Docker Basics Summary

1. Application Deployment Methods

There are 3 typical ways to deploy a web application or install a system application:

1.1 Physical Machine

You need to install:

• Hardware
• Operating system
• Servers
• Tools
• Libraries required to develop and run the application
• The application

1.2 Virtual Machine

You need to install:

• Hardware
• Physical operating system
• Virtual machine manager (like VirtualBox)
• Virtual operating system
• Servers on the virtual operating system
• Tools
• Libraries required to develop and run the application on the virtual operating system
• The application itself in your virtual machine

1.3 Container (Recommended)

You need to install:

• Hardware
• Physical operating system
• A container engine to manage containerized applications
• A container with a minimal set of tools, servers, the application and required libraries

Key Advantage: No kernel operating system is required because containers share the host operating system's kernel.

2. Disadvantages of Each Deployment Method

Physical Machines vs VMs/Containers

• Resource utilization: Physical computers often underutilize resources - not taking full advantage of CPU time, disk space or RAM installed
• Scalability: Scaling physical computers involves purchasing and configuring additional hardware, which is time-consuming and expensive
• Isolation and Security: Physical computers lack the isolation capabilities provided by virtual machines and containers, increasing risk of conflicts and security vulnerabilities
• Flexibility and Portability: Applications deployed on physical computers are tightly coupled with the underlying hardware, making them less portable and flexible

Virtual Machines vs Containers

• Resource utilization: Virtual machines consume more resources because each VM requires its own operating system instance. Part of RAM, storage, and CPU resources are used to run the OS, leading to less efficient resource utilization
• Slower Startup Times: VMs typically have slower startup times due to the time required to boot up the OS. Containers start almost instantly since they share the host OS kernel
• Complexity: Managing VMs involves dealing with full-fledged operating systems, which adds complexity to deployment, configuration, and maintenance
• Limited Scalability: While VMs offer scalability, they are less scalable than containers due to higher resource overhead per instance

Containers - General Advantages

✓ Faster
✓ More portable
✓ More efficient (CPU, RAM, disk)
✓ Highly scalable

Only problem: Isolation - Virtual machines provide the best isolation among applications, but containers provide better isolation than physical machines.

3. Container Engines

What is a Container Engine?

A container engine (also known as container runtime) is a software component responsible for creating, managing, and running containers on a host system.

Docker Engine

The most popular container engine:

• Utilizes the Docker daemon dockerd to manage containers and images
• Docker CLI docker for interacting with the daemon

Other Container Engines

• Containerd
• cri-o
• LXD
• Podman

4. Basic Docker Concepts

Key Terminology

• Docker: Platform for developing, shipping, and running applications in containers
• Container: Running instance of an image
• Image: Read-only template with instructions for creating a container
• Build: Process of creating an image from a Dockerfile
• Run: Execute a container from an image
• Dockerfile: Text file with instructions to build a Docker image
• DockerHub: Registry of Docker image repositories

Key Relationships

Dockerfile --[build]--> Image --[run]--> Container (instance)

Docker vs Virtual Machines

• VMs include a complete OS
• Containers share the host operating system's kernel
• Containers are lighter and faster

5. Docker Compose

Tool for defining and running multi-container Docker applications.

Main Features

1. Define Your Application Services: In a docker-compose.yml file (YAML format), you specify the services that make up your application. Each service typically corresponds to a container
2. Environment Variables and Overrides: Use environment variables and override settings using a separate .env file or environment variables passed directly to docker-compose
3. Start Your Application: Use docker-compose up command to:
   • Create necessary networks and volumes
   • Pull Docker images (if not already available locally)
   • Start containers in the correct order based on dependencies
4. Manage Your Application: Docker Compose provides commands for managing your multi-container application

Use Cases

Especially useful for applications with multiple interconnected services:

• Web servers
• Databases
• Caches
• Message brokers

6. Installing Docker Engine on Debian Linux

# Update repositories
sudo apt-get -y update

# Install dependencies
sudo apt-get -y install apt-transport-https ca-certificates curl gnupg2 gpg lsb-release

# Add Docker GPG key
curl -fsSL https://download.docker.com/linux/debian/gpg | sudo gpg --dearmor -o /usr/share/keyrings/docker.gpg

# Add Docker repository
echo "deb [arch=$(dpkg --print-architecture) signed-by=/usr/share/keyrings/docker.gpg] https://download.docker.com/linux/debian trixie stable" | sudo tee /etc/apt/sources.list.d/docker.list > /dev/null

# Update and install Docker
sudo apt-get -y update
sudo apt-get -y install docker-ce docker-ce-cli containerd.io docker-compose-plugin

Post-Installation

# Check Docker service status
sudo systemctl status docker
# Verify the service is started and enabled

# Add your user to the docker group
sudo gpasswd -a $USER docker

# Log out and log in, then verify membership
id
# Should show docker group (984)

# Check Docker version
docker --version

7. Essential Docker Commands

Image Management

Pulling an Image

docker pull <image_name:version>

# Examples:
docker pull photon:3.0
docker pull mysql:8.0

List Images

docker images
# Shows all downloaded images with repository, tag, image ID, created date, and size

Remove an Image

docker rmi <image_id>
# or
docker rmi <image_name:tag>

# Example:
docker rmi mysql:8.0

Search for Images

docker search <image_name>

# Example:
docker search nginx

Container Management

Run a Container

docker run <image_name>

# Run in detached mode (background)
docker run -d <image_name>

# Run with name
docker run --name <container_name> <image_name>

# Run with port mapping
docker run -p <host_port>:<container_port> <image_name>

# Example: Run nginx with port mapping
docker run -d -p 8080:80 --name my-nginx nginx

List Containers

# List running containers
docker ps

# List all containers (including stopped)
docker ps -a

# List only container IDs
docker ps -q

Stop a Container

docker stop <container_id>
# or
docker stop <container_name>

Start a Container

docker start <container_id>
# or
docker start <container_name>

Restart a Container

docker restart <container_id>

Remove a Container

# Must be stopped first
docker rm <container_id>

# Force remove (even if running)
docker rm -f <container_id>

Execute Commands in Running Container

docker exec <container_id> <command>

# Interactive shell
docker exec -it <container_id> /bin/bash
# or
docker exec -it <container_id> sh

View Container Logs

docker logs <container_id>

# Follow log output
docker logs -f <container_id>

# Show last N lines
docker logs --tail 100 <container_id>

Inspect Container

docker inspect <container_id>
# Returns detailed information in JSON format

System Management

View Docker System Information

docker info

View Docker Disk Usage

docker system df

Clean Up Unused Resources

# Remove all stopped containers
docker container prune

# Remove all unused images
docker image prune

# Remove all unused volumes
docker volume prune

# Remove all unused networks
docker network prune

# Clean up everything (use with caution!)
docker system prune

Volume Management

Create a Volume

docker volume create <volume_name>

List Volumes

docker volume ls

Remove a Volume

docker volume rm <volume_name>

Inspect a Volume

docker volume inspect <volume_name>

Network Management

List Networks

docker network ls

Create a Network

docker network create <network_name>

Remove a Network

docker network rm <network_name>

Inspect a Network

docker network inspect <network_name>

Building Images

Build from Dockerfile

docker build -t <image_name:tag> <path_to_dockerfile_directory>

# Example:
docker build -t myapp:1.0 .

Build with custom Dockerfile name

docker build -f <dockerfile_name> -t <image_name:tag> .

Docker Compose Commands

Start Services

docker-compose up

# Start in detached mode
docker-compose up -d

Stop Services

docker-compose down

# Stop and remove volumes
docker-compose down -v

View Logs

docker-compose logs

# Follow logs
docker-compose logs -f

List Services

docker-compose ps

Execute Command in Service

docker-compose exec <service_name> <command>

Quick Reference Table

Note: This document provides an overview of fundamental Docker concepts and commands as presented in https://www.collados.org/asix1/sm1/tasks/sm1act11/sm1act11.html