ffsshhttiikk Event Sourcing
What I do
I provide an architectural pattern where application state is stored as a sequence of events rather than as current state. Every state change is captured as an immutable event, creating a complete audit trail and enabling temporal queries. Event sourcing allows reconstructing any past state, provides full audit capability, and enables sophisticated event processing patterns. The event log becomes the source of truth.
When to use me
Use event sourcing when you need a complete audit trail, when temporal queries are important, or when business rules depend on event history. It's valuable for domain models where state transitions are complex. Event sourcing excels in systems requiring eventual consistency, audit compliance, or the ability to replay events. Avoid for simple CRUD applications or when you need only current state efficiently.
Core Concepts
- Event: Immutable record of something that happened
- Event Store: Specialized database for events
- Aggregate: Entity reconstructed from events
- Snapshot: Optimization to avoid replaying all events
- Projection: Building read models from events
- Event Versioning: Handling evolving event schemas
- Replay: Rebuilding state from event history
- Append-Only: Events are never modified or deleted
- Domain Events: Business events from domain model
- Integration Events: Events for external systems
Code Examples
Event Definitions
from abc import ABC
from dataclasses import dataclass, field
from datetime import datetime
from typing import Generic, TypeVar, Protocol
from uuid import UUID, uuid4
T = TypeVar("T", bound='Event')
@dataclass
class Event:
event_id: UUID
aggregate_id: UUID
event_type: str
occurred_at: datetime
version: int
def to_dict(self) -> dict:
return {
"event_id": str(self.event_id),
"aggregate_id": str(self.aggregate_id),
"event_type": self.event_type,
"occurred_at": self.occurred_at.isoformat(),
"version": self.version
}
@dataclass
class UserCreated(Event):
email: str
name: str
def __init__(
self,
aggregate_id: UUID,
email: str,
name: str,
version: int
):
super().__init__(
event_id=uuid4(),
aggregate_id=aggregate_id,
event_type="UserCreated",
occurred_at=datetime.utcnow(),
version=version
)
self.email = email
self.name = name
@dataclass
class UserEmailChanged(Event):
old_email: str
new_email: str
def __init__(
self,
aggregate_id: UUID,
old_email: str,
new_email: str,
version: int
):
super().__init__(
event_id=uuid4(),
aggregate_id=aggregate_id,
event_type="UserEmailChanged",
occurred_at=datetime.utcnow(),
version=version
)
self.old_email = old_email
self.new_email = new_email
@dataclass
class UserDeactivated(Event):
reason: str
def __init__(
self,
aggregate_id: UUID,
reason: str,
version: int
):
super().__init__(
event_id=uuid4(),
aggregate_id=aggregate_id,
event_type="UserDeactivated",
occurred_at=datetime.utcnow(),
version=version
)
self.reason = reason
@dataclass
class UserReactivated(Event):
reason: str
def __init__(
self,
aggregate_id: UUID,
reason: str,
version: int
):
super().__init__(
event_id=uuid4(),
aggregate,
event_type="UserReactivated_id=aggregate_id",
occurred_at=datetime.utcnow(),
version=version
)
self.reason = reason
Aggregate Root with Event Sourcing
from abc import ABC, abstractmethod
from typing import TypeVar, Generic
E = TypeVar("E", bound=Event)
class AggregateRoot(ABC, Generic[E]):
def __init__(self, aggregate_id: UUID):
self._id = aggregate_id
self._version = 0
self._events: list[E] = []
@property
def id(self) -> UUID:
return self._id
@property
def version(self) -> int:
return self._version
@property
def uncommitted_events(self) -> list[E]:
return self._events.copy()
def clear_uncommitted_events(self) -> None:
self._events.clear()
def _apply(self, event: E) -> None:
self._version += 1
self._events.append(event)
self._apply_event(event)
@abstractmethod
def _apply_event(self, event: E) -> None:
pass
class UserAggregate(AggregateRoot[Event]):
def __init__(self, user_id: UUID):
super().__init__(user_id)
self._email: str | None = None
self._name: str | None = None
self._is_active: bool = True
self._email_history: list[str] = []
@classmethod
def create(
cls,
email: str,
name: str
) -> 'UserAggregate':
aggregate = cls(uuid4())
event = UserCreated(
aggregate_id=aggregate._id,
email=email,
name=name,
version=1
)
aggregate._apply(event)
return aggregate
def change_email(self, new_email: str) -> None:
if not self._is_active:
raise ValueError("Cannot change email for inactive user")
if new_email == self._email:
return
event = UserEmailChanged(
aggregate_id=self._id,
old_email=self._email or "",
new_email=new_email,
version=self._version + 1
)
self._apply(event)
def deactivate(self, reason: str) -> None:
if not self._is_active:
raise ValueError("User already deactivated")
event = UserDeactivated(
aggregate_id=self._id,
reason=reason,
version=self._version + 1
)
self._apply(event)
def reactivate(self, reason: str) -> None:
if self._is_active:
raise ValueError("User already active")
event = UserReactivated(
aggregate_id=self._id,
reason=reason,
version=self._version + 1
)
self._apply(event)
def _apply_event(self, event: Event) -> None:
if isinstance(event, UserCreated):
self._email = event.email
self._name = event.name
self._is_active = True
self._email_history = [event.email]
elif isinstance(event, UserEmailChanged):
self._email = event.new_email
self._email_history.append(event.new_email)
elif isinstance(event, UserDeactivated):
self._is_active = False
elif isinstance(event, UserReactivated):
self._is_active = True
@property
def email(self) -> str | None:
return self._email
@property
def name(self) -> str | None:
return self._name
@property
def is_active(self) -> bool:
return self._is_active
@property
def email_history(self) -> list[str]:
return self._email_history.copy()
Event Store
from abc import ABC, abstractmethod
from typing import Protocol, TypeVar
E = TypeVar("E", bound=Event)
class EventStore(Protocol[E]):
@abstractmethod
def append(self, event: E) -> None:
pass
@abstractmethod
def get_events(self, aggregate_id: UUID) -> list[E]:
pass
@abstractmethod
def get_all_events(self, from_version: int = 0) -> list[E]:
pass
class InMemoryEventStore(EventStore[Event]):
def __init__(self):
self._events: list[Event] = []
self._by_aggregate: dict[UUID, list[Event]] = {}
def append(self, event: Event) -> None:
self._events.append(event)
if event.aggregate_id not in self._by_aggregate:
self._by_aggregate[event.aggregate_id] = []
self._by_aggregate[event.aggregate_id].append(event)
def get_events(self, aggregate_id: UUID) -> list[Event]:
return self._by_aggregate.get(aggregate_id, []).copy()
def get_all_events(self, from_version: int = 0) -> list[Event]:
return [e for e in self._events if e.version > from_version]
class SnapshotStore:
def __init__(self):
self._snapshots: dict[UUID, dict] = {}
def save_snapshot(self, aggregate_id: UUID, version: int, state: dict) -> None:
self._snapshots[aggregate_id] = {
"version": version,
"state": state
}
def get_snapshot(self, aggregate_id: UUID) -> dict | None:
return self._snapshots.get(aggregate_id)
Repository with Snapshots
class UserRepository:
def __init__(
self,
event_store: EventStore[Event],
snapshot_store: SnapshotStore | None = None,
snapshot_threshold: int = 10
):
self._event_store = event_store
self._snapshot_store = snapshot_store
self._snapshot_threshold = snapshot_threshold
def save(self, aggregate: UserAggregate) -> None:
for event in aggregate.uncommitted_events:
self._event_store.append(event)
aggregate.clear_uncommitted_events()
if self._snapshot_store and self._should_snapshot(aggregate):
self._save_snapshot(aggregate)
def load(self, aggregate_id: UUID) -> UserAggregate:
snapshot = None
if self._snapshot_store:
snapshot = self._snapshot_store.get_snapshot(aggregate_id)
if snapshot:
events = self._event_store.get_events(aggregate_id)
events_after_snapshot = [
e for e in events
if e.version > snapshot["version"]
]
aggregate = self._reconstitute(snapshot["state"])
for event in events_after_snapshot:
aggregate._apply(event)
return aggregate
events = self._event_store.get_events(aggregate_id)
return self._reconstitute_from_events(events)
def _should_snapshot(self, aggregate: UserAggregate) -> bool:
return (
self._snapshot_store and
aggregate.version % self._snapshot_threshold == 0
)
def _save_snapshot(self, aggregate: UserAggregate) -> None:
state = {
"email": aggregate.email,
"name": aggregate.name,
"is_active": aggregate.is_active,
"email_history": aggregate.email_history
}
self._snapshot_store.save_snapshot(
aggregate.id,
aggregate.version,
state
)
def _reconstitute(self, state: dict) -> UserAggregate:
aggregate = UserAggregate(uuid4())
aggregate._email = state["email"]
aggregate._name = state["name"]
aggregate._is_active = state["is_active"]
aggregate._email_history = state["email_history"]
return aggregate
def _reconstitute_from_events(self, events: list[Event]) -> UserAggregate:
aggregate = UserAggregate(uuid4())
for event in events:
aggregate._apply(event)
return aggregate
Projections for Read Models
from abc import ABC, abstractmethod
from dataclasses import dataclass, field
from datetime import datetime
from typing import Generic, TypeVar, Protocol
P = TypeVar("P", bound='Projection')
@dataclass
class UserView:
user_id: UUID
email: str
name: str
is_active: bool
created_at: datetime
updated_at: datetime
version: int
class Projection(ABC):
@abstractmethod
def apply(self, event: Event) -> None:
pass
class UserProjection(Projection):
def __init__(self):
self._users: dict[UUID, UserView] = {}
@property
def users(self) -> dict[UUID, UserView]:
return self._users.copy()
def get_user(self, user_id: UUID) -> UserView | None:
return self._users.get(user_id)
def get_active_users(self) -> list[UserView]:
return [u for u in self._users.values() if u.is_active]
def apply(self, event: Event) -> None:
if isinstance(event, UserCreated):
self._users[event.aggregate_id] = UserView(
user_id=event.aggregate_id,
email=event.email,
name=event.name,
is_active=True,
created_at=event.occurred_at,
updated_at=event.occurred_at,
version=event.version
)
elif isinstance(event, UserEmailChanged):
if user := self._users.get(event.aggregate_id):
user.email = event.new_email
user.updated_at = event.occurred_at
user.version = event.version
elif isinstance(event, UserDeactivated):
if user := self._users.get(event.aggregate_id):
user.is_active = False
user.updated_at = event.occurred_at
user.version = event.version
class ProjectionManager:
def __init__(self):
self._projections: list[Projection] = []
def add(self, projection: Projection) -> None:
self._projections.append(projection)
def apply(self, event: Event) -> None:
for projection in self._projections:
try:
projection.apply(event)
except Exception as e:
print(f"Projection failed: {e}")
class AsyncProjector:
def __init__(self, event_store: EventStore[Event]):
self._event_store = event_store
self._projection_manager = ProjectionManager()
self._last_processed_version: dict[str, int] = {}
def add_projection(self, projection: Projection, name: str) -> None:
self._projection_manager.add(projection)
def start(self) -> None:
import threading
def process():
while True:
for event in self._event_store.get_all_events(0):
self._projection_manager.apply(event)
threading.Thread(target=process, daemon=True).start()
Event Versioning
from abc import ABC, abstractmethod
from datetime import datetime
from typing import dict
class EventUpgrader(ABC):
@abstractmethod
def can_upgrade(self, event_type: str, version: int) -> bool:
pass
@abstractmethod
def upgrade(self, event: dict) -> dict:
pass
class UserEventUpgrader(EventUpgrader):
V1_TO_V2_MAPPING = {
"email": "contact_email"
}
def can_upgrade(self, event_type: str, version: int) -> bool:
return event_type == "UserCreated" and version == 1
def upgrade(self, event: dict) -> dict:
upgraded = event.copy()
upgraded["version"] = 2
for old_field, new_field in self.V1_TO_V2_MAPPING.items():
if old_field in upgraded:
upgraded[new_field] = upgraded.pop(old_field)
upgraded["metadata"] = {
"upgraded_from_version": 1,
"upgraded_at": datetime.utcnow().isoformat()
}
return upgraded
class EventUpgraderChain:
def __init__(self):
self._upgraders: list[EventUpgrader] = []
def add_upgrader(self, upgrader: EventUpgrader) -> None:
self._upgraders.append(upgrader)
def upgrade(self, event: dict) -> dict:
current = event
for upgrader in self._upgraders:
if upgrader.can_upgrade(
current["event_type"],
current["version"]
):
current = upgrader.upgrade(current)
return current
class VersionedEventStore:
def __init__(self, base_store: EventStore[Event]):
self._store = base_store
self._upgrader_chain = EventUpgraderChain()
self._upgrader_chain.add_upgrader(UserEventUpgrader())
def append(self, event: Event) -> None:
self._store.append(event)
def get_events(self, aggregate_id: UUID) -> list[dict]:
events = self._store.get_events(aggregate_id)
return [self._upgrader_chain.upgrade(e.to_dict()) for e in events]
Best Practices
- Immutable Events: Never modify or delete events
- Idempotent Appends: Handle duplicate event writes safely
- Version Events: Plan for event schema evolution
- Use Snapshots: Optimize loading large event histories
- Atomic Writes: Save events in transactions
- Compensating Events: For undo actions, emit inverse events
- Projection Async: Projections should not block writes
- Event Ordering: Preserve order within aggregates
- Backwards Compatibility: New code should read old events
- Testing: Test aggregates through their event history
- Governance: Document event schemas thoroughly
- Performance: Index events by aggregate ID and type