Docker Compose File Structure

This is what the equivalent YAML file looks like for the MongoDB and Mongo Express setup we did earlier with Docker commands:

version: '3.8'

services:
  mongo:
    image: mongo
    ports:
      - 27017:27017
    environment:
      MONGO_INITDB_ROOT_USERNAME: admin
      MONGO_INITDB_ROOT_PASSWORD: qwerty

  mongo-express:
    image: mongo-express
    ports:
      - 8081:8081
    environment:
      ME_CONFIG_MONGODB_ADMINUSERNAME: admin
      ME_CONFIG_MONGODB_ADMINPASSWORD: qwerty
      ME_CONFIG_MONGODB_URL: mongodb://admin:qwerty@mongo:27017/

Key Things to Note About the YAML File

1. Indentation is Important

Indentation is very important in YAML files — always use proper spacing (spaces, not tabs).

Even a small indentation mistake can break the configuration.

2. Services

All the containers we want to run are defined under services.

Each service represents one container.

Example:

3. No Network Defined

Notice there is no network option in the YAML file.

This is because Docker Compose automatically creates a default network for all containers defined in the YAML file.

So when using a Docker Compose file:

• All services are automatically connected
• Containers can communicate using service names
• No need to manually create a Docker network

Example:

mongo-express → connects to → mongo

Here mongo works as a hostname inside the Docker network.

4. Two Ways to Define Environment Variables

Method 1: List Style (- key=value)

environment:
  - MONGO_INITDB_ROOT_USERNAME=admin
  - MONGO_INITDB_ROOT_PASSWORD=qwerty

Method 2: Key-Value Style (Preferred)

environment:
  MONGO_INITDB_ROOT_USERNAME: admin
  MONGO_INITDB_ROOT_PASSWORD: qwerty

This second format is more readable and commonly preferred.

5. Adding More Services

If we want to add more services in the future (for example Redis, Node.js, or backend APIs), we simply define a new service in the same file.

Example

services:
  mongo:
    image: mongo

  mongo-express:
    image: mongo-express

  redis:
    image: redis

Each service can have:

• Its own image
• Ports
• Environment variables
• Volumes
• Networks (if needed)

6. Why This Structure is Powerful

Docker Compose file structure allows us to define an entire application stack in one place.

Instead of running multiple commands like:

docker run ...
docker run ...
docker network create ...

We define everything in a single YAML file and manage it as one system.

Summary

• Docker Compose uses a YAML file to define containers.
• Each container is called a service.
• Docker Compose automatically creates a default network.
• Environment variables can be defined in two formats.
• Multiple services can be added easily in the same file.
• The structure is simple, scalable, and easy to maintain.