Aggregate 5 - Immutable aggregate¶
This example shows another variation of the Dog
aggregate class used
in the tutorial and module docs.
Like in the previous example, this example also does not use the library
Aggregate
class. Instead, it defines its own Aggregate
and DomainEvent
base classes. In contrast to the previous examples, the aggregate is defined
as frozen dataclasses to be an immutable object. This has implications for
the aggregate command methods, which must simply return the events that they
trigger without mutating the state of the aggregate object.
The Dog
aggregate is an immutable frozen dataclass, but it otherwise similar
to the previous example. It explicitly defines event classes. And it explicitly
triggers events in command methods. However, it has a mutate()
method which
evolves aggregate state by constructing a new instance of the aggregate class
for each event.
The application code in this example must receive the new events that are triggered when calling the aggregate command methods. The aggregate projector function must also be supplied when getting an aggregate from the repository and when taking snapshots.
Domain model¶
from dataclasses import dataclass
from datetime import datetime, timezone
from typing import Any, Iterable, Optional, Tuple, Type, TypeVar
from uuid import UUID, uuid4
from eventsourcing.domain import Snapshot
@dataclass(frozen=True)
class DomainEvent:
originator_id: UUID
originator_version: int
timestamp: datetime
@staticmethod
def create_timestamp() -> datetime:
return datetime.now(tz=timezone.utc)
TAggregate = TypeVar("TAggregate", bound="Aggregate")
@dataclass(frozen=True)
class Aggregate:
id: UUID
version: int
created_on: datetime
def trigger_event(
self,
event_class: Type[DomainEvent],
**kwargs: Any,
) -> DomainEvent:
kwargs = kwargs.copy()
kwargs.update(
originator_id=self.id,
originator_version=self.version + 1,
timestamp=event_class.create_timestamp(),
)
return event_class(**kwargs)
@classmethod
def projector(
cls: Type[TAggregate],
aggregate: Optional[TAggregate],
events: Iterable[DomainEvent],
) -> Optional[TAggregate]:
for event in events:
aggregate = cls.mutate(event, aggregate)
return aggregate
@classmethod
def mutate(cls, event: DomainEvent, aggregate: Any) -> Any:
"""Mutates aggregate with event."""
@dataclass(frozen=True)
class Dog(Aggregate):
name: str
tricks: Tuple[str, ...]
@dataclass(frozen=True)
class Registered(DomainEvent):
name: str
@classmethod
def register(cls, name: str) -> DomainEvent:
return cls.Registered(
originator_id=uuid4(),
originator_version=1,
timestamp=DomainEvent.create_timestamp(),
name=name,
)
@dataclass(frozen=True)
class TrickAdded(DomainEvent):
trick: str
def add_trick(self, trick: str) -> DomainEvent:
return self.trigger_event(self.TrickAdded, trick=trick)
@classmethod
def mutate(cls, event: DomainEvent, aggregate: Optional["Dog"]) -> Optional["Dog"]:
"""Mutates aggregate with event."""
if isinstance(event, cls.Registered):
return Dog(
id=event.originator_id,
version=event.originator_version,
created_on=event.timestamp,
name=event.name,
tricks=tuple(),
)
elif isinstance(event, cls.TrickAdded):
assert aggregate is not None
return Dog(
id=aggregate.id,
version=event.originator_version,
created_on=event.timestamp,
name=aggregate.name,
tricks=aggregate.tricks + (event.trick,),
)
elif isinstance(event, Snapshot):
return Dog(
id=event.state["id"],
version=event.state["version"],
created_on=event.state["created_on"],
name=event.state["name"],
tricks=tuple(event.state["tricks"]), # comes back from JSON as a list
)
else:
raise NotImplementedError(event) # pragma: no cover
Application¶
from typing import Any, Dict
from uuid import UUID
from eventsourcing.application import Application
from eventsourcing.examples.aggregate5.domainmodel import Dog
class DogSchool(Application):
is_snapshotting_enabled = True
def register_dog(self, name: str) -> UUID:
event = Dog.register(name)
self.save(event)
return event.originator_id
def add_trick(self, dog_id: UUID, trick: str) -> None:
dog = self.repository.get(dog_id, projector_func=Dog.projector)
event = dog.add_trick(trick)
self.save(event)
def get_dog(self, dog_id: UUID) -> Dict[str, Any]:
dog = self.repository.get(dog_id, projector_func=Dog.projector)
return {"name": dog.name, "tricks": dog.tricks}
Test case¶
from unittest import TestCase
from eventsourcing.examples.aggregate5.application import DogSchool
from eventsourcing.examples.aggregate5.domainmodel import Dog
class TestDogSchool(TestCase):
def test_dog_school(self) -> None:
# Construct application object.
school = DogSchool()
# Evolve application state.
dog_id = school.register_dog("Fido")
school.add_trick(dog_id, "roll over")
school.add_trick(dog_id, "play dead")
# Query application state.
dog = school.get_dog(dog_id)
assert dog["name"] == "Fido"
assert dog["tricks"] == ("roll over", "play dead")
# Select notifications.
notifications = school.notification_log.select(start=1, limit=10)
assert len(notifications) == 3
# Take snapshot.
school.take_snapshot(dog_id, version=3, projector_func=Dog.projector)
dog = school.get_dog(dog_id)
assert dog["name"] == "Fido"
assert dog["tricks"] == ("roll over", "play dead")
# Continue with snapshotted aggregate.
school.add_trick(dog_id, "fetch ball")
dog = school.get_dog(dog_id)
assert dog["name"] == "Fido"
assert dog["tricks"] == ("roll over", "play dead", "fetch ball")