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Docker

Docker packages applications into containers — lightweight, isolated environments with everything needed to run (code, runtime, libraries). Key concepts: Image (blueprint, built from Dockerfile), Container (running instance of image), Docker Compose (multi-container orchestration). Containers share the host OS kernel (unlike VMs). Benefits: consistency across environments, fast startup, easy scaling.

Key Concepts

Deep Dive: Docker vs VMs 
VM:                          Docker:
┌──────────┐ ┌──────────┐   ┌──────────┐ ┌──────────┐
│  App A   │ │  App B   │   │  App A   │ │  App B   │
├──────────┤ ├──────────┤   ├──────────┤ ├──────────┤
│ Guest OS │ │ Guest OS │   │  Bins/   │ │  Bins/   │
├──────────┤ ├──────────┤   │  Libs    │ │  Libs    │
│ Hypervisor               │ ├──────────┴─┴──────────┤
│                          │ │    Docker Engine       │
├──────────────────────────┤ ├────────────────────────┤
│      Host OS             │ │      Host OS           │
└──────────────────────────┘ └────────────────────────┘
Feature VM Container
Size GBs MBs
Startup Minutes Seconds
Isolation Full OS Process-level
Performance Overhead Near-native
Deep Dive: Dockerfile 
# Multi-stage build for Java Spring Boot
FROM eclipse-temurin:21-jdk AS builder
WORKDIR /app
COPY pom.xml .
COPY src ./src
RUN mvn clean package -DskipTests

FROM eclipse-temurin:21-jre
WORKDIR /app
COPY --from=builder /app/target/*.jar app.jar
EXPOSE 8080
ENTRYPOINT ["java", "-jar", "app.jar"]

Key instructions: | Instruction | Purpose | |-------------|---------| | FROM | Base image | | WORKDIR | Set working directory | | COPY / ADD | Copy files into image | | RUN | Execute command during build | | EXPOSE | Document port (doesn't publish) | | ENTRYPOINT | Command to run on container start | | CMD | Default args for ENTRYPOINT | | ENV | Set environment variable |

Deep Dive: Docker Compose 
version: '3.8'
services:
  app:
    build: .
    ports:
      - "8080:8080"
    environment:
      - SPRING_PROFILES_ACTIVE=prod
      - DB_HOST=postgres
    depends_on:
      - postgres
      - redis

  postgres:
    image: postgres:15
    environment:
      POSTGRES_DB: mydb
      POSTGRES_PASSWORD: secret
    volumes:
      - pg_data:/var/lib/postgresql/data

  redis:
    image: redis:7-alpine
    ports:
      - "6379:6379"

volumes:
  pg_data:

Commands: docker compose up -d, docker compose down, docker compose logs

Deep Dive: Best Practices
  • Use multi-stage builds to reduce image size
  • Use .dockerignore to exclude unnecessary files
  • Don't run as root — use USER instruction
  • Pin versions — FROM node:18.17 not FROM node:latest
  • Minimize layers — combine RUN commands
  • Use health checks — HEALTHCHECK CMD curl -f http://localhost:8080/actuator/health
Common Interview Questions
  • What is Docker? How is it different from a VM?
  • What is a Dockerfile? Explain key instructions.
  • What is Docker Compose? When would you use it?
  • What is a multi-stage build? Why use it?
  • What is the difference between CMD and ENTRYPOINT?
  • How do you persist data in Docker?
  • What is a Docker volume?
  • How do containers communicate with each other?