uranusjr commented on code in PR #75184: URL: https://github.com/apache/airflow/pull/75184#discussion_r4397355996
########## task-sdk/src/airflow/sdk/coordinators/_xcom_backend.py: ########## @@ -0,0 +1,345 @@ +# +# 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. + + ``unchanged`` means the backend did not store the value, which then goes to the API server as it is. + """ + + value: JsonValue = None + unchanged: bool = False + 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 is_hook_overridden(xcom: type[BaseXCom], *hooks: str) -> bool: + """Whether ``xcom`` overrides any of ``hooks``, and so may do more than the default backend.""" + return xcom is not BaseXCom and any( + next(k for k in xcom.__mro__ if hook in vars(k)) is not BaseXCom for hook in hooks + ) + + [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 result reaches the supervisor through ``on_result``. Connection and Variable lookups of + the backend reach the API server through ``client``. + """ + + client: Client + on_result: Callable[[XComBackendResult], None] = attrs.field(repr=False) + """Called with the result of a call, and with an error when the process exits without answering one.""" + decoder: ClassVar[TypeAdapter[_FromXComBackendProcess]] = TypeAdapter(_FromXComBackendProcess) + + _call_request_id: int | None = attrs.field(default=None, init=False) + _queued_calls: deque[XComBackendCall] = attrs.field(factory=deque, init=False) + _unanswered_calls: int = attrs.field(default=0, init=False) + + @classmethod + def start( # type: ignore[override] + cls, + *, + id: UUID, + client: Client, + logger: FilteringBoundLogger, + selector: selectors.BaseSelector, + on_result: Callable[[XComBackendResult], None], + ) -> Self: + return super().start( + id=id, + client=client, + on_result=on_result, + # 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(), + ) + + def send_call(self, call: XComBackendCall) -> None: + """ + Send ``call`` to the process, which answers it through ``on_result``. + + The process runs one call at a time, so a call waits its turn and is answered in the order it + was sent. That is also how the first call of a task waits for the process to start. + """ + self._queued_calls.append(call) + self._unanswered_calls += 1 + self._send_next_call() + + def _send_next_call(self) -> None: + """Hand the oldest waiting call to the process, if it has asked for one.""" + if self._call_request_id is None or not self._queued_calls: + return + request_id, self._call_request_id = self._call_request_id, None + self.send_msg(self._queued_calls.popleft(), request_id=request_id) + + def stop(self) -> None: + """Let the helper process exit, and kill it if it does not exit in time.""" + # The task this process serves is over, so nothing waits for the calls it has not answered. + self._queued_calls.clear() + self._unanswered_calls = 0 + if self._exit_code is None and self._call_request_id is not None: + # The process may be dead already, such as when the task it serves was killed. + with contextlib.suppress(OSError): + self.send_msg(XComBackendCall(hook="exit"), request_id=self._call_request_id) + self._call_request_id = None + deadline = time.monotonic() + _EXIT_TIMEOUT + while self._open_sockets and (now := time.monotonic()) < deadline: + self._service_subprocess(max_wait_time=deadline - now) + if self._exit_code is None: + with contextlib.suppress(self._process.TimeoutExpired): + self._exit_code = self._process.wait(timeout=max(0.0, deadline - time.monotonic())) + if self._exit_code is None: + self.kill(signal.SIGTERM, force=True) + # Only this process's own sockets, since the selector belongs to the task it serves. + for sock in list(self._open_sockets): + self._on_socket_closed(sock) + sock.close() + + def _on_socket_closed(self, sock: socket) -> None: + super()._on_socket_closed(sock) + if sock is self.stdin: + # The process has exited, or is exiting, so it answers none of the calls it was still to run. + unanswered, self._unanswered_calls = self._unanswered_calls, 0 + for _ in range(unanswered): + self.on_result(XComBackendResult(error="The XCom backend process exited before it answered")) + + def _handle_await_call( + self, msg: AwaitXComBackendCall, log: FilteringBoundLogger, req_id: int + ) -> ResponseSent: + self._call_request_id = req_id + self._send_next_call() + return ResponseSent.ALREADY_SENT + + def _handle_result(self, msg: XComBackendResult, log: FilteringBoundLogger, req_id: int) -> ResponseSent: + self._call_request_id = req_id + self._unanswered_calls -= 1 + self._send_next_call() + self.on_result(msg) + return ResponseSent.ALREADY_SENT + + _request_handlers = { + **WatchedSubprocess._get_shared_request_handlers(GetConnection, GetVariable, MaskSecret), + **dict( + [ + register_request_method(AwaitXComBackendCall, _handle_await_call), + register_request_method(XComBackendResult, _handle_result), + ] + ), + } + + +def run_xcom_backend() -> None: + """Run the XCom backend calls that the supervisor sends, until it sends ``exit``.""" + # Lazy: these load the task runtime and the configured backend, which only the helper process needs. + from airflow.sdk.execution_time import task_runner + from airflow.sdk.execution_time.comms import CommsDecoder + from airflow.sdk.execution_time.xcom import XCom + + comms = CommsDecoder[_ToXComBackendProcess, _FromXComBackendProcess]( + log=log, body_decoder=TypeAdapter(_ToXComBackendProcess) + ) + # The secrets backends of a task process look up Connections and Variables through SUPERVISOR_COMMS. + task_runner.SUPERVISOR_COMMS = comms # type: ignore[assignment] + runner = _HookRunner(XCom) + call = comms.send(AwaitXComBackendCall()) + while isinstance(call, XComBackendCall) and call.hook != "exit": + call = comms.send(runner.run(call)) + + [email protected] +class _HookRunner: + """Runs the hooks of an XCom backend that the supervisor's calls name, one method per hook.""" + + xcom: type[BaseXCom] + + def run(self, call: XComBackendCall) -> XComBackendResult: + try: + if (hook := getattr(self, call.hook, None)) is None: + raise ValueError(f"Unknown XCom backend hook {call.hook!r}") + return hook(call) + except Exception as e: + log.exception("XCom backend call failed", hook=call.hook, key=call.key) + return XComBackendResult(error=f"{type(e).__name__}: {e}") + + def store(self, call: XComBackendCall) -> XComBackendResult: + # Lazy: only the helper process decodes and encodes XCom values. + from airflow.sdk.serde import deserialize + + if call.coordinate is None: + raise ValueError("A store call needs the coordinate of the XCom") + try: + value = deserialize(call.value) # type: ignore[type-var] + except (ImportError, TypeError, AttributeError, ValueError) as e: + # The default backend would keep such a value in the XCom row as it is. + log.warning( + "Cannot decode the XCom value; storing it without the XCom backend", key=call.key, error=e + ) + return XComBackendResult(unchanged=True) Review Comment: Falling back to storing the value in the DB defeats the purpose of the backend. People configure object storage precisely to keep payloads out of the metadata DB, and some for compliance. A Python task would fail here, so this should fail the push too. Raising is probably better. Same for the block in `_load_row`. -- 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]
