Multi-container apps

Adding a database

Most real apps need more than one container. Let's describe an app together with a MySQL database. Here each part is a service:

services:
  web:
    image: nginx
    ports:
      - "8080:80"

  db:
    image: mysql:8
    environment:
      MYSQL_ROOT_PASSWORD: secret
      MYSQL_DATABASE: app
    volumes:
      - db-data:/var/lib/mysql

volumes:
  db-data:

New pieces compared to the last lesson:

• A second service, db, using the mysql:8 image.
• environment: sets environment variables inside the container - here the database password and an initial database name. (This is the Compose version of the -e flag you used earlier.)
• volumes: under db mounts a volume at MySQL's data folder, so the data survives docker compose down, just like the volumes chapter taught.
• A top-level volumes: section declares the db-data volume so Compose creates and manages it.

Automatic networking

Here's the best part. You did not create a network, yet web and db can already reach each other. Compose automatically puts all services on a shared network and lets them find each other by service name. So your app would connect to the database using the host name db and port 3306 - no extra configuration.

This is the same name-based networking from the previous chapter, set up for you automatically.

Start the multi-container app

Start both services with one command:

docker compose up -d

Compose creates the network, the volume, and both containers. Check them:

docker compose ps

And when you're done:

docker compose down

To also delete the volume (and its data), add -v:

docker compose down -v

You can now describe a whole multi-part app in one file. Next, let's make that file cleaner and more flexible with build settings and environment variables.

The connection detail everyone gets wrong once

When your app connects to the database, the host is the service name (db), not localhost. This trips up almost everyone: inside a container, localhost means that same container, so pointing the app at localhost for the database fails. Use db (the service name) and the database's internal port 3306. Remember this and you'll skip the single most common Compose connection bug.

FAQ

How do containers in Docker Compose talk to each other?

Compose puts every service on a shared network automatically, and they reach each other by service name. Your app connects to the database at host db (the service name), with no manual network setup.

Why does my app connect to the database with the service name, not localhost?

Because localhost inside a container refers to that container itself. The database runs in a different container, so you address it by its service name (db) on the shared Compose network.

How do I persist database data in Docker Compose?

Give the database service a named volume mounted at its data folder (for MySQL, /var/lib/mysql) and declare that volume in the top-level volumes: section. The data then survives docker compose down.