🚀 Docker Compose for DevOps Engineers.

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🚀 What is Docker Compose?

Docker Compose is a tool designed to define and run multi-container applications using a simple YAML file. Instead of manually running multiple docker run commands, Compose lets you:

✅ Define services in a docker-compose.yml file
✅ Spin up all containers with a single command
✅ Easily manage dependencies between containers
✅ Pass environment variables and configurations efficiently

📜 What is YAML?

YAML (Yet Another Markup Language or YAML Ain’t Markup Language) is a human-readable format used for writing configuration files.
🔹 Uses .yml or .yaml extensions
🔹 Follows an indentation-based structure
🔹 Easily readable and widely used in DevOps

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🏗️ Task 1: Understanding docker-compose.yml

To get hands-on with Docker Compose, learn how to:

1️⃣ Set up an environment using docker-compose.yml
2️⃣ Configure multiple services within the file
3️⃣ Link containers so they communicate with each other
4️⃣ Use environment variables in the YAML file

🎯 Sample docker-compose.yml File

Here’s a simple example of a docker-compose.yml file that runs an NGINX server and a PostgreSQL database together:

version: '3.8'

networks:
  mynetwork:  # Custom network name

services:
  web:
    image: nginx:latest
    ports:
      - "8080:80"
    depends_on:
      - db
    networks:
      - mynetwork  # Attach web to custom network

  db:
    image: postgres:latest
    environment:
      POSTGRES_USER: myuser
      POSTGRES_PASSWORD: mypassword
      POSTGRES_DB: mydatabase
    networks:
      - mynetwork  # Attach db to the same network

🛠️ Breakdown

✅ networks

• networks: defines a custom bridge network called mynetwork.

• Both web and db services are attached to mynetwork.

• Now, web can connect to db using db:5432 (PostgreSQL default port).

✅ web (NGINX Service)

• Uses nginx:latest image.

• Exposes http://localhost:8080 by mapping host port 8080 → container port 80.

• depends_on: db ensures the database starts first (but doesn’t check if it’s fully ready).

✅ db (PostgreSQL Service)

• Uses postgres:latest image.

• Environment variables configure:

  • POSTGRES_USER: Username (myuser)

  • POSTGRES_PASSWORD: Password (mypassword)

  • POSTGRES_DB: Default database (mydatabase)

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🚀 How to Run?

1️⃣ Start containers:

docker-compose up -d

2️⃣ Stop and remove containers:

docker-compose down

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🛠️ Task 2: Pull and Run a Docker Image

Now, let’s pull and run a Docker container step by step:

1️⃣ Pull a Docker Image

Download an existing image (e.g., NGINX) from Docker Hub:

docker pull nginx:latest

2️⃣ Run the Container as a Non-Root User

By default, Docker runs as root, which is not secure. To run Docker without sudo, add your user to the Docker group:

sudo usermod -aG docker $USER

Reboot the machine to apply the changes:

sudo reboot

3️⃣ Start the Container

Run the NGINX container in detached mode (-d):

docker run -d --name my-nginx -p 8080:80 nginx

🌐 Now, visit http://localhost:8080 to see NGINX in action!

4️⃣ Inspect the Running Container

Check running processes and exposed ports:

docker inspect my-nginx

5️⃣ View Logs

Check the container logs to debug issues:

docker logs my-nginx

6️⃣ Stop and Restart the Container

To stop the container:

docker stop my-nginx

To start it again:

docker start my-nginx

7️⃣ Remove the Container

Once you're done, clean up the container:

docker rm my-nginx

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🎯 Summary

🔹 Docker Compose simplifies managing multi-container apps using a YAML file.
🔹 YAML is a human-friendly configuration format for DevOps tools.
🔹 Running Docker without sudo improves security and ease of use.

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