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