cqrs-implementation

# 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