projection-patterns

# 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
    global_position: int


class Projection(ABC):
    """Base class for projections."""

    @property
    @abstractmethod
    def name(self) -> str:
        """Unique projection name for checkpointing."""
        pass

    @abstractmethod
    def handles(self) -> List[str]:
        """List of event types this projection handles."""
        pass

    @abstractmethod
    async def apply(self, event: Event) -> None:
        """Apply event to the read model."""
        pass


class Projector:
    """Runs