Lee-W commented on code in PR #66584: URL: https://github.com/apache/airflow/pull/66584#discussion_r3266403915
########## airflow-core/src/airflow/triggers/shared_stream.py: ########## @@ -0,0 +1,276 @@ +# Licensed to the Apache Software Foundation (ASF) under one +# or more contributor license agreements. See the NOTICE file +# distributed with this work for additional information +# regarding copyright ownership. The ASF licenses this file +# to you under the Apache License, Version 2.0 (the +# "License"); you may not use this file except in compliance +# with the License. You may obtain a copy of the License at +# +# http://www.apache.org/licenses/LICENSE-2.0 +# +# Unless required by applicable law or agreed to in writing, +# software distributed under the License is distributed on an +# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY +# KIND, either express or implied. See the License for the +# specific language governing permissions and limitations +# under the License. +""" +Shared underlying I/O between :class:`BaseEventTrigger` instances in the triggerer. + +When multiple triggers declare the same non-``None`` +:meth:`~airflow.triggers.base.BaseEventTrigger.shared_stream_key`, the +triggerer routes them through :class:`SharedStreamManager` so that one +underlying poll loop produces raw events that are broadcast to every +participating trigger. Each trigger then runs +:meth:`~airflow.triggers.base.BaseEventTrigger.filter_shared_stream` to +convert the broadcast into its own :class:`~airflow.triggers.base.TriggerEvent` +instances. Triggers that opt out (the default) keep their independent +``run()``-based poll loops untouched. + +Lifecycle invariants +-------------------- + +The manager and groups cooperate to keep a single invariant true at every +``await``-point: + + A key is present in :attr:`SharedStreamManager._groups` only while its + group's poll task is alive and accepting new subscribers. + +This rules out the late-subscriber races that the naive design admits — a +new subscriber for a key whose poll has died or is in the middle of being +torn down always falls through to "create a fresh group" rather than +attaching to a dead one and hanging on an empty queue. The invariant is +maintained synchronously: + +* When ``_poll`` ends for any reason other than cancellation (the upstream + iterator raised, or returned), the group's ``finally`` block evicts the + key from ``_groups`` and broadcasts a terminal sentinel to current + subscribers — all without yielding, so no other coroutine can interleave. +* When the last subscriber leaves, :meth:`SharedStreamManager.unsubscribe` + evicts the key from ``_groups`` *before* awaiting ``group.stop()``, so a + new subscriber arriving while we wait for cancellation creates a fresh + group. +* :meth:`SharedStreamManager.stop_all` clears ``_groups`` in one synchronous + step before awaiting any stop, applying the same rule to shutdown. +""" + +from __future__ import annotations + +import asyncio +from collections.abc import AsyncIterator, Callable, Hashable +from contextlib import suppress +from typing import TYPE_CHECKING, Any + +import structlog + +if TYPE_CHECKING: + from structlog.stdlib import BoundLogger + + from airflow.triggers.base import BaseEventTrigger + +log = structlog.get_logger(__name__) + + +class _PollTerminated(Exception): + """ + Raised inside subscribers when ``open_shared_stream`` returns without yielding more events. + + Implementations are expected to run for the lifetime of the group; an + early return would otherwise leave subscribers waiting forever on an + empty queue. + """ + + +class _PollFailure: + """Sentinel propagated through subscriber queues when the shared poll ends.""" + + __slots__ = ("exc",) + + def __init__(self, exc: BaseException) -> None: + self.exc = exc + + +async def _drain(queue: asyncio.Queue) -> AsyncIterator[Any]: + """ + Yield items from ``queue`` until a poll termination sentinel arrives. + + Subscribers exit either by their consuming task being cancelled + (Airflow's standard idiom — :class:`CancelledError` propagates through + ``queue.get()``) or by the shared poll ending, in which case the + :class:`_PollFailure` sentinel re-raises here. + """ + while True: + item = await queue.get() + if isinstance(item, _PollFailure): + raise item.exc + yield item + + +class _SharedStreamGroup: + """One shared poll loop broadcasting raw events to N subscriber queues.""" + + def __init__( + self, + *, + key: Hashable, + trigger_class: type[BaseEventTrigger], + kwargs: dict[str, Any], + on_poll_terminate: Callable[[_SharedStreamGroup], None], + log: BoundLogger, + ) -> None: + self.key = key + self.trigger_class = trigger_class + self.kwargs = kwargs + self.log = log + self._on_poll_terminate = on_poll_terminate + self._subscribers: dict[int, asyncio.Queue] = {} + self._poll_task: asyncio.Task | None = None + + def start(self) -> None: + """Start the underlying poll loop. Call exactly once per group.""" + if self._poll_task is not None: + raise RuntimeError(f"Shared stream group {self.key!r} already started") + self._poll_task = asyncio.create_task( + self._poll(), + name=f"shared-stream-poll[{self.key!r}]", + ) + + async def _poll(self) -> None: + terminal_exc: BaseException | None = None + try: + async for raw_event in self.trigger_class.open_shared_stream(self.kwargs): + for queue in self._subscribers.values(): + queue.put_nowait(raw_event) + terminal_exc = _PollTerminated( + f"open_shared_stream for {self.key!r} returned without raising; " + "shared streams are expected to run for the lifetime of the group" + ) + except asyncio.CancelledError: + # ``stop()`` initiated this; the manager has already evicted the + # group and is awaiting our exit. Do not run the terminate path. + raise + except Exception as exc: + terminal_exc = exc + self.log.exception("Shared stream poll failed; propagating to subscribers", key=self.key) + finally: + if terminal_exc is not None: + # Synchronous: evict from the manager and broadcast the + # sentinel before returning to the loop, so no coroutine can + # observe ``_groups[key]`` pointing at a dead poll. + self._on_poll_terminate(self) + failure = _PollFailure(terminal_exc) + for queue in self._subscribers.values(): + queue.put_nowait(failure) + + def subscribe(self, trigger_id: int) -> AsyncIterator[Any]: + """Register ``trigger_id`` as a subscriber and return its raw event stream.""" + if trigger_id in self._subscribers: + raise RuntimeError(f"Trigger {trigger_id} already subscribed to shared stream {self.key!r}") + queue: asyncio.Queue = asyncio.Queue() Review Comment: Bounded now — `asyncio.Queue(maxsize=self._max_subscriber_queue)` per subscriber (default 1024, configurable via `[triggerer] shared_stream_subscriber_queue_size` -- This is an automated message from the Apache Git Service. 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