ffsshhttiikk Saga Pattern
What I do
I provide a pattern for managing distributed transactions across multiple services without traditional two-phase commit. Sagas coordinate a sequence of local transactions, where each transaction updates data and publishes an event to trigger the next step. If a step fails, compensating transactions undo previous steps, maintaining data consistency across services. This enables long-running business processes while preserving eventual consistency.
When to use me
Use sagas when you need to coordinate actions across multiple microservices or bounded contexts, especially when traditional distributed transactions are impractical. Sagas are ideal for long-running business workflows, order processing, booking systems, or any multi-step process spanning services. Avoid sagas when ACID transactions within a single service are sufficient, or when strict immediate consistency is required across all steps.
Core Concepts
- Saga: Sequence of local transactions with compensating actions
- Local Transaction: Single service operation with its own database
- Compensating Transaction: Action that undoes a local transaction
- Choreography: Distributed coordination via events
- Orchestration: Central coordinator managing saga flow
- Saga State: Tracking progress and handling failures
- Idempotency: Safe to execute steps multiple times
- Retry Strategies: Handling transient failures
- Timeout Management: Preventing hung sagas
- Checkpointing: Saving saga state for recovery
Code Examples
Choreography-Based Saga
from abc import ABC, abstractmethod
from dataclasses import dataclass
from datetime import datetime
from typing import Protocol, Callable
from uuid import UUID, uuid4
@dataclass
class OrderCreated:
order_id: UUID
customer_id: UUID
items: list[dict]
total: float
timestamp: datetime = datetime.utcnow()
@dataclass
class PaymentProcessed:
order_id: UUID
payment_id: UUID
amount: float
timestamp: datetime = datetime.utcnow()
@dataclass
class InventoryReserved:
order_id: UUID
reservation_id: UUID
items: list[dict]
timestamp: datetime = datetime.utcnow()
@dataclass
class OrderShipped:
order_id: UUID
tracking_number: str
carrier: str
timestamp: datetime = datetime.utcnow()
@dataclass
class OrderCancelled:
order_id: UUID
reason: str
timestamp: datetime = datetime.utcnow()
class EventBus(Protocol):
def publish(self, event: object) -> None:
pass
class OrderService:
def __init__(self, event_bus: EventBus):
self._event_bus = event_bus
self._orders: dict[UUID, dict] = {}
def create_order(self, customer_id: UUID, items: list[dict]) -> OrderCreated:
order_id = uuid4()
total = sum(item["price"] * item["quantity"] for item in items)
order = {
"order_id": order_id,
"customer_id": customer_id,
"items": items,
"total": total,
"status": "created"
}
self._orders[order_id] = order
event = OrderCreated(
order_id=order_id,
customer_id=customer_id,
items=items,
total=total
)
self._event_bus.publish(event)
return event
class PaymentService:
def __init__(self, event_bus: EventBus):
self._event_bus = event_bus
self._payments: dict[UUID, dict] = {}
def handle_order_created(self, event: OrderCreated) -> None:
payment_id = uuid4()
payment = {
"payment_id": payment_id,
"order_id": event.order_id,
"amount": event.total,
"status": "processing"
}
self._payments[payment_id] = payment
payment_event = PaymentProcessed(
order_id=event.order_id,
payment_id=payment_id,
amount=event.total
)
self._event_bus.publish(payment_event)
def refund(self, order_id: UUID) -> None:
for payment in self._payments.values():
if payment["order_id"] == order_id:
payment["status"] = "refunded"
class InventoryService:
def __init__(self, event_bus: EventBus):
self._event_bus = event_bus
self._reservations: dict[UUID, dict] = {}
self._inventory: dict[str, int] = {"P001": 100, "P002": 50}
def handle_order_created(self, event: OrderCreated) -> None:
reservation_id = uuid4()
reserved_items = []
for item in event.items:
product_id = item["product_id"]
if self._inventory.get(product_id, 0) >= item["quantity"]:
self._inventory[product_id] -= item["quantity"]
reserved_items.append(item)
reservation = {
"reservation_id": reservation_id,
"order_id": event.order_id,
"items": reserved_items,
"status": "reserved"
}
self._reservations[reservation_id] = reservation
self._event_bus.publish(InventoryReserved(
order_id=event.order_id,
reservation_id=reservation_id,
items=reserved_items
))
def release(self, order_id: UUID) -> None:
for reservation in self._reservations.values():
if reservation["order_id"] == order_id:
for item in reservation["items"]:
self._inventory[item["product_id"]] += item["quantity"]
reservation["status"] = "released"
class ShippingService:
def __init__(self, event_bus: EventBus):
self._event_bus = event_bus
self._shipments: dict[UUID, dict] = {}
def handle_inventory_reserved(self, event: InventoryReserved) -> None:
shipment_id = uuid4()
tracking = f"TRK-{uuid4().hex[:8].upper()}"
shipment = {
"shipment_id": shipment_id,
"order_id": event.order_id,
"tracking_number": tracking,
"status": "shipped"
}
self._shipments[shipment_id] = shipment
self._event_bus.publish(OrderShipped(
order_id=event.order_id,
tracking_number=tracking,
carrier="FEDEX"
))
Orchestration-Based Saga
from abc import ABC, abstractmethod
from dataclasses import dataclass, field
from datetime import datetime
from typing import Protocol, Optional
from uuid import UUID, uuid4
@dataclass
class SagaContext:
saga_id: UUID
order_id: UUID
customer_id: UUID
items: list[dict]
total: float
step_results: dict[str, dict] = field(default_factory=dict)
current_step: str = ""
is_completed: bool = False
is_compensating: bool = False
class SagaStep(ABC):
@property
@abstractmethod
def name(self) -> str:
pass
@abstractmethod
def execute(self, context: SagaContext) -> dict:
pass
@abstractmethod
def compensate(self, context: SagaContext, step_result: dict) -> None:
pass
class CreateOrderStep(SagaStep):
@property
def name(self) -> str:
return "create_order"
def execute(self, context: SagaContext) -> dict:
return {"order_id": str(context.order_id), "status": "created"}
def compensate(self, context: SagaContext, step_result: dict) -> None:
print(f"Compensating: Order {step_result['order_id']} cancelled")
class ProcessPaymentStep(SagaStep):
@property
def name(self) -> str:
return "process_payment"
def execute(self, context: SagaContext) -> dict:
return {"payment_id": str(uuid4()), "amount": context.total}
def compensate(self, context: SagaContext, step_result: dict) -> None:
print(f"Compensating: Refunding payment {step_result['payment_id']}")
class ReserveInventoryStep(SagaStep):
@property
def name(self) -> str:
return "reserve_inventory"
def execute(self, context: SagaContext) -> dict:
return {"reservation_id": str(uuid4()), "items": context.items}
def compensate(self, context: SagaContext, step_result: dict) -> None:
print(f"Compensating: Releasing inventory {step_result['reservation_id']}")
class ShipOrderStep(SagaStep):
@property
def name(self) -> str:
return "ship_order"
def execute(self, context: SagaContext) -> dict:
return {"tracking_number": f"TRK-{uuid4().hex[:8].upper()}"}
def compensate(self, context: SagaContext, step_result: dict) -> None:
print(f"Compensating: Canceling shipment {step_result['tracking_number']}")
class OrderProcessingSaga:
def __init__(self):
self._steps: list[SagaStep] = [
CreateOrderStep(),
ProcessPaymentStep(),
ReserveInventoryStep(),
ShipOrderStep()
]
def execute(self, context: SagaContext) -> bool:
try:
for step in self._steps:
context.current_step = step.name
result = step.execute(context)
context.step_results[step.name] = result
context.is_completed = True
return True
except Exception as e:
print(f"Saga failed at step {context.current_step}: {e}")
self._compensate(context)
return False
def _compensate(self, context: SagaContext) -> None:
context.is_compensating = True
completed_steps = [
name for name in self._steps
if name in context.step_results
]
for step_name in reversed(completed_steps):
step = next(s for s in self._steps if s.name == step_name)
result = context.step_results[step_name]
try:
step.compensate(context, result)
except Exception as e:
print(f"Compensation failed for {step_name}: {e}")
class SagaOrchestrator:
def __init__(self):
self._saga_store: dict[UUID, SagaContext] = {}
self._saga_factories: dict[str, callable] = {
"order": lambda: OrderProcessingSaga()
}
def start_saga(
self,
saga_type: str,
order_id: UUID,
customer_id: UUID,
items: list[dict],
total: float
) -> UUID:
saga_id = uuid4()
context = SagaContext(
saga_id=saga_id,
order_id=order_id,
customer_id=customer_id,
items=items,
total=total
)
self._saga_store[saga_id] = context
saga_factory = self._saga_factories.get(saga_type)
if not saga_factory:
raise ValueError(f"Unknown saga type: {saga_type}")
saga = saga_factory()
success = saga.execute(context)
if success:
print(f"Saga {saga_id} completed successfully")
else:
print(f"Saga {saga_id} failed and compensated")
return saga_id
Saga with Retry and Timeout
from abc import ABC, abstractmethod
from dataclasses import dataclass
from datetime import datetime, timedelta
from typing import Protocol
from uuid import UUID
@dataclass
class SagaStepResult:
step_name: str
success: bool
result: dict | None = None
error: str | None = None
retry_count: int = 0
class RetryableStep(ABC):
@property
@abstractmethod
def name(self) -> str:
pass
@property
@abstractmethod
def max_retries(self) -> int:
pass
@property
@abstractmethod
def retry_delay_seconds(self) -> int:
pass
@abstractmethod
def execute_with_retry(self, context: 'SagaContext') -> SagaStepResult:
pass
class ResilientSagaOrchestrator:
def __init__(self):
self._active_sagas: dict[UUID, 'SagaContext'] = {}
self._timeout_minutes: int = 30
def execute_step(
self,
context: 'SagaContext',
step: RetryableStep
) -> SagaStepResult:
result = SagaStepResult(step.name, False)
for attempt in range(step.max_retries + 1):
try:
result = SagaStepResult(
step_name=step.name,
success=True,
result=step.execute_with_retry(context),
retry_count=attempt
)
break
except TransientError as e:
result.error = str(e)
result.retry_count = attempt
if attempt < step.max_retries:
import time
time.sleep(step.retry_delay_seconds)
return result
def start_with_timeout(self, context: 'SagaContext') -> None:
self._active_sagas[context.saga_id] = context
import threading
timer = threading.Timer(
self._timeout_minutes * 60,
self._handle_timeout,
args=[context.saga_id]
)
timer.start()
def _handle_timeout(self, saga_id: UUID) -> None:
if saga_id in self._active_sagas:
context = self._active_sagas[saga_id]
print(f"Saga {saga_id} timed out, initiating compensation")
self._compensate_all(context)
del self._active_sagas[saga_id]
def _compensate_all(self, context: 'SagaContext') -> None:
pass
class TransientError(Exception):
pass
Saga Persistence and Recovery
from abc import ABC, abstractmethod
from dataclasses import dataclass
from datetime import datetime
from typing import Protocol
from uuid import UUID
@dataclass
class SagaState:
saga_id: UUID
saga_type: str
status: str
current_step: str
step_results: dict
created_at: datetime
updated_at: datetime
completed_at: datetime | None = None
class SagaStore(Protocol):
@abstractmethod
def save(self, state: SagaState) -> None:
pass
@abstractmethod
def load(self, saga_id: UUID) -> SagaState | None:
pass
@abstractmethod
def update_status(self, saga_id: UUID, status: str, current_step: str) -> None:
pass
@abstractmethod
def get_pending_sagas(self) -> list[SagaState]:
pass
class PersistentSagaOrchestrator:
def __init__(self, saga_store: SagaStore):
self._store = saga_store
self._active_sagas: dict[UUID, 'SagaContext'] = {}
def start_saga(
self,
saga_type: str,
saga_id: UUID,
initial_data: dict
) -> None:
context = SagaContext(
saga_id=saga_id,
order_id=initial_data.get("order_id", uuid4()),
customer_id=initial_data.get("customer_id", uuid4()),
items=initial_data.get("items", []),
total=initial_data.get("total", 0)
)
self._active_sagas[saga_id] = context
state = SagaState(
saga_id=saga_id,
saga_type=saga_type,
status="running",
current_step="",
step_results={},
created_at=datetime.utcnow(),
updated_at=datetime.utcnow()
)
self._store.save(state)
def recover_sagas(self) -> None:
pending = self._store.get_pending_sagas()
for state in pending:
self._resume_saga(state)
def _resume_saga(self, state: SagaState) -> None:
if state.saga_id not in self._active_sagas:
print(f"Resuming saga {state.saga_id}")
context = self._active_sagas.get(state.saga_id)
if context:
context.step_results = state.step_results
def record_step_start(self, saga_id: UUID, step_name: str) -> None:
self._store.update_status(saga_id, "running", step_name)
def record_step_complete(
self,
saga_id: UUID,
step_name: str,
result: dict
) -> None:
if saga_id in self._active_sagas:
self._active_sagas[saga_id].step_results[step_name] = result
self._store.save(SagaState(
saga_id=saga_id,
saga_type="",
status="running",
current_step=step_name,
step_results={},
created_at=datetime.utcnow(),
updated_at=datetime.utcnow()
))
Distributed Saga with Compensation
from abc import ABC, abstractmethod
from dataclasses import dataclass
from typing import Protocol
@dataclass
class BookingRequest:
hotel_id: str
flight_id: str
customer_id: str
start_date: str
end_date: str
@dataclass
class HotelBooking:
confirmation_number: str
@dataclass
class FlightBooking:
reservation_code: str
class HotelService(Protocol):
def book(self, request: BookingRequest) -> HotelBooking:
pass
def cancel(self, confirmation_number: str) -> None:
pass
class FlightService(Protocol):
def book(self, request: BookingRequest) -> FlightBooking:
pass
def cancel(self, reservation_code: str) -> None:
pass
class TripBookingSaga:
def __init__(
self,
hotel_service: HotelService,
flight_service: FlightService
):
self._hotel = hotel_service
self._flight = flight_service
self._completed_steps: list[str] = []
def execute(self, request: BookingRequest) -> dict:
try:
hotel_booking = self._hotel.book(request)
self._completed_steps.append("hotel")
flight_booking = self._flight.book(request)
self._completed_steps.append("flight")
return {
"status": "success",
"hotel_confirmation": hotel_booking.confirmation_number,
"flight_reservation": flight_booking.reservation_code
}
except Exception as e:
print(f"Trip booking failed: {e}")
self._compensate()
raise
def _compensate(self) -> None:
for step in reversed(self._completed_steps):
if step == "flight":
for booking in self._flight_bookings:
self._flight.cancel(booking.reservation_code)
elif step == "hotel":
for booking in self._hotel_bookings:
self._hotel.cancel(booking.confirmation_number)
self._completed_steps.clear()
@property
def _flight_bookings(self) -> list[FlightBooking]:
return []
@property
def _hotel_bookings(self) -> list[HotelBooking]:
return []
Best Practices
- Keep Sagas Short: Limit the number of steps
- Idempotent Steps: Handle retries safely
- Compensating Logic: Each step must have undo logic
- Timeout Handling: Prevent hung sagas
- Saga State Persistence: Recover from crashes
- Avoid Cross-Dependencies: Steps should be independent
- Eventual Consistency: Accept intermediate states
- Testing: Test happy path and all failure scenarios
- Monitoring: Track saga execution times
- Documentation: Document saga flows clearly
- Choreography vs Orchestration: Choose based on complexity
- Retry Strategies: Handle transient failures with backoff