Docker Hub and Docker Compose

Docker Hub

A public (or private) cloud registry of Docker images.

Used for searching and sharing ready-made images.

Address: https://hub.docker.com

How to Choose the Right Image?

When selecting an image, look for:

• Official Image
• Number of downloads
• Date of last update

How to Read Documentation on Docker Hub?

Each image has:

1. Overview: documentation about the image's content and usage
2. Tags
3. Dockerfile
4. Other resources

Python Container Image

On Docker Hub we can find an image with Python:

docker.io/library/python:3.12-slim

Breakdown:

How to Navigate Tags?

On the Python image page we can find e.g.:

• 3.12
• 3.12-slim
• 3.12-alpine
• 3.12-bookworm
• latest

🔎 Meaning of Tags:

How to Read a Dockerfile?

On the image's GitHub page we can find the Dockerfile.

Typical Dockerfile:

FROM debian:bookworm-slim

ENV PYTHON_VERSION 3.12.2

RUN apt-get update && apt-get install -y \
    build-essential \
    libssl-dev

CMD ["python3"]

What We Look For:

• FROM → base image
• RUN → what is installed
• ENV → environment variables
• CMD → default command

This tells us:

• what OS it is based on
• what packages it contains
• whether it is secure

Docker Compose

A tool for configuring multi-container applications.

Microservices

Web applications typically consist of multiple services:

• The application is divided into components that communicate with each other.
• Better scaling
• Simpler development

Components of a Web Application

    HTML
     |
+----------+
| Frontend |
+----------+
      | REST
+----------+  NFS  +----------+
|  Backend |-------| Storage  |
+----------+       +----------+
      | postgres
+----------+
| Database |
+----------+

A typical web application consists of multiple components. Each component has different requirements.

E.g.

1. SQL Database, accessible only from the internal network for application needs
2. Backend: Implements the REST interface and application logic.
3. Frontend: JS Single Page Application, communicates with the BACKEND via the REST interface.

What is Docker Compose?

Docker Compose allows you to:

• Define the entire application in a single YAML file
• Start everything with one command
• Easily manage networks, volumes, and configuration
• Reproducible environment (dev/test/prod)

Virtual Environment Configuration

Uses a YAML file:

docker-compose.yml

docker-compose.yaml

services:
  service_name:
    image: image_name
    ports:
      - "host_port:container_port"
    volumes:
      - volume_name:/path/in/container
    environment:
      VARIABLE: value
    depends_on:
      - other_service

volumes:
  volume_name:

Simple Virtual Environment Startup

Start the application:

docker compose up -d

Stop it:

docker compose down

Structure of compose.yaml

• services: containers, image, ports, environment
• networks: connectivity between services
• volumes: persistent data
• secrets/configs: configuration without baking into the image

Docker Service

• One DNS name
• One or more containers

Service Configuration

• image – Docker image
• ports – port mapping
• environment – environment variables
• volumes – persistent storage
• networks – networks
• cmd: how to start

Docker Named Volume

• Named volumes for mapping, managed by Docker

Where is it physically located?

On Linux, typically:

/var/lib/docker/volumes/

Docker Network

Custom networks between containers.

projectname_default

Within it:

• Each container has an internal IP
• Each service has a DNS record

Example: Deploying WordPress with Docker Compose

WordPress requires:

• PHP + Apache (wordpress image)
• MySQL database

Deployment using docker run

docker network create wordpress_net
docker volume create db_data
docker volume create wordpress_data
docker run -d \
  --name wordpress_db \
  --restart always \
  --network wordpress_net \
  -v db_data:/var/lib/mysql \
  -e MYSQL_ROOT_PASSWORD=example \
  -e MYSQL_DATABASE=wordpress \
  -e MYSQL_USER=wpuser \
  -e MYSQL_PASSWORD=wppassword \
  mysql:8.0
docker run -d \
  --name wordpress_app \
  --restart always \
  --network wordpress_net \
  -p 8080:80 \
  -v wordpress_data:/var/www/html \
  -e WORDPRESS_DB_HOST=wordpress_db:3306 \
  -e WORDPRESS_DB_USER=wpuser \
  -e WORDPRESS_DB_PASSWORD=wppassword \
  -e WORDPRESS_DB_NAME=wordpress \
  wordpress:latest

Deploying WordPress with Compose

docker-compose.yml for WordPress

services:
  db:
    image: mysql:8.0
    container_name: wordpress_db
    restart: always
    environment:
      MYSQL_ROOT_PASSWORD: example
      MYSQL_DATABASE: wordpress
      MYSQL_USER: wpuser
      MYSQL_PASSWORD: wppassword
    volumes:
      - db_data:/var/lib/mysql
  wordpress:
    image: wordpress:latest
    container_name: wordpress_app
    restart: always
    ports:
      - "8080:80"
    environment:
      WORDPRESS_DB_HOST: db:3306
      WORDPRESS_DB_USER: wpuser
      WORDPRESS_DB_PASSWORD: wppassword
      WORDPRESS_DB_NAME: wordpress
    depends_on:
      - db
    volumes:
      - wordpress_data:/var/www/html
volumes:
  db_data:
  wordpress_data:

Healthcheck

depends_on only waits for the container to start, not for when the database is ready.

Solution – healthcheck condition:

services:
  db:
    image: mysql:8.0
    healthcheck:
      test: ["CMD", "mysqladmin", "ping", "-h", "localhost"]
      interval: 10s
      timeout: 5s
      retries: 5

  wordpress:
    depends_on:
      db:
        condition: service_healthy

WordPress will only start once MySQL successfully responds to the ping.

Docker Swarm Mode

Docker's native tool for orchestrating containers in a cluster of multiple servers.

Allows:

• managing multiple nodes as a single unit
• high availability (High Availability)
• horizontal scaling of services
• rolling updates without downtime
• automatic placement of containers

Docker Compose

• Defining and running multi-container applications
• Typically a local development environment
• Single node (single Docker Engine)

Docker Swarm

• Container orchestration in a multi-node cluster
• Production environment
• High availability and scaling

Docker Swarm Mode

• One or more compute nodes
• Connected by a network
• Manager Node
• Worker Node

Docker Swarm Mode

        Internet

   +----------------+  +---------+  +---------+
   | Leader Manager |  | Manager |  | Manager |
   +----------------+  +---------+  +---------+

   +--------+  +--------+  +--------+
   | Worker |  | Worker |  | Worker |
   +--------+  +--------+  +--------+

   +-----+     +-----+
   | NAS |     | SAN |
   +-----+     +-----+

Docker Swarm Mode

• Service resilience against compute node failure.
• Scaling – adding nodes as needed.
• Upgrade or update compute nodes without service interruption.

Manager Node

• Maintains cluster state
• Allocates resources
• Maintains information about services and assigns DNS names.
• RAFT algorithm for electing the "leader" node.

Worker Node

• Starts and stops containers