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
- Projection Architecture
┌─────────────┐ ┌─────────────┐ ┌─────────────┐ │ Event Store │────►│ Projector │────►│ Read Model │ │ │ │ │ │ (Database) │ │ ┌─────────┐ │ │ ┌─────────┐ │ │ ┌─────────┐ │ │ │ Events │ │ │ │ Handler │ │ │ │ Tables │ │ │ └─────────┘ │ │ │ Logic │ │ │ │ Views │ │ │ │ │ └─────────┘ │ │ │ Cache │ │ └─────────────┘ └─────────────┘ └─────────────┘
- 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
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)
Template 2: Order Summary Projection
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",
"OrderShipped",
"OrderCompleted",
"OrderCancelled"
]
async def apply(self, event: Event) -> None:
handlers = {
"OrderCreated": self._handle_created,
"OrderItemAdded": self._handle_item_added,
"OrderItemRemoved": self._handle_item_removed,
"OrderShipped": self._handle_shipped,
"OrderCompleted": self._handle_completed,
"OrderCancelled": self._handle_cancelled,
}
handler = handlers.get(event.event_type)
if handler:
await handler(event)
async def _handle_created(self, event: Event):
async with self.pool.acquire() as conn:
await conn.execute(
"""
INSERT INTO order_summaries
(order_id, customer_id, status, total_amount, item_count, created_at)
VALUES ($1, $2, $3, $4, $5, $6)
""",
event.data['order_id'],
event.data['customer_id'],
'pending',
0,
0,
event.data['created_at']
)
async def _handle_item_added(self, event: Event):
async with self.pool.acquire() as conn:
await conn.execute(
"""
UPDATE order_summaries
SET total_amount = total_amount + $2,
item_count = item_count + 1,
updated_at = NOW()
WHERE order_id = $1
""",
event.data['order_id'],
event.data['price'] * event.data['quantity']
)
async def _handle_item_removed(self, event: Event):
async with self.pool.acquire() as conn:
await conn.execute(
"""
UPDATE order_summaries
SET total_amount = total_amount - $2,
item_count = item_count - 1,
updated_at = NOW()
WHERE order_id = $1
""",
event.data['order_id'],
event.data['price'] * event.data['quantity']
)
async def _handle_shipped(self, event: Event):
async with self.pool.acquire() as conn:
await conn.execute(
"""
UPDATE order_summaries
SET status = 'shipped',
shipped_at = $2,
updated_at = NOW()
WHERE order_id = $1
""",
event.data['order_id'],
event.data['shipped_at']
)
async def _handle_completed(self, event: Event):
async with self.pool.acquire() as conn:
await conn.execute(
"""
UPDATE order_summaries
SET status = 'completed',
completed_at = $2,
updated_at = NOW()
WHERE order_id = $1
""",
event.data['order_id'],
event.data['completed_at']
)
async def _handle_cancelled(self, event: Event):
async with self.pool.acquire() as conn:
await conn.execute(
"""
UPDATE order_summaries
SET status = 'cancelled',
cancelled_at = $2,
cancellation_reason = $3,
updated_at = NOW()
WHERE order_id = $1
""",
event.data['order_id'],
event.data['cancelled_at'],
event.data.get('reason')
)
Template 3: Elasticsearch Search Projection
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']
)
Template 4: Aggregating Projection
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']
)
Template 5: Multi-Table Projection
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",
"CustomerTierChanged"
]
async def apply(self, event: Event) -> None:
async with self.pool.acquire() as conn:
async with conn.transaction():
if event.event_type == "CustomerCreated":
# Insert into customers table
await conn.execute(
"""
INSERT INTO customers (customer_id, email, name, tier, created_at)
VALUES ($1, $2, $3, 'bronze', $4)
""",
event.data['customer_id'],
event.data['email'],
event.data['name'],
event.data['created_at']
)
# Initialize activity summary
await conn.execute(
"""
INSERT INTO customer_activity_summary
(customer_id, total_orders, total_spent, total_reviews)
VALUES ($1, 0, 0, 0)
""",
event.data['customer_id']
)
elif event.event_type == "OrderCompleted":
# Update activity summary
await conn.execute(
"""
UPDATE customer_activity_summary SET
total_orders = total_orders + 1,
total_spent = total_spent + $2,
last_order_at = $3
WHERE customer_id = $1
""",
event.data['customer_id'],
event.data['total_amount'],
event.data['completed_at']
)
# Insert into order history
await conn.execute(
"""
INSERT INTO customer_order_history
(customer_id, order_id, amount, completed_at)
VALUES ($1, $2, $3, $4)
""",
event.data['customer_id'],
event.data['order_id'],
event.data['total_amount'],
event.data['completed_at']
)
elif event.event_type == "ReviewSubmitted":
await conn.execute(
"""
UPDATE customer_activity_summary SET
total_reviews = total_reviews + 1,
last_review_at = $2
WHERE customer_id = $1
""",
event.data['customer_id'],
event.data['submitted_at']
)
elif event.event_type == "CustomerTierChanged":
await conn.execute(
"""
UPDATE customers SET tier = $2, updated_at = NOW()
WHERE customer_id = $1
""",
event.data['customer_id'],
event.data['new_tier']
)
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