Closer Look at the Compose File

The example file we’ve listed uses the Compose version 3.8 file format, defines two services, defines a network called counter-net, and defines a volume called counter-vol.

services

Most of the detail is in the services section, so let’s take a closer look at that.

The services section has two second-level keys:

• web-fe
• redis

Each of these defines a service (container) in the app. It’s important to understand that Compose will deploy each of these as a container, and it will use the name of the keys as part of the container names. In our example, we’ve defined two keys; web-fe and redis. This means Compose will deploy two containers, one will have web-fe in its name and the other will have redis.

web-fe

Within the definition of the web-fe service, we give Docker the following instructions:

• build: .

  This tells Docker to build a new image using the instructions in the Dockerfile in the current directory (.). The newly built image will be used in a later step to create the container for this service.

• command: python app.py

  This tells Docker to run a Python app called app.py as the main app in the container. The app.py file must exist in the image, and the image must contain Python. The Dockerfile takes care of both of these requirements.

• ports:

  This tells Docker to map port 5000 inside the container (target) to port 5000 on the host (published). This means that traffic sent to the Docker host on port 5000 will be directed to port 5000 on the container. The app inside the container listens on port 5000.

• networks:

  This tells Docker which network to attach the service’s container to. The network should already exist or be defined in the networks top-level key. If it’s an overlay network, it will need to have the attachable flag so that standalone containers can be attached to it (Compose deploys standalone containers instead of Docker Services).

• volumes:

  This tells Docker to mount the counter-vol volume (source:) to /code (target:) inside the container. The counter-vol volume needs to already exist, or be defined in the volumes top-level key at the bottom of the file.

In summary, Compose will instruct Docker to deploy a single standalone container for the web-fe service. It will be based on an image built from a Dockerfile in the same directory as the Compose file. This image will be started as a container and run app.py as its main app. It will expose itself on port 5000 on the host, attach to the counter-net network, and mount a volume to /code.

Note: Technically speaking, we don’t need the command: python app.py option. This is because, as you will soon see, the application’s Dockerfile already defines python app.py as the default app for the image. However, we’re showing it here so you know how it works. You can also use Compose to override CMD instructions set in Dockerfiles.

redis

The definition of the redis service is simpler:

• image: "redis:alpine"

  This tells Docker to start a standalone container called redis based on the redis:alpine image. This image will be pulled from Docker Hub.

• networks:

  The redis container will be attached to the counter-net network.

As both services will be deployed onto the same counter-net network, they will be able to resolve each other by name. This is important as the application is configured to communicate with the redis service by name.

Now that we understand how the Compose file works, let’s deploy it!