This document describes the current stable version of Celery (5.6). For development docs, go here.

# Source code for celery.worker.consumer.gossip

    """Worker <-> Worker communication Bootstep."""
    from collections import defaultdict
    from functools import partial
    from heapq import heappush
    from operator import itemgetter

    from kombu import Consumer
    from kombu.asynchronous.semaphore import DummyLock
    from kombu.exceptions import ContentDisallowed, DecodeError

    from celery import bootsteps
    from celery.utils.log import get_logger
    from celery.utils.objects import Bunch

    from .mingle import Mingle

    __all__ = ('Gossip',)

    logger = get_logger(__name__)
    debug, info = logger.debug, logger.info

    class Gossip(bootsteps.ConsumerStep):
        """Bootstep consuming events from other workers.

        This keeps the logical clock value up to date.
        """

        label = 'Gossip'
        requires = (Mingle,)
        _cons_stamp_fields = itemgetter(
            'id', 'clock', 'hostname', 'pid', 'topic', 'action', 'cver',
        )
        compatible_transports = {'amqp', 'redis'}

        def __init__(self, c, without_gossip=False,
                     interval=5.0, heartbeat_interval=2.0, **kwargs):
            self.enabled = not without_gossip and self.compatible_transport(c.app)
            self.app = c.app
            c.gossip = self
            self.Receiver = c.app.events.Receiver
            self.hostname = c.hostname
            self.full_hostname = '.'.join([self.hostname, str(c.pid)])
            self.on = Bunch(
                node_join=set(),
                node_leave=set(),
                node_lost=set(),
            )

            self.timer = c.timer
            if self.enabled:
                self.state = c.app.events.State(
                    on_node_join=self.on_node_join,
                    on_node_leave=self.on_node_leave,
                    max_tasks_in_memory=1,
                )
                if c.hub:
                    c._mutex = DummyLock()
                self.update_state = self.state.event
            self.interval = interval
            self.heartbeat_interval = heartbeat_interval
            self._tref = None
            self.consensus_requests = defaultdict(list)
            self.consensus_replies = {}
            self.event_handlers = {
                'worker.elect': self.on_elect,
                'worker.elect.ack': self.on_elect_ack,
            }
            self.clock = c.app.clock

            self.election_handlers = {
                'task': self.call_task
            }

            super().__init__(c, **kwargs)

        def compatible_transport(self, app):
            with app.connection_for_read() as conn:
                return conn.transport.driver_type in self.compatible_transports

        def election(self, id, topic, action=None):
            self.consensus_replies[id] = []
            self.dispatcher.send(
                'worker-elect',
                id=id, topic=topic, action=action, cver=1,
            )

        def call_task(self, task):
            try:
                self.app.signature(task).apply_async()
            except Exception as exc:  # pylint: disable=broad-except
                logger.exception('Could not call task: %r', exc)

        def on_elect(self, event):
            try:
                (id_, clock, hostname, pid,
                 topic, action, _) = self._cons_stamp_fields(event)
            except KeyError as exc:
                return logger.exception('election request missing field %s', exc)
            heappush(
                self.consensus_requests[id_],
                (clock, f'{hostname}.{pid}', topic, action),
            )
            self.dispatcher.send('worker-elect-ack', id=id_)

        def start(self, c):
            super().start(c)
            self.dispatcher = c.event_dispatcher

        def on_elect_ack(self, event):
            id = event['id']
            try:
                replies = self.consensus_replies[id]
            except KeyError:
                return  # not for us
            alive_workers = set(self.state.alive_workers())
            replies.append(event['hostname'])

            if len(replies) >= len(alive_workers):
                _, leader, topic, action = self.clock.sort_heap(
                    self.consensus_requests[id],
                )
                if leader == self.full_hostname:
                    info('I won the election %r', id)
                    try:
                        handler = self.election_handlers[topic]
                    except KeyError:
                        logger.exception('Unknown election topic %r', topic)
                    else:
                        handler(action)
                else:
                    info('node %s elected for %r', leader, id)
                self.consensus_requests.pop(id, None)
                self.consensus_replies.pop(id, None)

        def on_node_join(self, worker):
            debug('%s joined the party', worker.hostname)
            self._call_handlers(self.on.node_join, worker)

        def on_node_leave(self, worker):
            debug('%s left', worker.hostname)
            self._call_handlers(self.on.node_leave, worker)

        def on_node_lost(self, worker):
            info('missed heartbeat from %s', worker.hostname)
            self._call_handlers(self.on.node_lost, worker)

        def _call_handlers(self, handlers, *args, **kwargs):
            for handler in handlers:
                try:
                    handler(*args, **kwargs)
                except Exception as exc:  # pylint: disable=broad-except
                    logger.exception(
                        'Ignored error from handler %r: %r', handler, exc)

        def register_timer(self):
            if self._tref is not None:
                self._tref.cancel()
            self._tref = self.timer.call_repeatedly(self.interval, self.periodic)

        def periodic(self):
            workers = self.state.workers
            dirty = set()
            for worker in workers.values():
                if not worker.alive:
                    dirty.add(worker)
                    self.on_node_lost(worker)
            for worker in dirty:
                workers.pop(worker.hostname, None)

        def get_consumers(self, channel):
            self.register_timer()
            ev = self.Receiver(channel, routing_key='worker.#',
                               queue_ttl=self.heartbeat_interval)
            return [Consumer(
                channel,
                queues=[ev.queue],
                on_message=partial(self.on_message, ev.event_from_message),
                accept=ev.accept,
                no_ack=True
            )]

        def on_message(self, prepare, message):
            _type = message.delivery_info['routing_key']

            # For redis when `fanout_patterns=False` (See Issue #1882)
            if _type.split('.', 1)[0] == 'task':
                return
            try:
                handler = self.event_handlers[_type]
            except KeyError:
                pass
            else:
                return handler(message.payload)

            # proto2: hostname in header; proto1: in body
            hostname = (message.headers.get('hostname') or
                        message.payload['hostname'])
            if hostname != self.hostname:
                try:
                    _, event = prepare(message.payload)
                    self.update_state(event)
                except (DecodeError, ContentDisallowed, TypeError) as exc:
                    logger.error(exc)
            else:
                self.clock.forward()