---
name: Projection Patterns
slug: projection-patterns
category: Data
description: Projection Patterns builds read models and projections from event streams. Use it for implementing CQRS read sides, building materialized views, or optimizing query performance in event-sourced systems.
github: "https://github.com/wshobson/agents/tree/main/plugins/backend-development/skills/projection-patterns"
language: Python
stars: 38761
forks: 4132
install: "npx degit https://github.com/wshobson/agents/tree/main/plugins/backend-development/skills/projection-patterns ~/.claude/skills/projection-patterns"
installs_to: ~/.claude/skills/projection-patterns
source_path: plugins/backend-development/skills/projection-patterns/SKILL.md
collection_size: 25
category_size: 668
collection_url: "https://dirskills.com/collections/wshobson/agents"
added: 2026-08-13T07:37:07.889Z
last_synced: 2026-08-13T07:37:07.889Z
canonical_url: "https://dirskills.com/skills/projection-patterns"
---

# Projection Patterns

Projection Patterns builds read models and projections from event streams. Use it for implementing CQRS read sides, building materialized views, or optimizing query performance in event-sourced systems.

**Install:**

```bash
npx degit https://github.com/wshobson/agents/tree/main/plugins/backend-development/skills/projection-patterns ~/.claude/skills/projection-patterns
```

## README

# 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