Projection Patterns

From the author

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.

— the description by Seth Hobson

AutoGineer summary

Written by AutoGineer from the skill’s own files, not by the author

This skill provides a framework for building projections that transform event streams into queryable read models. It covers architectural concepts, different projection types, and implementation patterns for CQRS-based systems. The content includes best practices for ensuring idempotent processing, managing checkpoints, and handling event ordering, alongside code templates for building projectors in Python.

What it does

Explains the architecture of event-sourced projections
Categorizes projection types including live, catchup, persistent, and inline
Provides Python code templates for base projector classes
Offers implementation patterns for handling specific event types
Outlines best practices for idempotent event processing
Details strategies for managing checkpoints and projection rebuilds

When to use it

When implementing CQRS read sides
When creating materialized views from event streams
When optimizing query performance in event-sourced systems
When building search indexes or real-time dashboards from events
When aggregating data across multiple event streams

Advantages

Promotes decoupled projection design
Ensures reliable state reconstruction through checkpointing
Provides structured patterns for handling event-driven updates
Supports consistent read model maintenance

Usage examples

“How do I structure a class to handle multiple event types for a read model?”: The skill provides a base Projection class and an example OrderSummaryProjection that maps specific event types to dedicated handler methods.
“What are the best practices for ensuring my projections are robust?”: The skill advises making projections idempotent, using transactions for updates, storing checkpoints to resume after failures, and monitoring for processing lag.

Install

Download the skill as a ZIP:
https://mp-staging.autogineer.com/agent-skills/wshobson/projection-patterns/download

Unzip it into the cross-client skills folder of your project:

mkdir -p .agents/skills && unzip projection-patterns.zip -d .agents/skills

Documentation by Seth Hobson

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

Files

ATTRIBUTION.mdAdded by AutoGineer995 B
LICENSELicence file added by AutoGineer1.0 KiB
SKILL.md2.6 KiB
references/details.md13.9 KiB

LICENSE

MIT License

Copyright (c) 2024 Seth Hobson

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.

Requirements and compatibility

Works with: Claude Code, Claude apps, OpenAI Codex, GitHub Copilot, Cursor, Gemini CLI, Kiro, OpenCode, Goose, Amp, Roo Code, JetBrains Junie, Factory Droid, Devin (ex-Windsurf), Mistral Vibe, OpenHands, Letta

“Works with” means the tool reads the open Agent Skills format; a skill written for one tool may use features others lack.

Security

No issues found by our static scan· No issues found

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Download ZIP

The ZIP holds the skill’s files, its licence files and an ATTRIBUTION.md. Download projection-patterns.zip