What is Docker Compose?

The problem Compose solves

In the last chapter, running an app with a database meant: create a network, run the database with the right flags, run the app with the right flags and ports, and remember all of it. For anything beyond one container, that's a lot of commands to type and keep in sync.

Docker Compose lets you describe all of that in a single file called docker-compose.yml, and then bring the whole thing up (or down) with one command.

What Compose gives you

• One file describes every part of your app - each container, its image, ports, volumes and environment.
• One command starts everything: docker compose up.
• Automatic networking - Compose puts all your containers on a shared network, so they can reach each other by name automatically (no manual docker network create).
• Reproducible - the file lives in your project, so anyone can start the exact same setup.

Docker Compose services explained

In Compose, each container is called a service. A typical web app might have two services: app (your application) and db (its database). You describe each service once, and Compose handles running them together.

YAML in one minute

Compose files are written in YAML, a simple text format based on indentation. Two things to know:

• Indentation (spaces, not tabs) shows structure - items indented under a line "belong to" it.
• key: value sets a value; a - starts a list item.

You'll see plenty of examples in the next lessons, so it'll become familiar quickly.

Let's write our first docker-compose.yml.

The one thing that breaks Compose files: indentation

YAML uses spaces for structure, and it does not allow tabs. This bites almost everyone at least once: a stray tab or a misaligned line produces a confusing error and the file won't run. Use two spaces per level, stay consistent, and if Compose complains about the file, suspect indentation before anything else. Set your editor to insert spaces instead of tabs and most of this pain disappears.

FAQ

What is Docker Compose used for?

Running multi-container applications. You describe every service (container) in one docker-compose.yml file, then start them all together with docker compose up - including their networking and volumes - instead of typing many separate docker run commands.

Is Docker Compose separate from Docker?

Compose is part of the modern Docker install (the docker compose command). It builds on the same engine - it's just a friendlier way to run several containers at once.

What is a service in Docker Compose?

A service is one container definition in your Compose file - its image, ports, volumes, and environment. A typical app has a couple of services, such as app and db.