CQRS and Event Sourcing Intro

When to split read and write models in LLD discussions.

Expert14 min read

Read these first

Hexagonal Architecture

Why this matters

Command vs query — detailed pointwise breakdown in the section below.
Projections — detailed pointwise breakdown in the section below.
Event store sketch — detailed pointwise breakdown in the section below.
When not to — detailed pointwise breakdown in the section below.
Games and architecture patterns signal staff+ depth.

Command

Event storeAppend-only log

Read projection

Commands write events; projections build read models from the log.

Overview

When to split read and write models in LLD discussions. Study each section below in order — every list is interview-ready reference material.

Command vs query

Points to cover

Definition — what Command vs query 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

Projections

Points to cover

Definition — what Projections 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

Event store sketch

Points to cover

Definition — what Event store sketch 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

When not to

Points to cover

Definition — what When not to 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 "Command vs query" on paper first, then in code
public final class CqrsEventSourcingIntroExample {
    // 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.

Command vs query — review this slice before mock interviews.
Projections — review this slice before mock interviews.
Event store sketch — review this slice before mock interviews.
When not to — review this slice before mock interviews.

Test yourself

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