Handler Completion: first

first short-circuits event completion once the first successful non-None / non-undefined result is available.

Lifecycle impact

The first successful result wins (None/undefined and errors do not win).
In serial handler mode, remaining handlers are skipped once a winner appears.
In parallel handler mode, in-flight losers are cancelled or aborted.
Event completion resolves as soon as a winner is found (or all handlers fail).

Execution order example

Python
TypeScript
Go
Rust

import asyncio
from abxbus import BaseEvent, EventBus

class CompletionEvent(BaseEvent[str]):
    pass

bus = EventBus(
    'CompletionFirstBus',
    event_handler_concurrency='parallel',
    event_handler_completion='first',
)

state = {'slow_started': False, 'slow_cancelled': False}

async def fast_handler(_: CompletionEvent) -> str:
    await asyncio.sleep(0.01)
    return 'winner'

async def slow_handler(_: CompletionEvent) -> str:
    state['slow_started'] = True
    try:
        await asyncio.sleep(0.5)
        return 'slow'
    except asyncio.CancelledError:
        state['slow_cancelled'] = True
        raise

bus.on(CompletionEvent, slow_handler)
bus.on(CompletionEvent, fast_handler)

event = bus.emit(CompletionEvent())
await event.now()

value = await event.event_result(raise_if_any=False, raise_if_none=False)
assert value == 'winner'
assert state['slow_started'] is True
assert state['slow_cancelled'] is True

import { BaseEvent, EventBus } from 'abxbus'
import { z } from 'zod'

const CompletionEvent = BaseEvent.extend('CompletionEvent', { event_result_type: z.string() })
const bus = new EventBus('CompletionFirstBus', {
  event_handler_concurrency: 'parallel',
  event_handler_completion: 'first',
})

let slowStarted = false
let slowCompleted = false
const delay = (ms: number) => new Promise((resolve) => setTimeout(resolve, ms))

bus.on(CompletionEvent, async () => {
  slowStarted = true
  await delay(500)
  slowCompleted = true
  return 'slow'
})

bus.on(CompletionEvent, async () => {
  await delay(10)
  return 'winner'
})

const event = bus.emit(CompletionEvent({}))
await event.now()

if ((await event.eventResult({ raise_if_any: false, raise_if_none: false })) !== 'winner') {
  throw new Error('expected first winner result')
}
if (!slowStarted) throw new Error('expected slow handler to start')
if (slowCompleted) throw new Error('slow handler should not complete before event resolves')

package main

import (
	"time"

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

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

	slowStarted := make(chan struct{}, 1)
	slowCompleted := make(chan struct{}, 1)

	bus.On("CompletionEvent", "slow_handler", func(event *abxbus.BaseEvent) (any, error) {
		slowStarted <- struct{}{}
		time.Sleep(500 * time.Millisecond)
		slowCompleted <- struct{}{}
		return "slow", nil
	}, nil)

	bus.On("CompletionEvent", "fast_handler", func(event *abxbus.BaseEvent) (any, error) {
		time.Sleep(10 * time.Millisecond)
		return "winner", nil
	}, nil)

	event := bus.Emit(abxbus.NewBaseEvent("CompletionEvent", nil))
	_, err := event.Now(&abxbus.EventWaitOptions{FirstResult: true})
	if err != nil {
		panic(err)
	}
	value, err := event.EventResult(&abxbus.EventResultOptions{
		RaiseIfAny:  false,
		RaiseIfNone: false,
	})
	if err != nil {
		panic(err)
	}
	if value != "winner" {
		panic("expected first winner result")
	}
	select {
	case <-slowStarted:
	default:
		panic("expected slow handler to start")
	}
	select {
	case <-slowCompleted:
		panic("slow handler should not complete before event resolves")
	default:
	}
}

use abxbus::{
    event,
    event_bus::{EventBus, EventBusOptions},
    BaseEvent,
    types::{EventHandlerCompletionMode, EventHandlerConcurrencyMode},
};
use futures::executor::block_on;
use futures_timer::Delay;
use serde_json::json;
use std::time::Duration;

event! {
    struct CompletionEvent {
        event_result_type: serde_json::Value,
    }
}

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

bus.on(CompletionEvent, |_event: CompletionEvent| async move {
    Delay::new(Duration::from_millis(500)).await;
    Ok(json!("slow"))
});
bus.on(CompletionEvent, |_event: CompletionEvent| async move {
    Delay::new(Duration::from_millis(10)).await;
    Ok(json!("winner"))
});

let event = bus.emit(CompletionEvent { ..Default::default() });
let value = block_on(event.event_result())?;
assert_eq!(value, Some(json!("winner")));

Notes

This mode is useful for fallback chains and race-to-first-response patterns.
Use await event.now(first_result=True) / await event.now({ first_result: true }) / event.Now(&abxbus.EventWaitOptions{FirstResult: true}) / event.now_with_options(EventWaitOptions { first_result: true, ..Default::default() }) when you want to resolve as soon as a valid result is available while still allowing all handlers to continue. To stop running after the first valid handler result, set event_handler_completion: 'first' on the event definition.

Getting Started

Features

API Reference

Concurrency Control

Integrations

Further Reading

Handler Completion: first

Lifecycle impact

Execution order example

Notes

​Lifecycle impact

​Execution order example

​Notes

Lifecycle impact

Execution order example

Notes