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projection-patterns

projection-patterns

github.com/wshobsonDevelopment41.9k10.7k installsUpdated 2w ago
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About projection-patterns

A guide to designing projections and read models for event-sourced systems. It is intended for developers implementing CQRS, materialized views, and event-driven query models, and covers projection types, replay safety, checkpoints, and rebuilds.

Key features

  1. 01Projection architecture patterns
  2. 02Live, catchup, persistent, and inline types
  3. 03Idempotency and transaction guidance
  4. 04Checkpointing and lag monitoring
  5. 05Read model rebuild planning

Use cases

  1. 01Building CQRS read models
  2. 02Creating materialized views from events
  3. 03Generating search indexes from event streams

Install

Copy the command and run it in a terminal.

$npx skills add https://github.com/wshobson/agents --skill projection-patterns

After installing, type/projection-patternsin Claude Code to confirm it loaded.

Bundled filesreferences/details.md

Details

From the project 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
TypeDescriptionUse Case
LiveReal-time from subscriptionCurrent state queries
CatchupProcess historical eventsRebuilding read models
PersistentStores checkpointResume after restart
InlineSame transaction as writeStrong 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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