What is CQRS
Key idea:
CQRS (Command Query Responsibility Segregation) — a pattern where read and write operations use different data models, often different databases. Write-side handles commands (create/update), read-side — queries. Applied at high read-load (social feeds, analytics) or complex domains. Often paired with Event Sourcing. Complexity overhead high — unnecessary for CRUD apps.
Below: details, example, related terms, FAQ.
Details
- Command model: strong consistency, validation, business rules
- Query model: denormalized, optimized for specific read patterns
- Sync between them — via events (eventual consistency)
- Separate scaling: read side 10× replicas, write side stays small
- Popular frameworks: MediatR (.NET), EventStore, Axon (Java)
Example
// Command (write)
class CreateOrderCommand { orderId, customerId, items }
// Query (read)
class OrderSummaryView { orderId, total, status } // denormalizedRelated Terms
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Frequently Asked Questions
CQRS vs simple CRUD?
CRUD — one model for read and write. CQRS — separation, more code, better for complex domains. For simple apps — overkill.
Is Event Sourcing mandatory?
No. CQRS works with regular SQL too. But ES complements well — write side writes events, read side materialises views.
How to sync the read model?
Async via events (Kafka, RabbitMQ). Eventual consistency — read model lags seconds to minutes.