Demo Docker Compose

In this guide, you’ll learn how to migrate a basic Docker Compose setup from version 1 to version 3. We’ll remove deprecated options, configure environment variables for PostgreSQL, and explore build and deployment best practices.

1. Starting with the Basic Compose File (Version 1)

Here’s a simple Compose file in version 1 syntax defining five services:

redis:
  image: redis

db:
  image: postgres:9.4

vote:
  image: voting-app
  ports:
    - "5000:80"
  links:
    - redis

worker:
  image: worker-app
  links:
    - db
    - redis

result:
  image: result-app
  ports:
    - "5011:80"
  links:
    - db

Version 1 is straightforward but lacks support for:

• Automatic network creation
• Built-in DNS service discovery
• Named volumes and advanced deployment options

2. Upgrading to Version 3

Compose version 3 unlocks Docker Swarm compatibility, automatic networking, and enhanced resource definitions. Follow these steps:

1. Add version: "3" at the top.
2. Nest all services under the services: key.
3. Remove links:—service names now resolve via DNS.

version: "3"
services:
  redis:
    image: redis
  db:
    image: postgres:9.4
  vote:
    image: voting-app
    ports:
      - "5000:80"
  worker:
    image: worker-app
  result:
    image: result-app
    ports:
      - "5001:80"

In version 3, Docker Compose automatically creates a default network. Services communicate by name, e.g., redis ↔ db.

3. Specifying Build Options

You can define how images are built directly in your Compose file. Examples from Docker’s docs:

Simple build context:

version: "3.9"
services:
  webapp:
    build: ./dir

Custom Dockerfile and build args:

version: "3.9"
services:
  webapp:
    build:
      context: ./dir
      dockerfile: Dockerfile-alternate
      args:
        buildno: 1

Build with image tag:

services:
  webapp:
    build: ./dir
    image: webapp:tag

4. Deploying with Docker Compose

With your version 3 Compose file in place, start all services:

docker-compose up

Sample output:

WARNING: The Docker Engine you're using is running in swarm mode.
Compose does not use swarm mode to deploy services to multiple nodes
in a swarm. All containers will be scheduled on the current node.
To deploy your application across the swarm, use `docker stack deploy`.
Creating network "code_default" with the default driver
Creating code_redis_1    ... done
Creating code_db_1       ... done
Creating code_vote_1     ... done
Creating code_worker_1   ... done
Creating code_result_1   ... done

Here, code_default is the auto-generated network, and all containers are prefixed with the project name (code_).

docker-compose up -d

5. Handling PostgreSQL Initialization

On first run, PostgreSQL requires a superuser password. Without it, you’ll see:

db_1  | Error: Database is uninitialized and superuser password is not specified.
db_1  | You must specify POSTGRES_PASSWORD for the superuser. Use
db_1  | "-e POSTGRES_PASSWORD=password" to set it in "docker run".

Add environment variables under the db service:

version: "3"
services:
  redis:
    image: redis
  db:
    image: postgres:9.4
    environment:
      POSTGRES_USER: postgres
      POSTGRES_PASSWORD: postgres
  vote:
    image: voting-app
    ports:
      - "5000:80"
  worker:
    image: worker-app
  result:
    image: result-app
    ports:
      - "5001:80"

Restart in detached mode:

docker-compose up -d

PostgreSQL Environment Variables

6. Verifying the Setup

Once all containers are running, open your browser and test the apps:

• Voting app: http://localhost:5000
• Results app: http://localhost:5001

1. Cast a vote for one option (e.g., cats).
2. Confirm the result updates correctly.
3. Vote again (e.g., dogs) and verify real-time results.

Your Docker Compose setup is now:

• Version 3 compliant
• Automatically networked
• Securely initializing PostgreSQL

Links and References

• Docker Compose File Reference
• Compose and Docker Compatibility Matrix
• Docker Stack Deploy

Further Reading