cqrs-implementation

SKILL.md

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