Aggregate 5 - Immutable aggregate

This example shows the Dog class used in the tutorial and module docs.

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, both the event and the aggregate classes are defined as frozen dataclasses.

The Dog aggregate is a frozen dataclass, but it otherwise similar to the previous example. It explicitly defines event classes. And it explicitly triggers events in command methods. However, in this example the aggregate state is evolved by constructing a new instance of the aggregate class.

In contrast to the previous examples, the application code in this example must receive the new aggregate instance and the new domain events that are returned from 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 time import monotonic
from typing import Any, Iterable, List, Optional, Tuple, Type, TypeVar, cast
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.fromtimestamp(monotonic(), timezone.utc)


TAggregate = TypeVar("TAggregate", bound="Aggregate")


@dataclass(frozen=True)
class Aggregate:
    id: UUID
    version: int
    created_on: datetime

    def trigger_event(
        self: TAggregate,
        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) -> Tuple["Dog", List[DomainEvent]]:
        event = cls.Registered(
            originator_id=uuid4(),
            originator_version=1,
            timestamp=DomainEvent.create_timestamp(),
            name=name,
        )
        return cast(Dog, cls.mutate(event, None)), [event]

    @dataclass(frozen=True)
    class TrickAdded(DomainEvent):
        trick: str

    def add_trick(self, trick: str) -> Tuple["Dog", List[DomainEvent]]:
        event = self.trigger_event(self.TrickAdded, trick=trick)
        return cast(Dog, type(self).mutate(event, self)), [event]

    @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:
        dog, events = Dog.register(name)
        self.save(*events)
        return dog.id

    def add_trick(self, dog_id: UUID, trick: str) -> None:
        dog = self.repository.get(dog_id, projector_func=Dog.projector)
        dog, events = dog.add_trick(trick)
        self.save(*events)

    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")