projection-patterns

SKILL.md

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