aett-domain

Æt (Aett) is an Event Store for Python

Aett Domain provide base classes for aggregate and saga as encapsulations of business rules and processes, respectively.

Usage

The Aggregate class is abstract and a subtype aggregate would implement the external interfaces and internal behavior and define which events are raised as a response input.

from aett.domain.Domain import Aggregate
from dataclasses import dataclass
from aett.eventstore.EventStream import EventStream, Memento, DomainEvent
import datetime


@dataclass(frozen=True, kw_only=True)
class SampleEvent(DomainEvent):
    value: int


class ExampleAggregate(Aggregate[Memento]):
    def __init__(self, event_stream: EventStream, memento: Memento = None):
        self.value = 0
        super().__init__(event_stream, memento)

    def apply_memento(self, memento: Memento) -> None:
        if self.id != memento.id:
            raise ValueError("Memento id does not match aggregate id")
        self.value = memento.payload

    def get_memento(self) -> Memento:
        """
        The memento is a snapshot of the aggregate state. It is used to rehydrate the aggregate.
        
        It is backed by the Python __getstate__ and __setstate__ methods.
        """
        return Memento(id=self.id, version=self.version, payload={'key': self.value})

    def set_value(self, value: int) -> None:
        self.raise_event(
            SampleEvent(value=value, id=self.id, version=self.version,
                      timestamp=datetime.datetime.now(datetime.timezone.utc)))

    def _apply(self, event: SampleEvent) -> None:
        """
        The apply method is a convention named method to apply the event to the aggregate. It is called from the raise_event method using multiple dispatch
        """
        self.value = event.value

The saga class is likewise abstract and a subtype saga would implement the external interfaces and internal behavior, similar to the aggregate class.

from aett.domain.Domain import Saga
from aett.eventstore.EventStream import DomainEvent
from dataclasses import dataclass


@dataclass(frozen=True, kw_only=True)
class SampleEvent(DomainEvent):
    value: int

    
class SampleSaga(Saga):
    def _apply(self, event: SampleEvent) -> None:
        pass