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projection-patterns — Claude Code skill: projections and setup

Build read models from event streams with guidance on projection types and replay. Useful for CQRS read sides, materialized views, and search indexes.

Install into Claude Code · Plugin marketplace All methods and platforms ↓ 
/plugin marketplace add wshobson/agents
/plugin install backend-development@claude-code-workflows
Open on GitHub

What projection-patterns does

The skill walks through a projection pipeline: an Event Store feeds a projector, which updates a read model. It covers live projections for current-state queries, catchup projections for historical events, persistent projections with checkpoints, and inline projections that share a transaction with writes.

Its guidance covers safe replay, transactions for multi-table updates, lag monitoring, and rebuilding read models. Concrete templates and worked examples are in references/details.md. Events need to be processed in order, and failures should be logged and monitored. Install it when your project derives read models from events; it is not aimed at ordinary query work without event sourcing.

What it helps with

  • build a CQRS read model from events
  • rebuild a materialized view from event history
  • create a search index from an event stream
  • aggregate data across streams
  • resume a projection after failure using a checkpoint

Example prompts for the agent

  • Design a CQRS read model for these events.
  • Show how to rebuild this projection from history and store a checkpoint.
  • How should I create a search index from this event stream while preserving event order?

How to install projection-patterns

1Plugin marketplace

/plugin marketplace add wshobson/agents
/plugin install backend-development@claude-code-workflows

This installs the whole backend-development plugin with all its skills

2Skills CLI (Vercel)

npx skills add wshobson/agents --skill projection-patterns -a claude-code

3Manually: put the files in place

~/.claude/skills/projection-patterns/SKILL.md

Copy the whole skill folder; for one project use .claude/skills/projection-patterns/

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Questions about projection-patterns

How do I install projection-patterns in Claude Code?

Run npx skills add wshobson/agents --skill projection-patterns -a claude-code. The supplied commands also include /plugin marketplace add wshobson/agents followed by /plugin install backend-development@claude-code-workflows.

What does projection-patterns do?

It explains how to build read models and projections from events, covering current-state queries and rebuilding from historical events.

Where are the templates and worked examples?

They are in references/details.md; the skill points to that file for concrete templates.

Source text

SKILL.md · MIT 
---
name: projection-patterns
description: 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.
---

# 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

© Seth Hobson · MIT · wshobson/agents

See also

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