01 - Docker

Docker

Mental model Dockerfile = how to build an image. Docker Compose = how to run multiple containers together.

Dockerfile vs Docker Compose

Dockerfile Docker Compose

---

Builds an image Defines/runs services Describes application packaging Describes the local environment Usually one application image Can run many containers docker build docker compose up

flowchart LR
    Dockerfile --> Image["Docker Image"]
    Image --> Container["Application Container"]

    Compose["docker-compose.yml"] --> App["App"]
    Compose --> DB["PostgreSQL"]
    Compose --> Kafka["Kafka"]
    Compose --> Redis["Redis"]

Small Dockerfile

FROM eclipse-temurin:21-jre
 
WORKDIR /app
 
COPY target/order-service.jar app.jar
 
ENTRYPOINT ["java", "-jar", "app.jar"]

The important distinction:

Interview question Does a Dockerfile run PostgreSQL and

Kafka?

No. It describes how to build the application image. Compose can then run that application alongside PostgreSQL, Kafka, Redis, etc.

Compose Example

services:
  postgres:
    image: postgres:17
    environment:
      POSTGRES_DB: ecommerce
      POSTGRES_USER: ecommerce
      POSTGRES_PASSWORD: ecommerce
    ports:
      - "5432:5432"
 
  redis:
    image: redis:8
    ports:
      - "6379:6379"
 
  kafka:
    image: apache/kafka:latest
    ports:
      - "9092:9092"

Remember Dockerfile packages. Compose orchestrates the local stack.

Why We Use Compose

Without Compose:

docker run postgres ...
docker run redis ...
docker run kafka ...
docker run order-service ...

With Compose:

docker compose up

This gives every developer a repeatable local environment.

Production Connection

Compose is primarily useful for local development and simple environments.

For production we may use:

Docker image
    ↓
Kubernetes
    ↓
EKS

Interview Questions

Why containerize the application?

Containers give us a consistent runtime, isolate dependencies, simplify deployment, and make the application easier to move between environments.

Is Docker a VM?

No. Containers share the host kernel, whereas a traditional VM includes a guest operating system.

Why not just install PostgreSQL and Kafka locally?

Containers make the development environment reproducible and avoid polluting the developer’s machine with infrastructure-specific installations.

] → use Docker Compose to run Kafka

locally and connect Java/.NET services to it.