Layered Architecture in Practice

Controller, service, domain, repository — who calls whom.

Intermediate10 min read

Read these first

From Whiteboard to Java Skeleton

Why this matters

Layer responsibilities — detailed pointwise breakdown in the section below.
Dependency rule — detailed pointwise breakdown in the section below.
CampusOS booking service — detailed pointwise breakdown in the section below.
Thin controllers — detailed pointwise breakdown in the section below.
Production Java structure interviewers expect in backend LLD.

Dependency direction

Controller / FacadeHTTP or API entry

Serviceuse cases, orchestration

Domainentities, rules, invariants

Repositorypersistence port

Dependencies point inward — domain never imports infrastructure.

Overview

Controller, service, domain, repository — who calls whom. Study each section below in order — every list is interview-ready reference material.

Layer responsibilities

Points to cover

Definition — what Layer responsibilities means in one clear sentence
When to use — the force or smell that triggers this concept
CampusOS example — where this appears in parking, booking, or library modules
How to explain — pointwise bullets you can say aloud in an interview
Common mistake — what weak candidates do instead
Refactor move — one concrete change that improves the design

Dependency rule

Points to cover

Definition — what Dependency rule means in one clear sentence
When to use — the force or smell that triggers this concept
CampusOS example — where this appears in parking, booking, or library modules
How to explain — pointwise bullets you can say aloud in an interview
Common mistake — what weak candidates do instead
Refactor move — one concrete change that improves the design

CampusOS booking service

Points to cover

Definition — what CampusOS booking service means in one clear sentence
When to use — the force or smell that triggers this concept
CampusOS example — where this appears in parking, booking, or library modules
How to explain — pointwise bullets you can say aloud in an interview
Common mistake — what weak candidates do instead
Refactor move — one concrete change that improves the design

Thin controllers

Points to cover

Definition — what Thin controllers means in one clear sentence
When to use — the force or smell that triggers this concept
CampusOS example — where this appears in parking, booking, or library modules
How to explain — pointwise bullets you can say aloud in an interview
Common mistake — what weak candidates do instead
Refactor move — one concrete change that improves the design

Java

// CampusOS — apply "Layer responsibilities" on paper first, then in code
public final class LayeredArchitectureExample {
    // 1. Name entities from the problem statement
    // 2. Draw relationships before writing methods
    // 3. Introduce patterns only when a force appears
}

Quick recall

Everything you need if you only revisit this box.

Layer responsibilities — review this slice before mock interviews.
Dependency rule — review this slice before mock interviews.
CampusOS booking service — review this slice before mock interviews.
Thin controllers — review this slice before mock interviews.

Test yourself

Answer these before moving on — recall is what makes it stick.