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Essence of Backend Engineering: Docker

Essence of Backend Engineering: Docker

What is Docker?

From the official docs:

Docker is an open platform for developing, shipping, and running applications.

Docker simplifies the process of building, distributing, and running applications by packaging them into images that can be run consistently across different machines.

Docker allows you to:

  • Pull images for any OS or platform
  • Build your own application into an image
  • Run images as isolated containers

As long as Docker is installed, containers can run on any machine.

From Servers to Containers

Before Docker, applications were deployed on either physical servers or virtual machines.

  • Physical servers required one application per machine, each with its own OS and hardware resources, leading to high costs, difficult maintenance, and poor scalability.
  • Virtual machines improved utilization by running multiple OSs on the same hardware via a hypervisor, but each VM still required a full operating system, resulting in slow startup times and heavy resource usage.

Containers solve these problems by providing lightweight, isolated environments that share the host OS kernel instead of duplicating entire operating systems. This makes them faster to start, more efficient in resource usage, and easier to scale.

By virtualizing at the OS level rather than the hardware level, Docker enables consistent environments across development and deployment, effectively eliminating the classic “it works on my machine” problem.

Docker Images & Containers

Docker Image

A Docker image is a collection of configurations and instructions used to create containers.

  • Images are executed by Docker Engine
  • Many free images exist on Docker Hub, including:
    • Node.js
    • PostgreSQL
    • Java
    • Ubuntu

Docker Hub: https://hub.docker.com/ (opens in a new tab)

Docker Container

A container is a running instance of a Docker image.

Installing Docker

Windows

Mac

Linux

Create a free Docker Hub account at: https://hub.docker.com/ (opens in a new tab)

Docker CLI

The Docker CLI lets you manage images and containers from the terminal.

Common Commands

  • docker pull <image> – Download an image from Docker Hub
  • docker run --name <name> <image> – Run a container
    Useful flags:
    • -d: Run in background
    • -t: Allocate terminal
    • -p: Map ports
  • docker ps – View running containers
  • docker images – View local images
  • docker exec – Run a command inside a container

Dockerfiles

A Dockerfile is a text-based file containing instructions for building a Docker image.

Think of it as a recipe.

Dockerfile Notes

  • The first instruction must be FROM
  • Common instructions:
    • FROM, RUN, COPY, WORKDIR, CMD
  • Only one CMD instruction is allowed (the last one wins)

RUN vs CMD

  • RUN: executes commands during image build
  • CMD: specifies the command run when the container starts

Pushing Images to Docker Hub

Why Push?

  • Share images publicly
  • Allow others to run your app easily

Steps

docker login
docker tag <current-name> <username>/<image-name>:<tag>
docker push <username>/<image-name>:<tag>

View your image at: https://hub.docker.com/ (opens in a new tab)

Extending Docker

Docker Compose

  • Multi-container applications
  • Defined in a single YAML file

Kubernetes

  • Container orchestration
  • Handles scaling, deployment, and maintenance
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