Module 5 Lesson 1: Introduction to Docker Compose

Until now, we have been running single containers. But a real application usually needs multiple things:

• A Web Frontend.
• An API Backend.
• A Database.
• A Redis Cache.

Running these manually with docker run requires 4 terminal windows and a lot of typing. Docker Compose is the tool that lets you manage all of them as a single group.

1. What is Docker Compose?

Docker Compose is a tool for defining and running multi-container Docker applications. With Compose, you use a YAML file to configure your application’s services. Then, with a single command, you create and start all the services from your configuration.

The "Instruction Manual" Analogy:

• docker run: You are the cook, performing one step at a time ("Boil water", "Add pasta").
• docker-compose: You are the Head Chef. You provide a full recipe, and the kitchen (Docker) executes everything at once.

---

2. Why Use Compose?

1. Reproducibility: You can share one docker-compose.yml file with your whole team. Everyone will have the exact same Setup (Ports, Networks, Volumes).
2. Order: No more remembering which container needs to start first.
3. Efficiency: Stop all services with one command (docker-compose down) and start them all with another (docker-compose up).
4. Isolation: Compose automatically creates a dedicated network for your services so they can talk to each other without polluting your default bridge.

---

3. The YAML Format

Docker Compose uses YAML (Yet Another Markup Language). It is very sensitive to Indentation.

version: '3.8'           # The version of the Compose file format
services:                # Defining our "Apps"
  web:                   # A service named 'web'
    image: nginx         # Use the nginx image
    ports:               # Map ports
      - "80:80"

---

4. Compose vs. Orchestrators

• Docker Compose: Best for development and single-server production. It is "Small Scale."
• Kubernetes (K8s) / Docker Swarm: Best for managing thousands of containers across hundreds of servers. This is "Large Scale."

For 90% of developers, Docker Compose is the most important tool they will use every day.

---

Exercise: The Compare and Contrast

1. Look at the docker run command you wrote for WordPress in Module 4.
2. Now, imagine you also need a mysql database for that WordPress site.
3. How many different flags (ports, env-vars, networks, volumes) would you need to type to get both of them talking to each other manually?
4. Why is a "Text File" (YAML) better to store these settings than your terminal "Command History"?

---

Summary

Docker Compose turns "One-off commands" into "Documented Infrastructure." It is the bridge between a hobbyist running a container and a professional engineer managing an application stack.

Next Lesson: your first recipe: Writing your first docker-compose.yml.