CQRS Implementation
Comprehensive guide to implementing CQRS (Command Query Responsibility Segregation) patterns.
When to Use This Skill
- Separating read and write concerns
- Scaling reads independently from writes
- Building event-sourced systems
- Optimizing complex query scenarios
- Different read/write data models needed
- High-performance reporting requirements
Core Concepts
1. CQRS Architecture
┌─────────────┐
│ Client │
└──────┬──────┘
│
┌────────────┴────────────┐
│ │
▼ ▼
┌─────────────┐ ┌─────────────┐
│ Commands │ │ Queries │
│ API │ │ API │
└──────┬──────┘ └──────┬──────┘
│ │
▼ ▼
┌─────────────┐ ┌─────────────┐
│ Command │ │ Query │
│ Handlers │ │ Handlers │
└──────┬──────┘ └──────┬──────┘
│ │
▼ ▼
┌─────────────┐ ┌─────────────┐
│ Write │─────────►│ Read │
│ Model │ Events │ Model │
└─────────────┘ └─────────────┘
2. Key Components
| Component | Responsibility |
|---|---|
| Command | Intent to change state |
| Command Handler | Validates and executes commands |
| Event | Record of state change |
| Query | Request for data |
| Query Handler | Retrieves data from read model |
| Projector | Updates read model from events |
Templates and detailed worked examples
Full template library and detailed worked examples live in references/details.md. Read that file when you need the concrete templates.
Best Practices
Do's
- Separate command and query models - Different needs
- Use eventual consistency - Accept propagation delay
- Validate in command handlers - Before state change
- Denormalize read models - Optimize for queries
- Version your events - For schema evolution
Don'ts
- Don't query in commands - Use only for writes
- Don't couple read/write schemas - Independent evolution
- Don't over-engineer - Start simple
- Don't ignore consistency SLAs - Define acceptable lag