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Backend · Databases / AGENT SKILL
projection-patterns
0
Build read models, projections, and materialized views from event streams.
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.
Understand the trade-offs.
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Original instructions from the publisher’s 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
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