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  This tutorial aims to introduce fundamental concepts of Docker Compose by guiding you through the development of a basic Python web application.
  Using the Flask framework, the application features a hit counter in Redis, providing a practical example of how Docker Compose can be applied in web development scenarios.
  The concepts demonstrated here should be understandable even if you're not familiar with Python.
  This is a non-normative example that demonstrates core Compose functionality.
  Prerequisites
  Make sure you have:
  Installed the latest version of Docker Compose
  A basic understanding of Docker concepts and how Docker works
  Step 1: Set up
  Create a directory for the project:
  mkdir composetest
  composetest
  Create a file called
  app.py
  in your project directory and paste the following code in:
  import
  time
  import
  redis
  from
  flask
  import
  Flask
  Flask
  __name__
  cache
  redis
  Redis
  host
  'redis'
  port
  get_hit_count
  retries
  while
  True
  return
  cache
  incr
  'hits'
  except
  redis
  exceptions
  ConnectionError
  retries
  raise
  retries
  time
  sleep
  @app.route
  hello
  count
  get_hit_count
  return
  'Hello World! I have been seen
  count
  times.
  In this example,
  redis
  is the hostname of the redis container on the
  application's network and the default port,
  is used.
  Note
  Note the way the
  get_hit_count
  function is written. This basic retry
  loop attempts the 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.
  Create another file called
  requirements.txt
  in your project directory and
  paste the following code in:
  flask
  redis
  Create a
  Dockerfile
  and paste the following code in:
  # syntax=docker/dockerfile:1
  FROM
  python:3.10-alpine
  WORKDIR
  /code
  FLASK_APP
  app.py
  FLASK_RUN_HOST
  apk add --no-cache gcc musl-dev linux-headers
  COPY
  requirements.txt requirements.txt
  pip install -r requirements.txt
  EXPOSE
  COPY
  "flask"
  "run"
  "--debug"
  Understand the Dockerfile
  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 --debug
  Important
  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
  Dockerfile reference
  Step 2: Define services in a Compose file
  Compose simplifies the control of your entire application stack, making it easy to manage services, networks, and volumes in a single, comprehensible YAML configuration file.
  Create a file called
  compose.yaml
  in your project directory and paste
  the following:
  services
  build
  ports
  "8000:5000"
  redis
  image
  "redis:alpine"
  This Compose file defines two services:
  redis
  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,
  . This example service uses the default port for the Flask web server,
  redis
  service uses a public
  Redis
  image pulled from the Docker Hub registry.
  For more information on the
  compose.yaml
  file, see
  How Compose works
  Step 3: Build and run your app with Compose
  With a single command, you create and start all the services from your configuration file.
  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:8000/
  in a browser to see the application running.
  If this doesn't resolve, you can also try
  http://127.0.0.1:8000
  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
  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 4: Edit the Compose file to use Compose Watch
  Edit the
  compose.yaml
  file in your project directory to use
  watch
  so you can preview your running Compose services which are automatically updated as you edit and save your code:
  services
  build
  ports
  "8000:5000"
  develop
  watch
  action
  sync
  path
  target
  /code
  redis
  image
  "redis:alpine"
  Whenever a file is changed, Compose syncs the file to the corresponding location under
  /code
  inside the container. Once copied, the bundler updates the running application without a restart.
  For more information on how Compose Watch works, see
  Use Compose Watch
  . Alternatively, see
  Manage data in containers
  for other options.
  Note
  For this example to work, the
  --debug
  option is added to the
  Dockerfile
  . The
  --debug
  option in Flask enables automatic code reload, making it possible to work on the backend API without the need to restart or rebuild the container.
  After changing the
  file, subsequent API calls will use the new code, but the browser UI will not automatically refresh in this small example. Most frontend development servers include native live reload support that works with Compose.
  Step 5: Re-build and run the app with Compose
  From your project directory, type
  docker compose watch
  docker compose up --watch
  to build and launch the app and start the file watch mode.
  docker compose watch
  [+] Running 2/2
  â Container docs-redis-1 Created 0.0s
  â Container docs-web-1 Recreated 0.1s
  Attaching to redis-1, web-1
  â¦¿ watch enabled
  Check the
  Hello World
  message in a web browser again, and refresh to see the
  count increment.
  Step 6: Update the application
  To see Compose Watch in action:
  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
  count
  times.
  Refresh the app in your browser. The greeting should be updated, and the
  counter should still be incrementing.
  Once you're done, run
  docker compose down
  Step 7: Split up your services
  Using multiple Compose files lets you customize a Compose application for different environments or workflows. This is useful for large applications that may use dozens of containers, with ownership distributed across multiple teams.
  In your project folder, create a new Compose file called
  infra.yaml
  Cut the Redis service from your
  compose.yaml
  file and paste it into your new
  infra.yaml
  file. Make sure you add the
  services
  top-level attribute at the top of your file. Your
  infra.yaml
  file should now look like this:
  services
  redis
  image
  "redis:alpine"
  In your
  compose.yaml
  file, add the
  include
  top-level attribute along with the path to the
  infra.yaml
  file.
  include
  infra.yaml
  services
  build
  ports
  "8000:5000"
  develop
  watch
  action
  sync
  path
  target
  /code
  docker compose up
  to build the app with the updated Compose files, and run it. You should see the
  Hello world
  message in your browser.
  This is a simplified example, but it demonstrates the basic principle of
  include
  and how it can make it easier to modularize complex applications into sub-Compose files. For more information on
  include
  and working with multiple Compose files, see
  Working with multiple Compose files
  Step 8: Experiment with some other commands
  If you want to run your services in the background, you can pass the
  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 0.0.0.0:8000->5000/tcp
  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
  docker compose down
  command.
  Where to go next
  Try the
  Sample apps with Compose
  Explore the full list of Compose commands
  Explore the Compose file reference
  Check out the Learning Docker Compose video on LinkedIn Learning