Details

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 projections from event store."""

    def __init__(self, event_store, checkpoint_store):
        self.event_store = event_store
        self.checkpoint_store = checkpoint_store
        self.projections: List[Projection] = []

    def register(self, projection: Projection):
        self.projections.append(projection)

    async def run(self, batch_size: int = 100):
        """Run all projections continuously."""
        while True:
            for projection in self.projections:
                await self._run_projection(projection, batch_size)
            await asyncio.sleep(0.1)

    async def _run_projection(self, projection: Projection, batch_size: int):
        checkpoint = await self.checkpoint_store.get(projection.name)
        position = checkpoint or 0

        events = await self.event_store.read_all(position, batch_size)

        for event in events:
            if event.event_type in projection.handles():
                await projection.apply(event)

            await self.checkpoint_store.save(
                projection.name,
                event.global_position
            )

    async def rebuild(self, projection: Projection):
        """Rebuild a projection from scratch."""
        await self.checkpoint_store.delete(projection.name)
        # Optionally clear read model tables
        await self._run_projection(projection, batch_size=1000)

class OrderSummaryProjection(Projection):
    """Projects order events to a summary read model."""

    def __init__(self, db_pool: asyncpg.Pool):
        self.pool = db_pool

    @property
    def name(self) -> str:
        return "order_summary"

    def handles(self) -> List[str]:
        return [
            "OrderCreated",
            "OrderItemAdded",
            "OrderItemRemoved"

from elasticsearch import AsyncElasticsearch

class ProductSearchProjection(Projection):
    """Projects product events to Elasticsearch for full-text search."""

    def __init__(self, es_client: AsyncElasticsearch):
        self.es = es_client
        self.index = "products"

    @property
    def name(self) -> str:
        return "product_search"

    def handles(self) -> List[str]:
        return [
            "ProductCreated",
            "ProductUpdated",
            "ProductPriceChanged",
            "ProductDeleted"
        ]

    async def apply(self, event: Event) -> None:
        if event.event_type == "ProductCreated":
            await self.es.index(
                index=self.index,
                id=event.data['product_id'],
                document={
                    'name': event.data['name'],
                    'description': event.data['description'],
                    'category': event.data['category'],
                    'price': event.data['price'],
                    'tags': event.data.get('tags', []),
                    'created_at': event.data['created_at']
                }
            )

        elif event.event_type == "ProductUpdated":
            await self.es.update(
                index=self.index,
                id=event.data['product_id'],
                doc={
                    'name': event.data['name'],
                    'description': event.data['description'],
                    'category': event.data['category'],
                    'tags': event.data.get('tags', []),
                    'updated_at': event.data['updated_at']
                }
            )

        elif event.event_type == "ProductPriceChanged":
            await self.es.update(
                index=self.index,
                id=event.data['product_id'],
                doc={
                    'price': event.data['new_price'],
                    'price_updated_at': event.data['changed_at']
                }
            )

        elif event.event_type == "ProductDeleted":
            await self.es.delete(
                index=self.index,
                id=event.data['product_id']
            )

class DailySalesProjection(Projection):
    """Aggregates sales data by day for reporting."""

    def __init__(self, db_pool: asyncpg.Pool):
        self.pool = db_pool

    @property
    def name(self) -> str:
        return "daily_sales"

    def handles(self) -> List[str]:
        return ["OrderCompleted", "OrderRefunded"]

    async def apply(self, event: Event) -> None:
        if event.event_type == "OrderCompleted":
            await self._increment_sales(event)
        elif event.event_type == "OrderRefunded":
            await self._decrement_sales(event)

    async def _increment_sales(self, event: Event):
        date = event.data['completed_at'][:10]  # YYYY-MM-DD
        async with self.pool.acquire() as conn:
            await conn.execute(
                """
                INSERT INTO daily_sales (date, total_orders, total_revenue, total_items)
                VALUES ($1, 1, $2, $3)
                ON CONFLICT (date) DO UPDATE SET
                    total_orders = daily_sales.total_orders + 1,
                    total_revenue = daily_sales.total_revenue + $2,
                    total_items = daily_sales.total_items + $3,
                    updated_at = NOW()
                """,
                date,
                event.data['total_amount'],
                event.data['item_count']
            )

    async def _decrement_sales(self, event: Event):
        date = event.data['original_completed_at'][:10]
        async with self.pool.acquire() as conn:
            await conn.execute(
                """
                UPDATE daily_sales SET
                    total_orders = total_orders - 1,
                    total_revenue = total_revenue - $2,
                    total_refunds = total_refunds + $2,
                    updated_at = NOW()
                WHERE date = $1
                """,
                date,
                event.data['refund_amount']
            )

class CustomerActivityProjection(Projection):
    """Projects customer activity across multiple tables."""

    def __init__(self, db_pool: asyncpg.Pool):
        self.pool = db_pool

    @property
    def name(self) -> str:
        return "customer_activity"

    def handles(self) -> List[str]:
        return [
            "CustomerCreated",
            "OrderCompleted",
            "ReviewSubmitted"