Projection Patterns

by wshobson46891e7e60daNo licenseListed Oct 8, 2026Updated Oct 8, 2026

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.

Instructions onlySoftware Development

AI-generated overview

Guidance for building CQRS read models and projections from event streams in event-sourced systems.

What it does: Explains projection architecture, showing how events flow from an event store through a projector into a read model database. Describes projection types such as live, catchup, persistent and inline, and lists best practices and pitfalls for idempotency, checkpoints, ordering and rebuilds. Points to a bundled reference file for templates and worked examples.
When to use it: Use when implementing the read side of CQRS, building materialized views or search indexes from events, optimizing query performance, or aggregating data across event streams. Also relevant when designing real-time dashboards on event-sourced data. Not a fit for systems that are not event-sourced.
Requirements: No scripts or tools required; it is an instructions-only skill that references a bundled details document.

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   │ │└─────────────┘     └─────────────┘     └─────────────┘

┌─────────────┐     ┌─────────────┐     ┌─────────────┐│ 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

Source and attribution

Source:wshobson/agentsinplugins/backend-development/skills/projection-patternsat commit46891e7

License: No license

Content belongs to its original authors. SourceWeft indexes it from a public repository.

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Projection Patterns

Projection Patterns

When to Use This Skill

Core Concepts

1. Projection Architecture

2. Projection Types

Templates and detailed worked examples

Best Practices

Do's

Don'ts

Source and attribution

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