What is Docker and How It Works

Docker is one of the most widely used tools in backend development and DevOps. If you have worked with Node.js, NestJS, Python, or any backend framework, you have probably heard developers say:

“Just run it in Docker.”

But what is Docker exactly?

How does Docker work internally?

And how is Docker different from a virtual machine?

This guide explains Docker in simple terms with real-world backend examples.

What is Docker?

Docker is a containerisation platform that allows developers to package an application along with all its dependencies into a single unit called a container.

A Docker container includes:

Application code
Runtime
Libraries
Environment variables
System tools

This ensures the application runs the same way on every system.

Instead of saying:

“It works on my machine.”

Docker ensures it works on every machine.

Why Do We Need Docker?

Before Docker, developers faced common problems:

Different OS environments
Dependency conflicts
Version mismatches
Deployment failures
Server configuration issues

For example:

Your backend may run perfectly on your local system but fail in production because the server has a different Node.js version.

Docker solves this by creating isolated environments called containers.

How Docker Works Internally

To understand how Docker works, you need to understand three core components:

Docker Engine
Docker Image
Docker Container

1. Docker Image

A Docker image is a lightweight, read-only template used to create containers.

It contains:

Base OS (like Alpine Linux or Ubuntu)
Application code
Required dependencies

You define an image using a Dockerfile.

2. Docker Container

A Docker container is a running instance of a Docker image.

Think of it like:

Image → Blueprint

Container → Running App

Multiple containers can be created from a single image.

3. Docker Engine

Docker Engine is the core service that:

Builds images
Runs containers
Manages networking
Handles storage

It works directly with the host operating system kernel.

Docker vs Virtual Machine

Many developers ask:

What is the difference between Docker and a Virtual Machine?

Here is the key difference:

Virtual Machine:

Includes full OS
Heavy
Slower startup time
Uses more resources

Docker Container:

Shares host OS kernel
Lightweight
Starts in seconds
Uses fewer resources

Docker is more efficient because it does not need a full operating system inside each container.

How Docker Containers Share the Host OS

Docker uses Linux kernel features like:

Namespaces
Control Groups (cgroups)

Namespaces provide isolation between containers.

Cgroups control resource usage such as CPU and memory.

This allows multiple containers to run independently on the same system.

Real-World Use Case in Backend Development

In backend development, Docker is commonly used for:

Running Node.js applications
Deploying microservices
Setting up Redis, MongoDB, PostgreSQL
Creating consistent CI/CD pipelines
Containerizing APIs

For example, you can run:

Backend service
Database
Redis cache

All inside separate Docker containers using Docker Compose.

This makes development and deployment predictable and scalable.

Benefits of Using Docker

Using Docker provides:

Consistent development environment
Faster deployment
Easy scaling
Environment isolation
Simplified CI/CD integration

That’s why Docker is widely used in DevOps and backend engineering.

Conclusion

Docker is a containerization platform that packages applications and dependencies into lightweight containers.

It solves the “works on my machine” problem by ensuring consistent environments across development, testing, and production.

Understanding Docker and how it works is essential for backend developers, DevOps engineers, and anyone building scalable applications.