Docker Compose Development

Docker Compose Development Skill

This skill provides best practices and workflows for developing applications running in Docker Compose, specifically focusing on services that mount source code for development (hot-reload).

1. Context Awareness (First Step)

Before taking any action, ALWAYS analyze the docker-compose.yml file to understand the environment.

Checklist:

1. Identify Services: What services are defined?
2. Check Volumes: fast checks for volumes that mount local source code (e.g., ./backend:/app).
   • If mounted: Code changes usually reflect immediately (Hot Reload).
   • If NOT mounted: Code changes require image rebuild.
3. Check Service State:

   docker compose ps
   

2. Development Workflow

A. Code Changes (Hot Reload)

If a service mounts source code (e.g., volumes: - ./src:/app), DO NOT REBUILD the container for simple code changes.

1. Edit the code locally.
2. Wait a moment for auto-reload (if the framework supports it, like FastAPI/Next.js).
3. Verify the change (e.g., check logs or refresh browser).

B. Dependency Changes

If you modify requirements.txt, package.json, Dockerfile, or unmounted files:

Action: Rebuild ONLY the specific service.

docker compose up -d --build <service_name>

Note: This is faster than rebuilding everything.

C. Adding New Files

If you create a new file, ensure it is within the mounted directory.

• If valid: No action needed.
• If outside mount: You may need to adjust docker-compose.yml or rebuild.

3. Running Tests

CRITICAL: Always run tests INSIDE the container to ensure the environment matches production/CI. DO NOT run tests locally unless explicitly asked.

Syntax:

docker compose exec <service_name> <test_command>

Common Examples:

• Python/Pytest:

  docker compose exec <backend_service_name> pytest tests/ -v
  

• Node/Jest:

  docker compose exec <frontend_service_name> npm test
  

4. Troubleshooting & Operations

View Logs

To check for errors or verify startup:

docker compose logs --tail=100 -f <service_name>

Access Shell

To explore the container filesystem or debug manually:

docker compose exec <service_name> /bin/bash
# If bash is missing, try:
docker compose exec <service_name> sh

Restart Service

If hot-reload fails or application hangs:

docker compose restart <service_name>

Example Scenario

Scenario: You added a new endpoint to backend service (FastAPI).

1. Edit code: app/routers/new_api.py (Local file).
2. No Build Needed: Because ./app is mounted to container.
3. Check Logs: docker compose logs -f backend to see if it reloaded.
4. Test: docker compose exec backend pytest tests/test_new_api.py.

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