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What is Docker?

Docker is a container management service. The keywords of Docker are develop, ship, and run anywhere. The whole idea of Docker is for developers to easily develop applications, ship them into containers which can then be deployed anywhere.

Docker was first released in March 2013 and since then has become an essential tool for modern software development, especially in Agile-based projects.

Features of Docker

  • Reduced footprint - Docker reduces the size of development by providing a smaller footprint of the operating system via containers.

  • Seamless collaboration - With containers, it becomes easier for teams across different units (Development, QA, and Operations) to work seamlessly across applications.

  • Portable deployment - You can deploy Docker containers anywhere, on any physical and virtual machines, and even on the cloud.

  • Scalability - Since Docker containers are lightweight, they are very easily scalable.

What is a Container?

A container is a runnable instance of an image. You can create, start, stop, move, or delete a container using the Docker API or CLI.

Container Characteristics

  • Isolated - Containers are isolated from each other and run their own software, binaries, and configurations
  • Portable - Can be run on any OS
  • Ephemeral - Can be started and stopped quickly
  • Lightweight - Uses the host OS kernel, no full OS needed

What is a Container Image?

When running a container, it uses an isolated filesystem. This custom filesystem is provided by a container image. Since the image contains the container's filesystem, it must contain everything needed to run an application:

  • All dependencies
  • Configuration files
  • Scripts
  • Binaries
  • Environment variables
  • Default command to run
  • Other metadata

Docker vs Virtual Machines

Aspect Docker Containers Virtual Machines
OS Shares host OS kernel Full guest OS
Size Small (MBs) Large (GBs)
Startup Seconds Minutes
Performance Near-native Slight overhead
Isolation Process-level OS-level

Docker Architecture

┌─────────────────────────────────────────────────────────┐
│                     Docker Client                        │
│                    (CLI / API)                           │
└───────────────────────────┬─────────────────────────────┘
                            │
                            ▼
┌─────────────────────────────────────────────────────────┐
│                   Docker Daemon                         │
│              (Builds, runs, distributes)                │
└───────────────────────────┬─────────────────────────────┘
                            │
                            ▼
┌─────────────────────────────────────────────────────────┐
│                    Docker Registry                       │
│              (Docker Hub, private registry)              │
└─────────────────────────────────────────────────────────┘

Common Use Cases

  1. Development - Consistent development environments across teams

  2. CI/CD - Build, test, and deploy in containers

  3. Microservices - Each service in its own container

  4. Legacy Applications - Containerize old apps for easier deployment

  5. Data Processing - Isolated environments for data tasks

Key Docker Concepts

Concept Description
Image Read-only template with instructions for creating a container
Container Runnable instance of an image
Dockerfile Text document with instructions to build an image
Docker Compose Tool for defining and running multi-container apps
Registry Storage and distribution system for named Docker images

Next Steps