CQRS and Event Sourcing

CQRS and event sourcing are two of the most powerful — and most misunderstood — architectural patterns. They are often discussed together but are independent concepts. You can use CQRS without event sourcing and event sourcing without CQRS. Understanding each separately, then seeing how they combine, gives you the clearest mental model.

CQRS — Command Query Responsibility Segregation

The core idea is simple: use different models for reading and writing data.

In a traditional architecture, the same model handles both:

Traditional CRUD:
 
┌──────────┐     ┌──────────────┐     ┌──────────┐
│  Client   │────▶│   Service    │────▶│ Database │
│          │◀────│              │◀────│          │
│ Read AND │     │ Same model   │     │ Same     │
│ Write    │     │ for both     │