uranusjr commented on code in PR #75184: URL: https://github.com/apache/airflow/pull/75184#discussion_r4439675915
########## task-sdk/src/airflow/sdk/coordinators/_xcom_backend.py: ########## @@ -0,0 +1,337 @@ +# +# 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. +""" +The XCom backend for the tasks of a language SDK runtime. + +A language SDK runtime sends its XCom requests to the supervisor, which runs the configured XCom backend +on them before it forwards them to the API server. The backend runs in a helper process, started per task +the way the supervisor starts a Python task process, so the long-lived supervisor process keeps none of +the backend's state, such as a cached filesystem that a task process forked from it later could not use. + +The supervisor services the helper process from the selector of the task it serves, so a backend call +never blocks that task's request loop, which goes on heartbeating and enforcing ``execution_timeout``. +""" + +from __future__ import annotations + +import contextlib +import selectors +import signal +import time +from collections import deque +from collections.abc import Callable +from typing import TYPE_CHECKING, Annotated, ClassVar, Literal +from uuid import UUID + +import attrs +import structlog +from pydantic import BaseModel, Field, JsonValue, TypeAdapter + +from airflow.sdk.bases.xcom import BaseXCom, XComCoordinate +from airflow.sdk.execution_time.comms import ( + ConnectionResult, + ErrorResponse, + GetConnection, + GetVariable, + MaskSecret, + VariableResult, + XComResult, +) +from airflow.sdk.execution_time.supervisor import ( + ResponseSent, + WatchedSubprocess, + register_request_method, + task_process_uses_exec, +) + +if TYPE_CHECKING: + from socket import socket + from typing import Self + + from structlog.typing import FilteringBoundLogger + + from airflow.sdk.api.client import Client + +log: FilteringBoundLogger = structlog.get_logger(logger_name="xcom_backend") + +_EXIT_TIMEOUT = 10.0 + + +class XComBackendError(RuntimeError): + """ + An XCom backend call failed. + + For an exception raised in the helper process, the message holds its type and message, since the + exception itself does not cross the process boundary. + """ + + +class _Coordinate(BaseModel): + dag_id: str + run_id: str + task_id: str + ti_id: UUID | None = None + region_id: UUID | None = None + region_index: int | None = None + + +class XComBackendCall(BaseModel): + """ + A call of an XCom backend hook, sent to the helper process. + + ``value`` is a value as the runtime sends it for ``store``, the XCom row for ``load`` and ``purge``, + and a list of rows for ``load_many``. + """ + + hook: Literal["store", "load", "load_many", "purge", "exit"] + key: str = "" + value: JsonValue = None + coordinate: _Coordinate | None = None + map_index: int = -1 + type: Literal["XComBackendCall"] = "XComBackendCall" + + @classmethod + def store(cls, coordinate: XComCoordinate, map_index: int, key: str, value: JsonValue) -> Self: + return cls( + hook="store", + key=key, + value=value, + coordinate=_Coordinate.model_validate(attrs.asdict(coordinate)), + map_index=map_index, + ) + + +class XComBackendResult(BaseModel): + """The outcome of an XCom backend call, sent by the helper process.""" + + value: JsonValue = None + error: str | None = None + type: Literal["XComBackendResult"] = "XComBackendResult" + + +class AwaitXComBackendCall(BaseModel): + """Sent by the helper process when it is ready for its first call.""" + + type: Literal["AwaitXComBackendCall"] = "AwaitXComBackendCall" + + +_ToXComBackendProcess = Annotated[ + XComBackendCall | ConnectionResult | VariableResult | ErrorResponse, + Field(discriminator="type"), +] +_FromXComBackendProcess = Annotated[ + AwaitXComBackendCall | XComBackendResult | GetConnection | GetVariable | MaskSecret, + Field(discriminator="type"), +] + + +def runs_in_helper(xcom: type[BaseXCom]) -> bool: + """Whether task XComs go through the helper process, which runs every backend but the default one.""" + return xcom is not BaseXCom + + +_PendingCall = tuple[XComBackendCall, Callable[[XComBackendResult], None]] + + [email protected](kw_only=True) +class XComBackendProcess(WatchedSubprocess): + """ + The helper process that runs XCom backend calls for the task of a language SDK runtime. + + The process asks for a call, runs it, and answers with its result, which is also how it asks for the + next call. Each call carries the callback that takes its result, so a caller never has to match an + answer to its own call. Connection and Variable lookups of the backend reach the API server through + ``client``. + """ + + client: Client + decoder: ClassVar[TypeAdapter[_FromXComBackendProcess]] = TypeAdapter(_FromXComBackendProcess) + + _call_request_id: int | None = attrs.field(default=None, init=False) + _queued_calls: deque[_PendingCall] = attrs.field(factory=deque, init=False) + _running_call: Callable[[XComBackendResult], None] | None = attrs.field(default=None, init=False) + + @classmethod + def start( # type: ignore[override] + cls, + *, + id: UUID, + client: Client, + logger: FilteringBoundLogger, + selector: selectors.BaseSelector, + ) -> Self: + return super().start( + id=id, + client=client, + # The selector of the task this process serves, so that the task's request loop services both. + selector=selector, + logger=logger, + target=run_xcom_backend, + use_exec=task_process_uses_exec(), + ) + + @property + def can_run_a_call(self) -> bool: + """Whether the process can still be sent a call, which a process that has exited cannot.""" + return self.stdin in self._open_sockets + + def send_call(self, call: XComBackendCall, finish: Callable[[XComBackendResult], None]) -> None: + """ + Send ``call`` to the process, which answers it by calling ``finish`` with its result. + + The process runs one call at a time, so a call waits its turn. ``finish`` also takes the error of + a call the process never answers, such as when it exits while the call is waiting. + """ + self._queued_calls.append((call, finish)) + self._send_next_call() + + def _send_next_call(self) -> None: + """Hand the oldest waiting call to the process, if it has asked for one and runs none.""" + if self._call_request_id is None or self._running_call is not None or not self._queued_calls: + return + call, self._running_call = self._queued_calls.popleft() + request_id, self._call_request_id = self._call_request_id, None + self.send_msg(call, request_id=request_id) + + def _fail_waiting_calls(self, error: str) -> None: + """Answer every call the process has not answered with ``error``.""" + running, self._running_call = self._running_call, None + waiting = [running] if running is not None else [] + waiting += [finish for _, finish in self._queued_calls] + self._queued_calls.clear() + for finish in waiting: + finish(XComBackendResult(error=error)) + + def stop(self) -> None: + """Let the helper process exit, and kill it if it does not exit in time.""" + # The caller gives up on the calls it has not answered, so they are dropped rather than failed. Review Comment: Dropping the waiting calls instead of failing them is safe for the current callers, but a future caller that stops the process mid-task would leave the runtime waiting forever. -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: [email protected] For queries about this service, please contact Infrastructure at: [email protected]
