projection-patterns

★ 37,559

by wshobson · part of wshobson/agents

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.

🧩 One of 7 skills in the wshobson/agents package — works on its own, and pairs well with its siblings.

This is the playbook your agent receives when the skill activates — you don't need to read it to use the skill, but it's here to audit before installing.

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