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

text

                    ┌─────────────┐
                    │   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