MatrixSynapse/synapse/util/task_scheduler.py

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# Copyright 2023 The Matrix.org Foundation C.I.C.
#
# Licensed 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.
import logging
from typing import TYPE_CHECKING, Awaitable, Callable, Dict, List, Optional, Set, Tuple
from prometheus_client import Gauge
from twisted.python.failure import Failure
from synapse.logging.context import nested_logging_context
from synapse.metrics.background_process_metrics import run_as_background_process
from synapse.types import JsonMapping, ScheduledTask, TaskStatus
from synapse.util.stringutils import random_string
if TYPE_CHECKING:
from synapse.server import HomeServer
logger = logging.getLogger(__name__)
running_tasks_gauge = Gauge(
"synapse_scheduler_running_tasks",
"The number of concurrent running tasks handled by the TaskScheduler",
)
class TaskScheduler:
"""
This is a simple task sheduler aimed at resumable tasks: usually we use `run_in_background`
to launch a background task, or Twisted `deferLater` if we want to do so later on.
The problem with that is that the tasks will just stop and never be resumed if synapse
is stopped for whatever reason.
How this works:
- A function mapped to a named action should first be registered with `register_action`.
This function will be called when trying to resuming tasks after a synapse shutdown,
so this registration should happen when synapse is initialised, NOT right before scheduling
a task.
- A task can then be launched using this named action with `schedule_task`. A `params` dict
can be passed, and it will be available to the registered function when launched. This task
can be launch either now-ish, or later on by giving a `timestamp` parameter.
The function may call `update_task` at any time to update the `result` of the task,
and this can be used to resume the task at a specific point and/or to convey a result to
the code launching the task.
You can also specify the `result` (and/or an `error`) when returning from the function.
The reconciliation loop runs every minute, so this is not a precise scheduler.
There is a limit of 10 concurrent tasks, so tasks may be delayed if the pool is already
full. In this regard, please take great care that scheduled tasks can actually finished.
For now there is no mechanism to stop a running task if it is stuck.
Tasks will be run on the worker specified with `run_background_tasks_on` config,
or the main one by default.
"""
# Precision of the scheduler, evaluation of tasks to run will only happen
# every `SCHEDULE_INTERVAL_MS` ms
SCHEDULE_INTERVAL_MS = 1 * 60 * 1000 # 1mn
# Time before a complete or failed task is deleted from the DB
KEEP_TASKS_FOR_MS = 7 * 24 * 60 * 60 * 1000 # 1 week
# Maximum number of tasks that can run at the same time
MAX_CONCURRENT_RUNNING_TASKS = 10
# Time from the last task update after which we will log a warning
LAST_UPDATE_BEFORE_WARNING_MS = 24 * 60 * 60 * 1000 # 24hrs
def __init__(self, hs: "HomeServer"):
self._hs = hs
self._store = hs.get_datastores().main
self._clock = hs.get_clock()
self._running_tasks: Set[str] = set()
# A map between action names and their registered function
self._actions: Dict[
str,
Callable[
[ScheduledTask],
Awaitable[Tuple[TaskStatus, Optional[JsonMapping], Optional[str]]],
],
] = {}
self._run_background_tasks = hs.config.worker.run_background_tasks
if self._run_background_tasks:
self._clock.looping_call(
run_as_background_process,
TaskScheduler.SCHEDULE_INTERVAL_MS,
"handle_scheduled_tasks",
self._handle_scheduled_tasks,
)
def register_action(
self,
function: Callable[
[ScheduledTask],
Awaitable[Tuple[TaskStatus, Optional[JsonMapping], Optional[str]]],
],
action_name: str,
) -> None:
"""Register a function to be executed when an action is scheduled with
the specified action name.
Actions need to be registered as early as possible so that a resumed action
can find its matching function. It's usually better to NOT do that right before
calling `schedule_task` but rather in an `__init__` method.
Args:
function: The function to be executed for this action. The parameter
passed to the function when launched is the `ScheduledTask` being run.
The function should return a tuple of new `status`, `result`
and `error` as specified in `ScheduledTask`.
action_name: The name of the action to be associated with the function
"""
self._actions[action_name] = function
async def schedule_task(
self,
action: str,
*,
resource_id: Optional[str] = None,
timestamp: Optional[int] = None,
params: Optional[JsonMapping] = None,
) -> str:
"""Schedule a new potentially resumable task. A function matching the specified
`action` should have be registered with `register_action` before the task is run.
Args:
action: the name of a previously registered action
resource_id: a task can be associated with a resource id to facilitate
getting all tasks associated with a specific resource
timestamp: if `None`, the task will be launched as soon as possible, otherwise it
will be launch as soon as possible after the `timestamp` value.
Note that this scheduler is not meant to be precise, and the scheduling
could be delayed if too many tasks are already running
params: a set of parameters that can be easily accessed from inside the
executed function
Returns:
The id of the scheduled task
"""
status = TaskStatus.SCHEDULED
if timestamp is None or timestamp < self._clock.time_msec():
timestamp = self._clock.time_msec()
status = TaskStatus.ACTIVE
task = ScheduledTask(
random_string(16),
action,
status,
timestamp,
resource_id,
params,
result=None,
error=None,
)
await self._store.insert_scheduled_task(task)
if status == TaskStatus.ACTIVE:
if self._run_background_tasks:
await self._launch_task(task)
else:
self._hs.get_replication_command_handler().send_new_active_task(task.id)
return task.id
async def update_task(
self,
id: str,
*,
timestamp: Optional[int] = None,
status: Optional[TaskStatus] = None,
result: Optional[JsonMapping] = None,
error: Optional[str] = None,
) -> bool:
"""Update some task associated values. This is exposed publically so it can
be used inside task functions, mainly to update the result and be able to
resume a task at a specific step after a restart of synapse.
It can also be used to stage a task, by setting the `status` to `SCHEDULED` with
a new timestamp.
The `status` can only be set to `ACTIVE` or `SCHEDULED`, `COMPLETE` and `FAILED`
are terminal status and can only be set by returning it in the function.
Args:
id: the id of the task to update
timestamp: useful to schedule a new stage of the task at a later date
status: the new `TaskStatus` of the task
result: the new result of the task
error: the new error of the task
"""
if status == TaskStatus.COMPLETE or status == TaskStatus.FAILED:
raise Exception(
"update_task can't be called with a FAILED or COMPLETE status"
)
if timestamp is None:
timestamp = self._clock.time_msec()
return await self._store.update_scheduled_task(
id,
timestamp,
status=status,
result=result,
error=error,
)
async def get_task(self, id: str) -> Optional[ScheduledTask]:
"""Get a specific task description by id.
Args:
id: the id of the task to retrieve
Returns:
The task information or `None` if it doesn't exist or it has
already been removed because it's too old.
"""
return await self._store.get_scheduled_task(id)
async def get_tasks(
self,
*,
actions: Optional[List[str]] = None,
resource_id: Optional[str] = None,
statuses: Optional[List[TaskStatus]] = None,
max_timestamp: Optional[int] = None,
) -> List[ScheduledTask]:
"""Get a list of tasks. Returns all the tasks if no args is provided.
If an arg is `None` all tasks matching the other args will be selected.
If an arg is an empty list, the corresponding value of the task needs
to be `None` to be selected.
Args:
actions: Limit the returned tasks to those specific action names
resource_id: Limit the returned tasks to the specific resource id, if specified
statuses: Limit the returned tasks to the specific statuses
max_timestamp: Limit the returned tasks to the ones that have
a timestamp inferior to the specified one
Returns
A list of `ScheduledTask`, ordered by increasing timestamps
"""
return await self._store.get_scheduled_tasks(
actions=actions,
resource_id=resource_id,
statuses=statuses,
max_timestamp=max_timestamp,
)
async def delete_task(self, id: str) -> None:
"""Delete a task. Running tasks can't be deleted.
Can only be called from the worker handling the task scheduling.
Args:
id: id of the task to delete
"""
task = await self.get_task(id)
if task is None:
raise Exception(f"Task {id} does not exist")
if task.status == TaskStatus.ACTIVE:
raise Exception(f"Task {id} is currently ACTIVE and can't be deleted")
await self._store.delete_scheduled_task(id)
async def _handle_scheduled_tasks(self) -> None:
"""Main loop taking care of launching tasks and cleaning up old ones."""
await self._launch_scheduled_tasks()
await self._clean_scheduled_tasks()
async def _launch_scheduled_tasks(self) -> None:
"""Retrieve and launch scheduled tasks that should be running at that time."""
for task in await self.get_tasks(statuses=[TaskStatus.ACTIVE]):
await self._launch_task(task)
for task in await self.get_tasks(
statuses=[TaskStatus.SCHEDULED], max_timestamp=self._clock.time_msec()
):
await self._launch_task(task)
running_tasks_gauge.set(len(self._running_tasks))
async def _clean_scheduled_tasks(self) -> None:
"""Clean old complete or failed jobs to avoid clutter the DB."""
for task in await self._store.get_scheduled_tasks(
statuses=[TaskStatus.FAILED, TaskStatus.COMPLETE]
):
# FAILED and COMPLETE tasks should never be running
assert task.id not in self._running_tasks
if (
self._clock.time_msec()
> task.timestamp + TaskScheduler.KEEP_TASKS_FOR_MS
):
await self._store.delete_scheduled_task(task.id)
async def _launch_task(self, task: ScheduledTask) -> None:
"""Launch a scheduled task now.
Args:
task: the task to launch
"""
assert self._run_background_tasks
if task.action not in self._actions:
raise Exception(
f"No function associated with action {task.action} of the scheduled task {task.id}"
)
function = self._actions[task.action]
async def wrapper() -> None:
with nested_logging_context(task.id):
try:
(status, result, error) = await function(task)
except Exception:
f = Failure()
logger.error(
f"scheduled task {task.id} failed",
exc_info=(f.type, f.value, f.getTracebackObject()),
)
status = TaskStatus.FAILED
result = None
error = f.getErrorMessage()
await self._store.update_scheduled_task(
task.id,
self._clock.time_msec(),
status=status,
result=result,
error=error,
)
self._running_tasks.remove(task.id)
if len(self._running_tasks) >= TaskScheduler.MAX_CONCURRENT_RUNNING_TASKS:
return
if (
self._clock.time_msec()
> task.timestamp + TaskScheduler.LAST_UPDATE_BEFORE_WARNING_MS
):
logger.warn(
f"Task {task.id} (action {task.action}) has seen no update for more than 24h and may be stuck"
)
if task.id in self._running_tasks:
return
self._running_tasks.add(task.id)
await self.update_task(task.id, status=TaskStatus.ACTIVE)
run_as_background_process(task.action, wrapper)