projection-patternslisted

# 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 ### Template 1: Basic Projector ```python from abc import ABC, abstractmethod from dataclasses import dataclass from typing import Dict, Any, Callable, List import asyncpg @dataclass class Event: stream_id: str event_type: str data: dict version: int gl