Projection Patterns

---
name: projection-patterns
description: "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."
risk: critical
source: community
date_added: "2026-02-27"
---

# Projection Patterns

Comprehensive guide to building projections and read models for event-sourced systems.

## Use this skill when

- 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

## Do not use this skill when

- The task is unrelated to projection patterns
- You need a different domain or tool outside this scope

## Instructions

- Clarify goals, constraints, and required inputs.
- Apply relevant best practices and validate outcomes.
- Provide actionable steps and verification.
- If detailed examples are required, open `resources/implementation-playbook.md`.

## Resources

- `resources/implementation-playbook.md` for detailed patterns and examples.

## Limitations
- Use this skill only when the task clearly matches the scope described above.
- Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
- Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.

## Example

For `OrderItemAdded` at position 42, apply the increment and persist checkpoint 42
in one transaction. Replay position 42 and verify no second increment; crash
before commit and verify both the row update and checkpoint roll back together.