Implement Command Query Responsibility Segregation

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