CQRS (Command Query Responsibility Segregation)

CQRS is a pattern that separates read and update operations for a data store. Instead of using a single model for both, you have a Command Model (Write) and a Query Model (Read).

When to Use

• High Disparity: Read load is 1000x higher than Write load (or vice versa).
• Complex UI: The UI needs data shaped differently than the way it’s stored (e.g., Dashboard aggregation).
• Event Sourcing: CQRS is almost mandatory for Event Sourcing.

Quick Start (Conceptual)

// Command Side (Write) - Optimized for Integrity
class CreateOrderCommand { ... }
class OrderAggregate {
    create(cmd: CreateOrderCommand) {
        // Validate business rules
        // Save to DB (Normalized / Event Store)
    }
}

// Query Side (Read) - Optimized for Speed
class GetOrderSummaryQuery { ... }
class OrderSummaryProjector {
    // Listens to "OrderCreated" event
    on(event) {
        // Update a flat "Read DB" (e.g., ElasticSearch, Redis, Document DB)
        // optimized for the specific UI view
    }
}

Core Concepts

Separation

• Commands: Intent to change state. Void return type (or Ack). Strict validation.
• Queries: Request for data. No side effects. Returns DTOs.

Synchronization

The Read DB is eventually consistent with the Write DB. Logic (Projectors) syncs them via Events.

Best Practices

Do:

• Start with Logical CQRS (Separate classes, same DB) before Physical CQRS (Separate DBs).
• accept Eventual Consistency in the UI (Optimistic UI updates).

Don’t:

• Don’t use CQRS for simple CRUD (It adds massive complexity).
• Don’t try to make the Read side fully real-time synchronized (You’ll lose the scaling usage).

Troubleshooting

References

• MSDN CQRS Guide
• Greg Young – CQRS Documents