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

Where to go next

Next, try the Sample apps with Compose
Explore the full list of Compose commands
Compose configuration file reference
To learn more about volumes and bind mounts, see Manage data in Docker

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