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3. Docker Compose

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

3.1 Step 1: Setup

Define the application dependencies.

  1. Create a directory for the project:
$ mkdir composetest
$ cd composetest
  1. 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.

  1. Create another file called requirements.txt in your project directory and paste this in:

    flask
redis

3.2 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
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 so Python packages such as MarkupSafe and SQLAlchemy can compile speedups.
  • Copy requirements.txt and install the Python dependencies.
  • Copy the current directory . in the project to the workdir . in the image.
  • Set the default command for the container to flask run.

3.3 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'
services:
  web:
    build: .
    ports:
      - "8080:5000"
  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 port 8080 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.

3.4 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 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.

  1. Enter http://localhost:8080/ 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:8080 to find the Hello World message. If this doesn't resolve, you can also try http://127.0.0.1:8080.

    Hint: In AWS Cloud9 use Tools -> Preview -> Preview running applications to open the browser on the appropriate remote address.

    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.

Hello World! I have been seen 2 times.

  1. 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>`.
  1. 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.

3.5 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'
services:
  web:
    build: .
    ports:
      - "8080:5000"
    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.

3.6 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.

3.7 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.

  1. 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)
  1. Refresh the app in your browser. The greeting should be updated, and the counter should still be incrementing.

3.8 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   /usr/local/bin/run         Up
composetest_web_1     /bin/sh -c python app.py   Up      8080->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

At this point, you have seen the basics of how Compose works. Go to next section