Overview

Quick setup
Middleware interface
Lifecycle
Built-in classes

Middlewares can observe event lifecycle transitions, react to handler registration changes, and add cross-cutting behavior around event execution.

Quick setup

Python
TypeScript

from abxbus import EventBus
from abxbus.middlewares import LoggerEventBusMiddleware, WALEventBusMiddleware

bus = EventBus(
    name='MyBus',
    middlewares=[
        WALEventBusMiddleware('./events.jsonl'),
        LoggerEventBusMiddleware('./events.log'),
    ],
)

import { BaseEvent, EventBus, type EventBusMiddleware, type EventStatus } from 'abxbus'

class LoggingMiddleware implements EventBusMiddleware {
  async onEventChange(eventbus: EventBus, event: BaseEvent, status: EventStatus): Promise<void> {
    if (status === 'completed') {
      console.log(`[${eventbus.label}] ${event.event_type}#${event.event_id.slice(-4)}`)
    }
  }
}

const bus = new EventBus('MyBus', {
  middlewares: [LoggingMiddleware],
})

Middleware interface

Use three optional hooks:

Event lifecycle: on_event_change / onEventChange
Handler-result lifecycle: on_event_result_change / onEventResultChange
Handler registration lifecycle: on_bus_handlers_change / onBusHandlersChange

See EventBusMiddleware for full signatures and custom middleware examples.

Lifecycle

Middleware hooks receive lifecycle statuses in strict order:

Event hooks: pending -> started -> completed
Event-result hooks: pending -> started -> completed
Handler registration hooks: called when handlers are added or removed via on(...) and off(...)

status passed to lifecycle hooks is never error. Handler failures are exposed on event_result.status and event_result.error during the completed callback.

Built-in classes

Python
TypeScript

retry

OtelTracingMiddleware

Getting Started

Features

API Reference

Concurrency Control

Integrations

Further Reading

Quick setup

Middleware interface

Lifecycle

Built-in classes

Getting Started

Features

API Reference

Concurrency Control

Integrations

Further Reading

​Quick setup

​Middleware interface

​Lifecycle

​Built-in classes

Quick setup

Middleware interface

Lifecycle

Built-in classes