Module 8 Lesson 2: Docker Compose for Local Development

When developing locally, your docker-compose.yml serves as your "Dependency Map." It defines everything your app needs to function that isn't the code itself.

1. The "Thin App" Pattern

The most common pattern is to run your "Databases" in Docker but run your "App" natively on your laptop for the fastest debugging.

# docker-compose.yml
services:
  db:
    image: postgres:15
    ports:
      - "5432:5432"
    environment:
      POSTGRES_PASSWORD: development_pass

  cache:
    image: redis:alpine
    ports:
      - "6379:6379"

Result: You have a production-grade database running in 1 second. Your code (running in VS Code or IntelliJ) connects to localhost:5432 just like it would if you had installed Postgres manually.

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2. Using docker-compose.override.yml

Docker Compose has a secret weapon: Automatic Merging. If you have a file named docker-compose.override.yml, Compose will automatically merge it with your main file.

• docker-compose.yml: Contains settings that work for everyone (e.g., the images).
• docker-compose.override.yml: Contains settings specific to YOUR laptop (e.g., mapping your local folder to a volume).

Tip: You usually put the override file in .gitignore so every developer can have their own personal settings.

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3. Speeding Up with --profile

What if your project has 20 services, but you are only working on the "Search" feature today? You can group services into Profiles.

services:
  app:
    # ...
  database:
    # ...
  search-engine:
    profiles: ["search"]
  heavy-batch-process:
    profiles: ["background"]

• Running docker-compose up will only start app and database.
• Running docker-compose --profile search up will start the search engine too.

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4. Managing Logs

In dev, you want to see what's happening.

• docker-compose logs -f: Streams logs from ALL services.
• docker-compose logs -f app: Only shows your app's logs.

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Exercise: The Personal Override

1. Create a simple docker-compose.yml with one service (nginx) using a static image.
2. Now create a docker-compose.override.yml and add a ports mapping to port 9000:80.
3. Run docker-compose up -d.
4. Check localhost:9000. Did it work?
5. Run docker-compose config. This shows you the "Final Result" of how Docker merged both files. Very useful for debugging!

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Summary

Docker Compose is the "glue" that holds your local environment together. By using overrides and profiles, you can tailor your infrastructure to your specific task without breaking it for the rest of your team.

Next Lesson: See changes instantly: Hot reloading and bind mounts.