What is Docker?

Definition

Docker is an open-source platform that automates application deployment using containerization. A container packages an application with all its dependencies (libraries, runtime, system tools) into a single, portable unit that runs consistently on any system with Docker installed.

Unlike virtual machines, containers share the host OS kernel, making them significantly lighter and faster to start. Docker was released in 2013 and has become the de facto standard for container technology.

Core Concepts

• Images: Read-only templates that define a container's filesystem and configuration. Built from Dockerfiles.
• Containers: Running instances of images. Isolated, lightweight, and ephemeral.
• Dockerfile: A text file with instructions for building an image (base image, dependencies, commands).
• Docker Compose: A tool for defining and running multi-container applications using a YAML file.
• Volumes: Persistent data storage that survives container restarts.
• Networks: Isolated networking for containers to communicate securely.
• Registry: A repository for storing and distributing images (Docker Hub, GitHub Container Registry).

Why Use Docker

Docker solves the "works on my machine" problem and brings several benefits:

• Environment consistency: Development, staging, and production use identical containers.
• Isolation: Each service runs in its own container with its own dependencies.
• Fast deployment: Containers start in seconds, enabling rapid scaling and rollbacks.
• Resource efficiency: Containers use far less memory and CPU than virtual machines.
• CI/CD integration: Containers fit naturally into continuous integration and deployment pipelines.
• Microservices: Each microservice can be containerized and scaled independently.

Code Example

# Dockerfile for a Python FastAPI application
FROM python:3.12-slim

WORKDIR /app

COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt

COPY . .

EXPOSE 8000

CMD ["uvicorn", "main:app", "--host", "0.0.0.0", "--port", "8000"]

Frequently Asked Questions

What is the difference between Docker and Kubernetes?

Docker creates and runs containers. Kubernetes orchestrates containers at scale - managing deployment, scaling, networking, and health monitoring across multiple hosts. Docker is for individual containers; Kubernetes manages fleets of them. They're complementary, not competitors.

Is Docker necessary for development?

Not strictly necessary, but highly recommended. Docker ensures consistent environments across team members and between development and production. It eliminates dependency conflicts and makes onboarding new developers faster.