Defining services in Docker - Commands & Configuration

Introduction

When you want to run multiple containers together that work as parts of one app, you define services. Services let you describe each container's settings in one file so you can start them all easily.

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

When you need to start multiple containers with one command instead of many commands.

When you want to share network settings between containers automatically.

When you want to set environment variables or volumes for containers in one place.

When you want to stop or restart all parts of your app together.

Config File - docker-compose.yml

docker-compose.yml

version: '3.9'
services:
  web:
    image: nginx:1.23
    ports:
      - "8080:80"
  db:
    image: postgres:15
    environment:
      POSTGRES_USER: exampleuser
      POSTGRES_PASSWORD: examplepass
      POSTGRES_DB: exampledb
    volumes:
      - db-data:/var/lib/postgresql/data
volumes:
  db-data:

This file defines two services: web and db.

The web service uses the nginx image and maps port 8080 on your computer to port 80 in the container.

The db service uses the postgres image with environment variables to set up the database user, password, and database name. It also uses a volume to save data persistently.

The volumes section creates a named volume db-data to keep database files even if the container stops.

Commands

This command starts all services defined in the docker-compose.yml file in detached mode, so they run in the background.

Terminal

docker-compose up -d

Expected Output

Creating network "default" with the default driver

Creating volume "db-data" with default driver

Creating example_web_1 ... done

Creating example_db_1  ... done

→

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

This command lists all running services started by docker-compose, showing their status and ports.

Terminal

docker-compose ps

Expected Output

    Name                  Command               State           Ports         

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

example_db_1    docker-entrypoint.sh postgres    Up      5432/tcp        

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

This command shows the logs of the web service to help you check if it started correctly or if there are errors.

Terminal

docker-compose logs web

Expected Output

example_web_1  | /docker-entrypoint.sh: /docker-entrypoint.d/ is not empty, will attempt to perform configuration

example_web_1  | /docker-entrypoint.sh: Looking for shell scripts in /docker-entrypoint.d/

example_web_1  | /docker-entrypoint.sh: Launching /docker-entrypoint.d/10-listen-on-ipv6-by-default.sh

example_web_1  | 10-listen-on-ipv6-by-default.sh: Getting the checksum of /etc/nginx/conf.d/default.conf

example_web_1  | 10-listen-on-ipv6-by-default.sh: Enabled listen on IPv6 in /etc/nginx/conf.d/default.conf

example_web_1  | /docker-entrypoint.sh: Launching /docker-entrypoint.d/20-envsubst-on-templates.sh

example_web_1  | /docker-entrypoint.sh: Configuration complete; ready for start up

This command stops and removes all containers, networks, and volumes created by docker-compose for this project.

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: defining services in a docker-compose.yml file lets you start and manage multiple containers as one app easily.

Common Mistakes

Not mapping ports in the service definition

Without port mapping, you cannot access the service from your computer's browser or other tools.

Always include the ports section with host:container port mapping for services you want to access externally.

Forgetting to create volumes for databases

Without volumes, database data is lost when the container stops or is removed.

Define volumes in the compose file and attach them to database services to keep data persistent.

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

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

Use the -d flag to run containers in the background so they keep running after you close the terminal.

Summary

Define each container as a service in docker-compose.yml with image, ports, environment, and volumes.

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

Use 'docker-compose ps' to check running services and 'docker-compose logs' to see their output.

Use 'docker-compose down' to stop and clean up all services and networks.