System Design Article

Event Sourcing & CQRS

Difficulty: Hard

Event Sourcing stores every change to your application state as an immutable event, and the current state is what you get when you replay them. CQRS splits the read and write paths so each can be optimized independently. Together they unlock auditability, time travel, and read/write scaling that traditional CRUD cannot. They also introduce eventual consistency, schema evolution pain, and a steep operational learning curve. This lesson teaches the mechanics, the implementation patterns (event store, snapshots, projections, sagas), and the honest answer to when these patterns are worth the cost (financial ledgers, audit-heavy domains, complex business workflows) and when they are over-engineering (a typical SaaS CRUD app).