spakky-event 6.1.2

Event handling stereotype for Spakky Framework (@EventHandler, @on_event)

Spakky Event

Event handling stereotype for Spakky Framework.

Installation

pip install spakky-event

Features

• @EventHandler: Stereotype for event handler classes
• @on_event: Decorator for marking methods as event handlers
• Type-safe: Full type hint support for event types
• Async support: Native async/await for event processing

Quick Start

Define Events

Events should extend one of the event base classes from spakky-domain:

• Use AbstractDomainEvent for events within a bounded context (in-process domain events)
• Use AbstractIntegrationEvent for cross-boundary events (message broker integration)

When using message broker plugins (RabbitMQ / Kafka), define events as AbstractIntegrationEvent.

from spakky.core.common.mutability import immutable
from spakky.domain.models.event import AbstractIntegrationEvent


@immutable
class UserCreatedEvent(AbstractIntegrationEvent):
    user_id: str
    email: str


@immutable
class UserDeletedEvent(AbstractIntegrationEvent):
    user_id: str

Create Event Handler

Use @EventHandler stereotype with @on_event decorators:

from spakky.event.stereotype.event_handler import EventHandler, on_event


@EventHandler()
class UserEventHandler:
    def __init__(self, notification_service: NotificationService) -> None:
        self.notification_service = notification_service

    @on_event(UserCreatedEvent)
    async def on_user_created(self, event: UserCreatedEvent) -> None:
        await self.notification_service.send_welcome_email(event.email)

    @on_event(UserDeletedEvent)
    async def on_user_deleted(self, event: UserDeletedEvent) -> None:
        await self.notification_service.send_goodbye_email(event.user_id)

Integration with Message Brokers

The @EventHandler stereotype works with Spakky's message broker plugins:

RabbitMQ

pip install spakky-rabbitmq

from spakky.core.application.application import SpakkyApplication
from spakky.core.application.application_context import ApplicationContext

app = (
    SpakkyApplication(ApplicationContext())
    .load_plugins()  # Loads spakky-rabbitmq plugin
    .scan()
    .start()
)

Kafka

pip install spakky-kafka

from spakky.core.application.application import SpakkyApplication
from spakky.core.application.application_context import ApplicationContext

app = (
    SpakkyApplication(ApplicationContext())
    .load_plugins()  # Loads spakky-kafka plugin
    .scan()
    .start()
)

API Reference

Stereotypes

Decorator	Description
@EventHandler()	Marks a class as an event handler (extends @Pod)
@on_event(EventType)	Marks a method as handler for specific event type

Interfaces

Interface	Description
IEventPublisher / IAsyncEventPublisher	Event publish entry point (type-based routing)
IEventBus / IAsyncEventBus	Integration event send entry point (Outbox seam)
IEventTransport / IAsyncEventTransport	Actual message broker transport
IEventConsumer / IAsyncEventConsumer	Handler callback registration
IEventDispatcher / IAsyncEventDispatcher	In-process handler dispatch

Implementations

Class	Description
EventMediator / AsyncEventMediator	Consumer + Dispatcher combined (in-process)
EventPublisher / AsyncEventPublisher	Type-based router (DomainEvent→Mediator, IntegrationEvent→EventBus)
DirectEventBus / AsyncDirectEventBus	Default EventBus → EventTransport delegation
EventHandlerRegistrationPostProcessor	Auto-registers @EventHandler methods

Types

Type	Description
EventRoute	Annotation class for event routing metadata
EventT_contra	TypeVar bound to AbstractEvent
EventHandlerCallback	Type alias for sync event callbacks
AsyncEventHandlerCallback	Type alias for async event callbacks

Errors

Class	Description
AbstractSpakkyEventError	Base error for event operations
InvalidMessageError	Raised when message is malformed

Related Packages

Package	Description
spakky-domain	DDD building blocks including AbstractEvent, AbstractDomainEvent, AbstractIntegrationEvent
spakky-rabbitmq	RabbitMQ transport (RabbitMQEventTransport)
spakky-kafka	Kafka transport (KafkaEventTransport)

In-process Domain Event Publishing

For events within a bounded context (DomainEvents), use the in-process publisher:

from spakky.core.application.application import SpakkyApplication
from spakky.core.application.application_context import ApplicationContext
from spakky.event import IAsyncEventPublisher

# Bootstrap application (load_plugins auto-registers event components)
app = (
    SpakkyApplication(ApplicationContext())
    .load_plugins()
    .scan()
    .start()
)

# Get publisher from container
publisher = app.container.get(IAsyncEventPublisher)
await publisher.publish(UserCreatedEvent(user_id="123", email="[e-mail]"))

Architecture (ISP Compliant)

The in-process event system follows Interface Segregation Principle:

• Consumer: Registers event handlers (register() method)
• Dispatcher: Dispatches events to handlers (dispatch() method)
• Mediator: Combines both interfaces in a single implementation
• Publisher: Depends only on Dispatcher (not Consumer)

┌─────────────────┐     ┌─────────────────┐
│   Publisher     │────▶│   Dispatcher    │
└─────────────────┘     └────────┬────────┘
                                 │
                        ┌────────▼────────┐
                        │    Mediator     │
                        │ (Consumer +     │
                        │  Dispatcher)    │
                        └────────┬────────┘
                                 │
                        ┌────────▼────────┐
                        │  EventHandler   │
                        │  @on_event()    │
                        └─────────────────┘

License

MIT License