# Projection Patterns

> Build read models and projections from event streams. Use when implementing CQRS read sides, building materialized views, or optimizing query performance in event-sourced systems.

Source: https://skillsagentes.com/skills/wshobson/agents/projection-patterns
Repository: https://github.com/wshobson/agents
Author: wshobson
License: MIT
Updated: hace 2 meses
Context cost: 45 tok installed, 667 tok once triggered, 4.2k tok with every bundled file
Bundle: 2 files, 17 KB
Permissions requested: none declared

## Install

```bash
npx -y skills add wshobson/agents --skill projection-patterns --agent claude-code
```

## What it does

- Enseña arquitectura de proyectores para transformar eventos en modelos de lectura
- Define tipos de proyección: Live, Catchup, Persistent e Inline
- Da buenas prácticas: idempotencia, transacciones, checkpoints, monitoreo de lag
- Provee plantillas detalladas en references/details.md para casos concretos

## Use it when

- Construir modelos de lectura CQRS
- Crear vistas materializadas a partir de eventos
- Optimizar rendimiento de consultas
- Implementar dashboards en tiempo real o índices de búsqueda a partir de eventos

## What triggers it

- "Ayúdame a diseñar un modelo de lectura CQRS a partir de mi event store"
- "Necesito crear una vista materializada desde eventos"
- "Cómo reconstruyo mis proyecciones tras un fallo"

## Before you install

- Requiere un sistema con event sourcing y acceso a `references/details.md` para las plantillas completas.

## Files

- SKILL.md — 3 KB
- references/details.md — 14 KB

## SKILL.md

Reproduced verbatim from wshobson/agents under MIT. This section is the upstream document and is in English.

# Projection Patterns

Comprehensive guide to building projections and read models for event-sourced systems.

## When to Use This Skill

- Building CQRS read models
- Creating materialized views from events
- Optimizing query performance
- Implementing real-time dashboards
- Building search indexes from events
- Aggregating data across streams

## Core Concepts

### 1. Projection Architecture

```
┌─────────────┐     ┌─────────────┐     ┌─────────────┐
│ Event Store │────►│ Projector   │────►│ Read Model  │
│             │     │             │     │ (Database)  │
│ ┌─────────┐ │     │ ┌─────────┐ │     │ ┌─────────┐ │
│ │ Events  │ │     │ │ Handler │ │     │ │ Tables  │ │
│ └─────────┘ │     │ │ Logic   │ │     │ │ Views   │ │
│             │     │ └─────────┘ │     │ │ Cache   │ │
└─────────────┘     └─────────────┘     └─────────────┘
```

### 2. Projection Types

| Type           | Description                 | Use Case               |
| -------------- | --------------------------- | ---------------------- |
| **Live**       | Real-time from subscription | Current state queries  |
| **Catchup**    | Process historical events   | Rebuilding read models |
| **Persistent** | Stores checkpoint           | Resume after restart   |
| **Inline**     | Same transaction as write   | Strong consistency     |

## 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

- **Make projections idempotent** - Safe to replay
- **Use transactions** - For multi-table updates
- **Store checkpoints** - Resume after failures
- **Monitor lag** - Alert on projection delays
- **Plan for rebuilds** - Design for reconstruction

### Don'ts

- **Don't couple projections** - Each is independent
- **Don't skip error handling** - Log and alert on failures
- **Don't ignore ordering** - Events must be processed in order
- **Don't over-normalize** - Denormalize for query patterns

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Skills Agentes — https://skillsagentes.com/skills/wshobson/agents/projection-patterns