Introduction

Defining your multi-container application in a docker-compose.yml file is the first step. The next is effectively managing its lifecycle. This lesson focuses on the practical commands for starting, stopping, scaling, and monitoring your Compose-defined applications. We'll cover essential operations that enable you to interact with your entire application stack as a single unit.

Key Concepts

Starting and Stopping Services

• docker compose up: The primary command to create and start services.

It performs several actions:

1. Looks for docker-compose.yml (or specified -f file).

2. Builds any images defined with build instructions.

3. Creates networks and volumes if they don't exist.

4. Starts containers for all services.

   • Use docker compose up -d to run in detached mode (background).
   • Use docker compose up --build to force image rebuilding before starting.

• docker compose stop: Stops running containers gracefully without removing them. You can later restart them with docker compose start.

• docker compose start: Starts stopped services.

• docker compose restart: Restarts all services or specified services.

• docker compose down: Stops and removes containers, networks, and (by default) named volumes associated with your Compose application. This is typically used for a full cleanup.

  • docker compose down --volumes: Also removes named volumes.
  • docker compose down --rmi all: Removes images used by any service.

Monitoring and Logging

• docker compose ps: Lists the containers managed by Compose, showing their state, ports, and attached commands.

• docker compose logs: Displays aggregated logs from all services or a specific service. This is invaluable for debugging.

  • docker compose logs -f: Follows the logs in real-time.
  • docker compose logs --tail 100: Shows the last 100 lines.

Executing Commands in Services

• docker compose exec [service_name] [command]: Executes a command inside a running service container. Similar to docker exec but references the service name.
  • Example: docker compose exec api bash to get a shell inside your api service.

Scaling Services

• docker compose up --scale [service_name]=[count]: Scales a service to a desired number of instances. This is a powerful feature for simple horizontal scaling on a single host.
  • Example: docker compose up --scale api=3 to run three instances of your api service.

Example/Code

Assume you have the full-stack docker-compose.yml from the previous lesson.

1. Start the application in detached mode: This will build images (if necessary) and start all services in the background.

   bash
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docker compose up -d ```

2. Check the status of services: See which containers are running.

   bash
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docker compose ps ```

3. View logs from all services: Follow logs in real-time.

   bash
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docker compose logs -f ```

4. Execute a command in the database container: Access the PostgreSQL client.

   bash
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docker compose exec database psql -U admin myappdb ```

5. Scale the API service: Run two instances of your api service.

   bash
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docker compose up -d --scale api=2 ```

6. Stop and clean up the application: Remove all containers, networks, and named volumes.

   bash
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docker compose down --volumes ```

Summary/Key Takeaways

• docker compose up (with -d or --build) starts your application stack.

• docker compose stop, start, restart, down manage the lifecycle of services.

• docker compose ps and docker compose logs are vital for monitoring.

• docker compose exec allows running commands inside a service container.

• docker compose up --scale enables basic horizontal scaling of services.