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Backend · Databases / AGENT SKILL
cqrs-implementation
0
Implement CQRS to separate read and write models and improve query performance.
Implement Command Query Responsibility Segregation for scalable architectures. Use when separating read and write models, optimizing query performance, or building event-sourced systems.
Understand the trade-offs.
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Original instructions from the publisher’s SKILL.md
# 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