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
Go
Rust

import asyncio
from abxbus 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.now()

assert max_in_flight >= 2

import { BaseEvent, EventBus } from 'abxbus'

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.now()

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

package main

import (
	"sync"

	abxbus "github.com/ArchiveBox/abxbus/abxbus-go"
)

func main() {
	bus := abxbus.NewEventBus("ParallelHandlerBus", &abxbus.EventBusOptions{
		EventHandlerConcurrency: abxbus.EventHandlerConcurrencyParallel,
	})

	var mu sync.Mutex
	inFlight := 0
	maxInFlight := 0
	started := make(chan struct{}, 2)
	release := make(chan struct{})

	tracked := func(event *abxbus.BaseEvent) (any, error) {
		mu.Lock()
		inFlight++
		if inFlight > maxInFlight {
			maxInFlight = inFlight
		}
		mu.Unlock()

		started <- struct{}{}
		<-release

		mu.Lock()
		inFlight--
		mu.Unlock()
		return nil, nil
	}

	bus.On("HandlerEvent", "tracked_a", tracked, nil)
	bus.On("HandlerEvent", "tracked_b", tracked, nil)

	event := bus.Emit(abxbus.NewBaseEvent("HandlerEvent", nil))
	<-started
	<-started
	close(release)
	if _, err := event.Now(); err != nil {
		panic(err)
	}

	if maxInFlight < 2 {
		panic("expected overlapping handlers")
	}
}

use abxbus::{
    event,
    event_bus::{EventBus, EventBusOptions},
    BaseEvent,
    types::EventHandlerConcurrencyMode,
};
use futures::executor::block_on;

event! {
    struct HandlerEvent {
        event_result_type: (),
    }
}

let bus = EventBus::new_with_options(
    Some("ParallelHandlerBus".to_string()),
    EventBusOptions {
        event_handler_concurrency: EventHandlerConcurrencyMode::Parallel,
        ..EventBusOptions::default()
    },
);

bus.on(HandlerEvent, |_event: HandlerEvent| async move { Ok(()) });
bus.on(HandlerEvent, |_event: HandlerEvent| async move { Ok(()) });

let event = bus.emit(HandlerEvent { ..Default::default() });
block_on(event.now());

Notes

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

​Lifecycle impact

​Execution order example

​Notes

Lifecycle impact

Execution order example

Notes