my-docs/Containerization & Orchestration/Docker/4-Docker-Compose.md

Docker Compose Guide

Docker Compose is a tool for defining and running multi-container Docker applications. It allows you to manage services, networks, and volumes using a YAML configuration file.

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1. Basic Docker Compose Structure

Before defining the services in Docker Compose, we need to specify the Docker Compose version and the services we want to run. Here's a basic YAML template:

version: "<python-version>"  

services:
    <service-name>:  # The name of your service (e.g., web, db)
        container_name: <container-name>  # The name of the container
        image: <image-name>  # Docker image to be used for this service
        restart: always  # Ensure the container restarts if it stops or fails
        ports:
            - "<sv-port>:<container-port>"  # Map the host port to the container port
        volumes: 
            - <vol-name>:<container-loc>  # Attach a volume to a specific location in the container
        environment:
            - <env1>=<value1>  # Environment variables to be passed to the container

Explanation:

• services:: The core section where you define different services (containers) that make up your application.
• <service-name>: Replace with the name of the service (e.g., web, database). Each service corresponds to a container.
• container_name: The name given to the container.
• image: The Docker image used to run the service (e.g., python:3.9, nginx).
• restart: always: Ensures the container will always restart if it stops, providing higher availability.
• ports: Maps ports from the host to the container, allowing the container to be accessed externally. The syntax is <host-port>:<container-port>.
• volumes: Links a Docker volume or host directory to a directory inside the container, enabling persistent data or sharing of resources. Example: myvolume:/usr/src/app.
• environment: Defines environment variables to be passed to the container at runtime. For example, setting an environment variable like DB_HOST=localhost.

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2. Defining Volumes

Docker Compose allows you to define persistent volumes that can be attached to containers. Here's how to define a volume:

volumes:
    <vol-name>:  # Define the volume here

Explanation:

• volumes:: This section allows you to define named volumes that can be used in the services.
• <vol-name>: Replace this with the name of the volume (e.g., data-volume, db-volume). The volume can be attached to different services to persist data beyond the container's lifecycle.

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3. Useful Docker Compose Commands

3.1 Start the Docker Compose Application

To bring up the application defined in the docker-compose.yml file, use:

docker compose up

3.2 Run Docker Compose in Detached Mode (Background)

To run your Docker Compose services in the background (detached mode):

docker compose up -d

3.3 Stop and Remove Docker Compose Services

To stop the services and remove the containers, networks, and volumes created by Docker Compose:

docker compose down

Explanation of Commands:

• docker compose up: Builds, (re)creates, and starts all services defined in the docker-compose.yml file.
• docker compose up -d: Runs the services in the background, keeping your terminal free while the containers continue running.
• docker compose down: Stops and removes all running services, containers, and networks created by Docker Compose. You can add the -v flag to remove volumes as well.