Docker Compose

Docker Compose is a tool for defining and running multi-container Docker applications. It uses a configuration file to provide all the necessary details about the services, networks, and volumes. Let’s dive into the essential aspects of Docker Compose for programmers.

Building Images

Building images is an essential step in the Docker Compose workflow. You can define the build context, Dockerfile path, and other parameters in the docker-compose.yml file for each service.

version: '3'
services:
  webapp:
    build:
      context: ./path_to_code
      dockerfile: Dockerfile.custom_name

This defines a service named webapp that uses the build context from ./path_to_code and a Dockerfile named Dockerfile.custom_name.

Running Images

Once images are built, you can run them as containers using Docker Compose. The image to run can be specified in the docker-compose.yml file.

version: '3'
services:
  webapp:
    image: my_custom_image:latest

Here, the webapp service runs the image my_custom_image with the latest tag.

Basic Commands

Docker Compose comes with a suite of commands to manage the lifecycle of your containers.

docker-compose up

This command starts all the services defined in your docker-compose.yml file.

docker-compose up

When run, it reads the docker-compose.yml file and starts the services accordingly. You can also use -d flag to run services in detached mode.

docker-compose down

Stops and removes all the services that are running.

docker-compose down

This command stops the services and also removes the containers, networks, and volumes defined in the docker-compose.yml file.

docker-compose logs

Displays the logs of your services.

docker-compose logs webapp

In this case, it will show the logs for the service named webapp.

docker-compose build

Builds the service images.

docker-compose build webapp

It builds the webapp service using the specified build context and Dockerfile in the docker-compose.yml file.

docker-compose ps

List the services.

docker-compose ps

This command provides an overview of the current state of the services.

docker-compose.yml

The docker-compose.yml file is a YAML file that defines the services, networks, and volumes for your application.

Defining Services

Services are the heart of your application, representing the containers that will run.

services:
  webapp:
    image: my_custom_image:latest
    ports:
      - "5000:5000"

This defines a service webapp that runs the image my_custom_image:latest and maps port 5000 inside the container to port 5000 on the host.

Defining Networks

Networks facilitate communication between containers.

services:
  webapp:
    image: my_custom_image
    networks:
      - my_custom_network

networks:
  my_custom_network:

This configuration sets up a custom network named my_custom_network and associates the webapp service with this network.

Common Keys

There are multiple keys in the docker-compose.yml file that dictate the behavior of services, networks, and volumes.

For services, some common keys include:

• image: The Docker image to use for the service.
• build: Instructions to build the Docker image.
• ports: Ports mapping between the host and the container.
• volumes: Mount paths for data persistence.

Volumes

Volumes are used for data persistence and sharing data among containers.

services:
  webapp:
    image: my_custom_image
    volumes:
      - my_data:/app/data

volumes:
  my_data:

Here, a volume named my_data is defined and mounted to /app/data in the webapp service container.

Secrets

Secrets are used to manage sensitive data.

services:
  webapp:
    image: my_custom_image
    secrets:
      - my_secret

secrets:
  my_secret:
    file: ./path_to_secret_file

In this configuration, a secret named my_secret is defined, and its value is taken from the file at ./path_to_secret_file. The webapp service can then access this secret.

With this cheat sheet, you now have a comprehensive overview of the core components and functionalities of Docker Compose. Happy containerizing!