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CQRS Pattern
Definition
Command Query Responsibility Segregation (CQRS) separates read and write operations into different models. Commands handle updates and writes, while queries handle reads. This separation allows independent optimization, scaling, and security for each operation type.
When to Use It
- Read and write workloads have different performance characteristics
- You need to scale reads and writes independently
- Complex business logic for updates but simple reads
- Different teams work on read and write sides
- You need to support multiple query models
- Domain-driven design with complex aggregates
Pros & Cons
✓ Pros
- Independent scaling of reads and writes
- Optimized data models for each operation
- Improved performance and scalability
- Better security segregation
- Simplified complex domain models
- Multiple read models possible
✗ Cons
- Increased system complexity
- Eventual consistency between models
- More code to maintain
- Synchronization overhead
- Learning curve for the team
- Not suitable for simple CRUD apps
Architecture Diagram
┌──────────┐
│ Client │
└─────┬────┘
│
┌─────────────┴─────────────┐
│ │
Write │ (Commands) Read │ (Queries)
▼ ▼
┌───────────────┐ ┌───────────────┐
│ Command │ │ Query │
│ Handler │ │ Handler │
└───────┬───────┘ └───────┬───────┘
│ │
▼ │
┌───────────────┐ │
│ Write Model │ │
│ (Domain) │ │
└───────┬───────┘ │
│ │
▼ │
┌───────────────┐ │
│ Write DB │ │
│ (Normalized) │ │
└───────┬───────┘ │
│ │
│ Event/Sync │
▼ │
┌───────────────┐ │
│ Event Stream │ │
│ (Propagation) │ │
└───────┬───────┘ │
│ │
│ Update │
▼ │
┌───────────────┐ │
│ Read Model │ │
│ (Denormalized)│◄────────────────┘
│ │
│ • Read DB 1 │
│ • Read DB 2 │
│ • Cache │
└───────────────┘
Key Points:
• Commands change state, don't return data
• Queries return data, don't change state
• Read models are eventually consistent
• Multiple optimized read models possible