Introduction to Docker Compose

Intro to Docker Compose
In this session we will learn the basics of Docker Compose and how to manage multi-container applications easily.

What are Multi-Container Applications?
Modern cloud-native applications are often composed of multiple smaller services that work together to form a complete application.

This is known as the microservices pattern. These could include:

Web front-end
Database
Authentication service
and so on.

Challenges with Microservices:
Managing and deploying multiple microservices can be complex and cumbersome, requiring careful orchestration of each service.

What is Docker Compose?
Docker Compose is a tool that allows you to define and manage multi-container Docker applications. It uses a declarative configuration file, typically in YAML format, to specify the services, networks, and volumes required for the application.

Benefits of Docker Compose:
Simplifies the orchestration of multi-container applications.
Allows for a single configuration file to define and manage all services.
Integrates with version control systems for better management.
Basic Docker Compose Commands
Check Installation:

docker compose version

Start an Application:

docker compose up

Stop an Application:

docker compose down

View Container Status:

docker compose ps

Simple Docker Compose Example
Let's create a basic Docker Compose setup with a web server using nginx:alpine image and a redis service using redis:alpine image.

Step 1: Create a Directory
Create a directory for your project.

mkdir myapp
cd myapp

Step 2: Create Docker Compose File
Create a file named docker-compose.yml and add the following content:

version: '3.8'

services:
  web:
    image: nginx:alpine
    ports:
      - "80:80"
  redis:
    image: redis:alpine

This file defines two services: a web server (nginx) and a Redis server.

Step 3: Start the Application
Run the following command to start the services:

docker compose up

You should see output indicating the services are starting.

Expected output:

Step 4: Verify the Setup
Check localhost:

curl localhost

You should see the default Nginx welcome page.

Expected output:

Step 5: View Status
Check the status of the running containers:

docker compose ps
#or
docker ps

Expected output:

Step 6: Stop the Application
Stop the services with:

docker compose down

=====####====
Let's discuss Another Example:
Let's investigate docker-compose.yml to understand.

version: '3.5'
services:
  api:
    build: .
    volumes:
      - "./app:/src/app"
    ports:
      - "1338:1338"
    depends_on:
      - db
      - cache
    networks:
        - test_nw
    environment:
      - DATABASE_HOST=mongodb://db:27017
      - REDIS_CACHE_HOST=redis://cache:6379
      - PORT=1338
  db:
    image: mongo:latest
    ports:
      - "27017:27017"
    networks:
        - test_nw
  cache:
    image: redis:latest
    ports:
      - "6379:6379"
    networks:
      - test_nw
networks:
  test_nw:
    driver: bridge

This is a simple docker-compse.yml example to deploy a Nodejs backend with MongoDb and Redis.

The Commands
Of course docker-compose has commands. Whaat! Okay I' will give you the most basic commands. I'm not super duper software engineer and I can live my life with these commands.

docker compose up

is a command that will look for docker-compose.yml by default and will process the docker-compose.yml, create the environment and run the services.
-d means that terminal is yours, it runs the command detachable mode
-f #non-standard-compose.yml-name# means that you can pass a compose.yml file with different name. Usually projects contains more than one docker-compose files. You can have compose file for production and development or you can seperate applications and tools in a different compose files.

docker compose down

is a command that will look for running compose.yml file and shutdown containers then remove all of them including networks, volumes etc.
docker compose log

is a command that will look for running compose.yml file and displays log which are generated by the containers.
Top Level Definitions

• version :
  Defined by the Compose Specification for backward compatibility. It is only informative, and you'll receive a warning message that it is obsolete if used.

• services :
  A service is an abstract definition of a computing resource within an application which can be scaled or replaced independently from other components. These services run in their containers and can communicate with each other.

• network :
  A layer that allows containers to communicate with each other.

• volume:
  Volumes are persistent data stores implemented by the container engine. Compose offers a neutral way for services to mount volumes, and configuration parameters to allocate them to infrastructure.

Service Definitions

• build:
  Lets you define the Dockerfile path to build when the compose file is being processed.

• volumes :
  Lets you define service-level persisted volumes to mount local files and folders.

• ports:
  Lets you expose container ports. The left-hand of the definition is the localhost address and the right-hand will be the container port. It basically means binding port 1338 of the container to localhost:1338.

• depends_on :
  option in Docker Compose is used to specify the dependencies between different services defined in your docker-compose.yml file.

• networks:
  option in Docker Compose is used to specify which network will be used by this container.

• environment:
  option in Docker Compose is used to specify environment variables which will be passed to container.

Conclusion
You have learned the basics of Docker Compose, created a simple multi-container application, and practiced using basic commands. This knowledge will help you manage more complex applications in the future.