• Understand what a software container is
• Get familiar with the Docker environment
• Learn how to build, publish, and deploy containers

Tasks

Normally, in order to run containers, we need to execute the corresponding run command (docker run) and set all the necessary parameters. This process can be difficult and repetitive when we need to start multiple containers. One way to “save” the running configuration is to create scripts. The problem with running multiple scripts is the loss of uniformity in configuration (which container connects to which network, who communicates with whom, etc.).

Docker Compose is a utility created by Docker that is used to centralise the configuration process of a container-based application in a declarative manner, using Yet Another Markup Language (YAML) configuration files.

Moreover, the format for Compose files is also used in Docker Swarm, the orchestrator created by Docker for managing Docker services, which we will discuss in lab 5.

For Windows and MacOS, Docker Compose is part of the Docker Desktop installation. For Linux, the installation is done according to the official guide.

YAML files are usually used to write declarative configurations. The format is very easy to understand and employ, as follows:

• “key:value” elements are used
• indented paragraphs are children properties of the previous paragraphs
• lists are delimited by ”-”.

# docker-compose.yml
version: "3.8"

services:
    api:
        build: . # builds the image from a Dockerfile
        image: register-image-name:version # uses an image from a registry
        environment:
            ENVIRONMENT_VARIABLE: value
        ports:
            - "5000:80"
        networks:
            - lab4-network

    postgres:
        image: postgres:12
        volumes:
            - lab4-volume:/var/lib/postgresql/ data
            - ./scripts/init-db.sql:/docker-entrypoint-init.d/init-db.sql
        networks:
            - lab4-network

volumes:
    lab4-volume:

networks:
    lab4-network:

The “version” attribute describes what functionalities will be loaded when running the Docker Compose utility.

The services attribute describes the services/containers that will run after the configuration is started by Compose. Each service represents a container that will have the name and configuration of the service. In the example above, the containers will be named api and postgres. The most important properties of services are the following:

• build - specifies the path where the Dockerfile the container will be built from is located
• image - specifies the name of the image used to run the container
• ports - a list of entries with the format “host_port:service_port”, which specifies which ports are exposed and/or mapped
• volumes - a list of entries with the format “host_volume:service_path” where the volume mappings are specified; the same rules that apply to CLI commands are maintained here as well; “host_volume” can be a standard volume or a bind mount
• networks - the list of networks which the service/container belongs to
• environment - object with entries of type “service_variable_name: value” which injects the environment variables specified when running the service/container.

The volumes attribute describes the volumes used in the configuration. Volumes are passed as objects. If we do not want to change the default configuration, the value is an empty field.

The networks attribute describes the networks used in the configuration. Networks are passed in the form of objects. If we do not want to change the default configuration, the value is an empty field. An example of a network configuration is the following (where we use a network that already exists because it was created independently of the Docker Compose file):

networks:
    my-network-that-actually-exists:
        external: true
        name: the-original-network-that-already-exists

In the example above, my-network-that-actually-exists is just a “renaming” of an existing network.

The commands for interacting with Docker Compose are syntactically similar to the classic Docker commands for both version 1 and version 2. Below, you can see the most used commands. For version 1, the utility is docker-compose, while for version 2 the utility is docker with the compose parameter. For more information, you can read the official documentation for version 1 and the official version 2 documentation. The main difference between versions 1 and 2 is that there are several additional commands in version 2.

$ docker-compose start # starts containers (V1)
$ docker-compose stop # stops containers (V1)
$ docker-compose pause # pauses containers using a SIGPAUSE signal (V1)
$ docker-compose unpause # removes containers from the pause state (V1)
$ docker-compose ps # lists active containers (V1)
$ docker-compose up # builds, recreates, starts and attaches containers to a service (V1)
$ docker-compose up -d # starts services in the background, detached from the terminal that initialised them (V1)
$ docker-compose up --build # create images before starting containers (V1)
$ docker-compose -f my-compose.yml up # uses the specified Compose file instead of the default one (V1)
$ docker-compose down # stops containers and deletes them, along with the networks, volumes, and images created (V1)
$ docker-compose rm # deletes all stopped containers (V1)
$ docker-compose rm -s -v # with -s all containers are stopped and with -v the anonymous volumes attached are also deleted (V1)
 
$ docker compose start # starts containers (V2)
$ docker compose stop # stops containers (V2)
$ docker compound pause # pauses containers using a SIGPAUSE signal (V2)
$ docker compose unpause # removes containers from the pause state (V2)
$ docker compose ps # lists active containers (V2)
$ docker compose ls # lists all container stacks (V2)
$ docker compose -p my-project -f my-compose.yml up # uses the specified Compose file instead of the default one and sets a project name (V2)
$ docker compose down # stops containers and deletes them, along with the networks, volumes, and images created (V2)
$ docker compose rm # deletes all stopped containers (V2)
$ docker compose rm -s -v # with -s all containers are stopped and with -v the anonymous volumes attached are also deleted (V2)

Docker Compose is recommended for use only at the local development stage. Docker Swarm (or other orchestrators, such as Kubernetes) should be used for testing or production environments.

Precisely because it is used in the development stage, Docker Compose has a mechanism to combine several Compose files to create different running configurations without replicating the common parts.

To run a Compose configuration based on multiple YAML files, you can use the command below:

$ docker-compose -f file-compose-1.yml -f file-compose-2.yml up --build # V1
$ docker compose -f file-compose-1.yml -f file-compose-2.yml up --build # V2

In the command above, the information in file-compose-2.yml will overwrite or complete the information in file-compose-1.yml. This is useful for quickly testing various configurations (or combinations pf configurations).