docker-compose.yml structure - Commands & Configuration

Introduction

Docker Compose helps you run multiple containers together easily. It uses a file called docker-compose.yml to define how containers work together, like setting their names, ports, and shared networks.

When you want to start a web app and a database together on your computer.

When you need to test how different services talk to each other locally.

When you want to share your app setup with teammates so they can run it exactly the same way.

When you want to stop and start all parts of your app with one command.

When you want to keep your container settings in one easy-to-read file.

Config File - docker-compose.yml

docker-compose.yml

version: '3.8'
services:
  web:
    image: nginx:1.23
    ports:
      - "8080:80"
    volumes:
      - ./html:/usr/share/nginx/html
  db:
    image: postgres:15
    environment:
      POSTGRES_USER: user
      POSTGRES_PASSWORD: password
      POSTGRES_DB: exampledb
    ports:
      - "5432:5432"

version: Defines the Compose file format version.

services: Lists the containers to run.

web: A service using the nginx image, exposing port 80 inside the container to port 8080 on the host, and sharing local html files.

db: A PostgreSQL database service with environment variables for user, password, and database name, exposing its default port.

Commands

Starts all services defined in docker-compose.yml in the background so your app and database run together.

Terminal

docker-compose up -d

Expected Output

Creating network "default" with the default driver

Creating volume "default" with default driver

Creating example_db_1 ... done

Creating example_web_1 ... done

→

-d - Run containers in detached mode (in the background)

Shows the status of all running services defined in your docker-compose.yml file.

Terminal

docker-compose ps

Expected Output

    Name                  Command               State           Ports         

----------------------------------------------------------------------------

example_db_1    docker-entrypoint.sh postgres    Up      0.0.0.0:5432->5432/tcp

example_web_1   nginx -g 'daemon off;'          Up      0.0.0.0:8080->80/tcp

Stops and removes all containers, networks, and volumes created by docker-compose up.

Terminal

docker-compose down

Expected Output

Stopping example_web_1 ... done

Stopping example_db_1  ... done

Removing example_web_1 ... done

Removing example_db_1  ... done

Removing network default

Key Concept

If you remember nothing else from this pattern, remember: docker-compose.yml defines multiple containers and how they work together in one simple file.

Common Mistakes

Using tabs instead of spaces in the docker-compose.yml file

YAML files require spaces for indentation; tabs cause parsing errors.

Always use spaces (usually 2) for indentation in docker-compose.yml.

Not exposing container ports to the host in the ports section

Without port mapping, you cannot access the service from your computer.

Map container ports to host ports using the format 'hostPort:containerPort' under ports.

Running docker-compose up without the -d flag and closing the terminal

The containers stop when the terminal closes if not run in detached mode.

Use 'docker-compose up -d' to keep containers running in the background.

Summary

docker-compose.yml defines multiple containers and their settings in one file.

Use 'docker-compose up -d' to start all services in the background.

Use 'docker-compose ps' to check running containers and 'docker-compose down' to stop and clean up.