Source code for distributed.semaphore

from __future__ import annotations

import asyncio
import logging
import uuid
import warnings
from asyncio import TimeoutError
from collections import defaultdict, deque

import dask
from dask.utils import parse_timedelta

from distributed.compatibility import PeriodicCallback
from distributed.metrics import time
from distributed.utils import Deadline, SyncMethodMixin, log_errors, wait_for
from distributed.utils_comm import retry_operation
from distributed.worker import get_client, get_worker

logger = logging.getLogger(__name__)

class SemaphoreExtension:
    """An extension for the scheduler to manage Semaphores

    This adds the following routes to the scheduler

    * semaphore_acquire
    * semaphore_release
    * semaphore_close
    * semaphore_refresh_leases
    * semaphore_register

    def __init__(self, scheduler):
        self.scheduler = scheduler

        # {semaphore_name: asyncio.Event} = defaultdict(asyncio.Event)
        # {semaphore_name: max_leases}
        self.max_leases = dict()
        # {semaphore_name: {lease_id: lease_last_seen_timestamp}}
        self.leases = defaultdict(dict)

                "semaphore_register": self.create,
                "semaphore_acquire": self.acquire,
                "semaphore_release": self.release,
                "semaphore_close": self.close,
                "semaphore_refresh_leases": self.refresh_leases,
                "semaphore_value": self.get_value,

        # {metric_name: {semaphore_name: metric}}
        self.metrics = {
            "acquire_total": defaultdict(int),  # counter
            "release_total": defaultdict(int),  # counter
            "average_pending_lease_time": defaultdict(float),  # gauge
            "pending": defaultdict(int),  # gauge

        validation_callback_time = parse_timedelta(
        ] = pc = PeriodicCallback(
            self._check_lease_timeout, validation_callback_time * 1000
        self.lease_timeout = parse_timedelta(
            dask.config.get(""), default="s"

    def get_value(self, name=None):
        return len(self.leases[name])

    # `comm` here is required by the handler interface
    def create(self, name=None, max_leases=None):
        # We use `self.max_leases` as the point of truth to find out if a semaphore with a specific
        # `name` has been created.
        if name not in self.max_leases:
            assert isinstance(max_leases, int), max_leases
            self.max_leases[name] = max_leases
            if max_leases != self.max_leases[name]:
                raise ValueError(
                    "Inconsistent max leases: %s, expected: %s"
                    % (max_leases, self.max_leases[name])

    def refresh_leases(self, name=None, lease_ids=None):
        now = time()
        logger.debug("Refresh leases for %s with ids %s at %s", name, lease_ids, now)
        for id_ in lease_ids:
            if id_ not in self.leases[name]:
                    f"Refreshing an unknown lease ID {id_} for {name}. This might be due to leases "
                    f"timing out and may cause overbooking of the semaphore!"
                    f"This is often caused by long-running GIL-holding in the task which acquired the lease."
            self.leases[name][id_] = now

    def _get_lease(self, name, lease_id):
        result = True

        if (
            # This allows request idempotency
            lease_id in self.leases[name]
            or len(self.leases[name]) < self.max_leases[name]
            now = time()
            logger.debug("Acquire lease %s for %s at %s", lease_id, name, now)
            self.leases[name][lease_id] = now
            self.metrics["acquire_total"][name] += 1
            result = False
        return result

    def _semaphore_exists(self, name):
        if name not in self.max_leases:
            return False
        return True

    async def acquire(self, name=None, timeout=None, lease_id=None):
        if not self._semaphore_exists(name):
            raise RuntimeError(f"Semaphore `{name}` not known or already closed.")

        if isinstance(name, list):
            name = tuple(name)
        deadline = Deadline.after(timeout)

        self.metrics["pending"][name] += 1
        while True:
                "Trying to acquire %s for %s with %s seconds left.",
            # Reset the event and try to get a release. The event will be set if the state
            # is changed and helps to identify when it is worth to retry an acquire

            result = self._get_lease(name, lease_id)

            # If acquiring fails, we wait for the event to be set, i.e. something has
            # been released and we can try to acquire again (continue loop)
            if not result:
                future = wait_for([name].wait(), timeout=deadline.remaining)
                    await future
                except TimeoutError:
                    result = False
                "Acquisition of lease %s for %s is %s after waiting for %ss.",
            # We're about to return, so the lease is no longer "pending"
            self.metrics["average_pending_lease_time"][name] = (
                self.metrics["average_pending_lease_time"][name] + deadline.elapsed
            ) / 2
            self.metrics["pending"][name] -= 1

            return result

    def release(self, name=None, lease_id=None):
        if not self._semaphore_exists(name):
                f"Tried to release semaphore `{name}` but it is not known or already closed."
        if isinstance(name, list):
            name = tuple(name)
        if name in self.leases and lease_id in self.leases[name]:
            self._release_value(name, lease_id)
                "Tried to release semaphore but it was already released: "
                f"{name=}, {lease_id=}. "
                "This can happen if the semaphore timed out before."

    def _release_value(self, name, lease_id):
        logger.debug("Releasing %s for %s", lease_id, name)
        # Everything needs to be atomic here.
        del self.leases[name][lease_id][name].set()
        self.metrics["release_total"][name] += 1

    def _check_lease_timeout(self):
        now = time()
        semaphore_names = list(self.leases.keys())
        for name in semaphore_names:
            ids = list(self.leases[name])
                "Validating leases for %s at time %s. Currently known %s",
            for _id in ids:
                time_since_refresh = now - self.leases[name][_id]
                if time_since_refresh > self.lease_timeout:
                        "Lease %s for %s timed out after %ss.",
                    self._release_value(name=name, lease_id=_id)

    def close(self, name=None):
        """Hard close the semaphore without warning clients which still hold a lease."""
        if not self._semaphore_exists(name):

        del self.max_leases[name]
        if name in
        if name in self.leases:
            if self.leases[name]:
                    f"Closing semaphore {name} but there remain unreleased leases {sorted(self.leases[name])}",
            del self.leases[name]
        if name in self.metrics["pending"]:
            if self.metrics["pending"][name]:
                    f"Closing semaphore {name} but there remain pending leases",
        # Clean-up state of semaphore metrics
        for _, metric_dict in self.metrics.items():
            if name in metric_dict:
                del metric_dict[name]

[docs]class Semaphore(SyncMethodMixin): """Semaphore This `semaphore <>`_ will track leases on the scheduler which can be acquired and released by an instance of this class. If the maximum amount of leases are already acquired, it is not possible to acquire more and the caller waits until another lease has been released. The lifetime or leases are controlled using a timeout. This timeout is refreshed in regular intervals by the ``Client`` of this instance and provides protection from deadlocks or resource starvation in case of worker failure. The timeout can be controlled using the configuration option ```` and the interval in which the scheduler verifies the timeout is set using the option ````. A noticeable difference to the Semaphore of the python standard library is that this implementation does not allow to release more often than it was acquired. If this happens, a warning is emitted but the internal state is not modified. .. warning:: This implementation is susceptible to lease overbooking in case of lease timeouts. It is advised to monitor log information and adjust above configuration options to suitable values for the user application. Parameters ---------- max_leases: int (optional) The maximum amount of leases that may be granted at the same time. This effectively sets an upper limit to the amount of parallel access to a specific resource. Defaults to 1. name: string (optional) Name of the semaphore to acquire. Choosing the same name allows two disconnected processes to coordinate. If not given, a random name will be generated. register: bool If True, register the semaphore with the scheduler. This needs to be done before any leases can be acquired. If not done during initialization, this can also be done by calling the register method of this class. When registering, this needs to be awaited. scheduler_rpc: ConnectionPool The ConnectionPool to connect to the scheduler. If None is provided, it uses the worker or client pool. This parameter is mostly used for testing. loop: IOLoop The event loop this instance is using. If None is provided, reuse the loop of the active worker or client. Examples -------- >>> from distributed import Semaphore ... sem = Semaphore(max_leases=2, name='my_database') ... ... def access_resource(s, sem): ... # This automatically acquires a lease from the semaphore (if available) which will be ... # released when leaving the context manager. ... with sem: ... pass ... ... futures =, range(10), sem=sem) ... client.gather(futures) ... # Once done, close the semaphore to clean up the state on scheduler side. ... sem.close() Notes ----- If a client attempts to release the semaphore but doesn't have a lease acquired, this will raise an exception. When a semaphore is closed, if, for that closed semaphore, a client attempts to: - Acquire a lease: an exception will be raised. - Release: a warning will be logged. - Close: nothing will happen. dask executes functions by default assuming they are pure, when using semaphore acquire/releases inside such a function, it must be noted that there *are* in fact side-effects, thus, the function can no longer be considered pure. If this is not taken into account, this may lead to unexpected behavior. """ def __init__( self, max_leases=1, name=None, register=True, scheduler_rpc=None, loop=None, ): self._scheduler = scheduler_rpc self._loop = loop = name or "semaphore-" + uuid.uuid4().hex self.max_leases = max_leases = uuid.uuid4().hex self._leases = deque() self.refresh_leases = True self._registered = None if register: self._registered = self.register() # this should give ample time to refresh without introducing another # config parameter since this *must* be smaller than the timeout anyhow refresh_leases_interval = ( parse_timedelta( dask.config.get(""), default="s", ) / 5 ) pc = PeriodicCallback( self._refresh_leases, callback_time=refresh_leases_interval * 1000 ) self.refresh_callback = pc # Need to start the callback using IOLoop.add_callback to ensure that the # PC uses the correct event loop. if self.loop is not None: self.loop.add_callback(pc.start) @property def scheduler(self): self._bind_late() return self._scheduler @property def loop(self): self._bind_late() return self._loop def _bind_late(self): if self._scheduler is None or self._loop is None: try: try: worker = get_worker() self._scheduler = self._scheduler or worker.scheduler self._loop = self._loop or worker.loop except ValueError: client = get_client() self._scheduler = self._scheduler or client.scheduler self._loop = self._loop or client.loop except ValueError: pass def _verify_running(self): if not self.scheduler or not self.loop: raise RuntimeError( f"{type(self)} object not properly initialized. This can happen if the object is being deserialized outside of the context of a Client or Worker." ) async def _register(self): await retry_operation( self.scheduler.semaphore_register,, max_leases=self.max_leases, operation=f"semaphore register id={} name={}", ) def register(self, **kwargs): return self.sync(self._register) def __await__(self): async def create_semaphore(): if self._registered: await self._registered return self return create_semaphore().__await__() async def _refresh_leases(self): if self.refresh_leases and self._leases: logger.debug( "%s refreshing leases for %s with IDs %s",,, self._leases, ) await retry_operation( self.scheduler.semaphore_refresh_leases, lease_ids=list(self._leases),, operation="semaphore refresh leases: id=%s, lease_ids=%s, name=%s" % (, list(self._leases),, ) async def _acquire(self, timeout=None): lease_id = uuid.uuid4().hex logger.debug( "%s requests lease for %s with ID %s",,, lease_id ) # Using a unique lease id generated here allows us to retry since the # server handle is idempotent result = await retry_operation( self.scheduler.semaphore_acquire,, timeout=timeout, lease_id=lease_id, operation="semaphore acquire: id=%s, lease_id=%s, name=%s" % (, lease_id,, ) if result: self._leases.append(lease_id) return result
[docs] def acquire(self, timeout=None): """ Acquire a semaphore. If the internal counter is greater than zero, decrement it by one and return True immediately. If it is zero, wait until a release() is called and return True. Parameters ---------- timeout : number or string or timedelta, optional Seconds to wait on acquiring the semaphore. This does not include local coroutine time, network transfer time, etc.. Instead of number of seconds, it is also possible to specify a timedelta in string format, e.g. "200ms". """ self._verify_running() timeout = parse_timedelta(timeout) return self.sync(self._acquire, timeout=timeout)
async def _release(self, lease_id): try: await retry_operation( self.scheduler.semaphore_release,, lease_id=lease_id, operation="semaphore release: id=%s, lease_id=%s, name=%s" % (, lease_id,, ) return True except Exception: # Release fails for whatever reason logger.error( "Release failed for id=%s, lease_id=%s, name=%s. Cluster network might be unstable?" % (, lease_id,, exc_info=True, ) return False
[docs] def release(self): """ Release the semaphore. Returns ------- bool This value indicates whether a lease was released immediately or not. Note that a user should *not* retry this operation. Under certain circumstances (e.g. scheduler overload) the lease may not be released immediately, but it will always be automatically released after a specific interval configured using "" and "". """ self._verify_running() if not self._leases: raise RuntimeError("Released too often") # popleft to release the oldest lease first lease_id = self._leases.popleft() logger.debug("%s releases %s for %s",, lease_id, return self.sync(self._release, lease_id=lease_id)
[docs] def get_value(self): """ Return the number of currently registered leases. """ self._verify_running() return self.sync(self.scheduler.semaphore_value,
def __enter__(self): self._verify_running() self.acquire() return self def __exit__(self, exc_type, exc_value, traceback): self.release() async def __aenter__(self): self._verify_running() await self.acquire() return self async def __aexit__(self, exc_type, exc_value, traceback): await self.release() def __getstate__(self): # Do not serialize the address since workers may have different # addresses for the scheduler (e.g. if a proxy is between them) return (, self.max_leases) def __setstate__(self, state): name, max_leases = state self.__init__( name=name, max_leases=max_leases, register=False, ) def close(self): self._verify_running() self.refresh_callback.stop() return self.sync(self.scheduler.semaphore_close, def __del__(self): if hasattr(self, "refresh_callback"): self.refresh_callback.stop()