/cqrs-implementation

name: cqrs-implementation
description: Implement Command Query Responsibility Segregation for scalable architectures. Use when separating read and write models, optimizing query performance, or building event-sourced systems.

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