Handlers: parallel

parallel allows multiple handlers for the same event to run at the same time. Companion runnable example:

Lifecycle impact

Event starts processing.
All applicable handlers are scheduled concurrently.
Event completion waits based on completion mode (all or first).
Per-handler timeout/error behavior remains independent per handler.

Execution order example

Python
TypeScript

import asyncio
from bubus import BaseEvent, EventBus

class HandlerEvent(BaseEvent):
    pass

bus = EventBus('ParallelHandlerBus', event_handler_concurrency='parallel')

in_flight = 0
max_in_flight = 0
release = asyncio.Event()

async def tracked(_: HandlerEvent) -> None:
    global in_flight, max_in_flight
    in_flight += 1
    max_in_flight = max(max_in_flight, in_flight)
    await release.wait()
    in_flight -= 1

bus.on(HandlerEvent, tracked)
bus.on(HandlerEvent, tracked)

event = bus.emit(HandlerEvent())
await asyncio.sleep(0)
release.set()
await event

assert max_in_flight >= 2

import { BaseEvent, EventBus } from 'bubus'

const HandlerEvent = BaseEvent.extend('HandlerEvent', {})
const bus = new EventBus('ParallelHandlerBus', { event_handler_concurrency: 'parallel' })

let inFlight = 0
let maxInFlight = 0
let release!: () => void
const gate = new Promise<void>((resolve) => {
  release = resolve
})

const tracked = async () => {
  inFlight += 1
  maxInFlight = Math.max(maxInFlight, inFlight)
  await gate
  inFlight -= 1
}

bus.on(HandlerEvent, tracked)
bus.on(HandlerEvent, tracked)

const event = bus.emit(HandlerEvent({}))
await new Promise((resolve) => setTimeout(resolve, 0))
release()
await event.done()

if (maxInFlight < 2) throw new Error('expected overlapping handlers')

Notes

Best for independent I/O-bound handlers where overlap reduces total latency.
If handlers mutate shared resources, add explicit synchronization.

Getting Started

Features

API Reference

Concurrency Control

Integrations

Further Reading

Handlers: parallel

Lifecycle impact

Execution order example

Notes

Getting Started

Features

API Reference

Concurrency Control

Integrations

Further Reading

​Lifecycle impact

​Execution order example

​Notes

Lifecycle impact

Execution order example

Notes