This document describes the current stable version of Celery (5.4). For development docs, go here.

Source code for celery.backends.rpc

"""The ``RPC`` result backend for AMQP brokers.

RPC-style result backend, using reply-to and one queue per client.
import time

import kombu
from kombu.common import maybe_declare
from kombu.utils.compat import register_after_fork
from kombu.utils.objects import cached_property

from celery import states
from celery._state import current_task, task_join_will_block

from . import base
from .asynchronous import AsyncBackendMixin, BaseResultConsumer

__all__ = ('BacklogLimitExceeded', 'RPCBackend')

The "rpc" result backend does not support chords!

Note that a group chained with a task is also upgraded to be a chord,
as this pattern requires synchronization.

Result backends that supports chords: Redis, Database, Memcached, and more.

[docs] class BacklogLimitExceeded(Exception): """Too much state history to fast-forward."""
def _on_after_fork_cleanup_backend(backend): backend._after_fork() class ResultConsumer(BaseResultConsumer): Consumer = kombu.Consumer _connection = None _consumer = None def __init__(self, *args, **kwargs): super().__init__(*args, **kwargs) self._create_binding = self.backend._create_binding def start(self, initial_task_id, no_ack=True, **kwargs): self._connection = initial_queue = self._create_binding(initial_task_id) self._consumer = self.Consumer( self._connection.default_channel, [initial_queue], callbacks=[self.on_state_change], no_ack=no_ack, accept=self.accept) self._consumer.consume() def drain_events(self, timeout=None): if self._connection: return self._connection.drain_events(timeout=timeout) elif timeout: time.sleep(timeout) def stop(self): try: self._consumer.cancel() finally: self._connection.close() def on_after_fork(self): self._consumer = None if self._connection is not None: self._connection.collect() self._connection = None def consume_from(self, task_id): if self._consumer is None: return self.start(task_id) queue = self._create_binding(task_id) if not self._consumer.consuming_from(queue): self._consumer.add_queue(queue) self._consumer.consume() def cancel_for(self, task_id): if self._consumer: self._consumer.cancel_by_queue(self._create_binding(task_id).name)
[docs] class RPCBackend(base.Backend, AsyncBackendMixin): """Base class for the RPC result backend.""" Exchange = kombu.Exchange Producer = kombu.Producer ResultConsumer = ResultConsumer #: Exception raised when there are too many messages for a task id. BacklogLimitExceeded = BacklogLimitExceeded persistent = False supports_autoexpire = True supports_native_join = True retry_policy = { 'max_retries': 20, 'interval_start': 0, 'interval_step': 1, 'interval_max': 1, }
[docs] class Consumer(kombu.Consumer): """Consumer that requires manual declaration of queues.""" auto_declare = False
[docs] class Queue(kombu.Queue): """Queue that never caches declaration.""" can_cache_declaration = False
def __init__(self, app, connection=None, exchange=None, exchange_type=None, persistent=None, serializer=None, auto_delete=True, **kwargs): super().__init__(app, **kwargs) conf = self._connection = connection self._out_of_band = {} self.persistent = self.prepare_persistent(persistent) self.delivery_mode = 2 if self.persistent else 1 exchange = exchange or conf.result_exchange exchange_type = exchange_type or conf.result_exchange_type = self._create_exchange( exchange, exchange_type, self.delivery_mode, ) self.serializer = serializer or conf.result_serializer self.auto_delete = auto_delete self.result_consumer = self.ResultConsumer( self,, self.accept, self._pending_results, self._pending_messages, ) if register_after_fork is not None: register_after_fork(self, _on_after_fork_cleanup_backend) def _after_fork(self): # clear state for child processes. self._pending_results.clear() self.result_consumer._after_fork() def _create_exchange(self, name, type='direct', delivery_mode=2): # uses direct to queue routing (anon exchange). return self.Exchange(None) def _create_binding(self, task_id): """Create new binding for task with id.""" # RPC backend caches the binding, as one queue is used for all tasks. return self.binding
[docs] def ensure_chords_allowed(self): raise NotImplementedError(E_NO_CHORD_SUPPORT.strip())
[docs] def on_task_call(self, producer, task_id): # Called every time a task is sent when using this backend. # We declare the queue we receive replies on in advance of sending # the message, but we skip this if running in the prefork pool # (task_join_will_block), as we know the queue is already declared. if not task_join_will_block(): maybe_declare(self.binding(, retry=True)
[docs] def destination_for(self, task_id, request): """Get the destination for result by task id. Returns: Tuple[str, str]: tuple of ``(reply_to, correlation_id)``. """ # Backends didn't always receive the `request`, so we must still # support old code that relies on current_task. try: request = request or current_task.request except AttributeError: raise RuntimeError( f'RPC backend missing task request for {task_id!r}') return request.reply_to, request.correlation_id or task_id
[docs] def on_reply_declare(self, task_id): # Return value here is used as the `declare=` argument # for Producer.publish. # By default we don't have to declare anything when sending a result. pass
[docs] def on_result_fulfilled(self, result): # This usually cancels the queue after the result is received, # but we don't have to cancel since we have one queue per process. pass
[docs] def as_uri(self, include_password=True): return 'rpc://'
[docs] def store_result(self, task_id, result, state, traceback=None, request=None, **kwargs): """Send task return value and state.""" routing_key, correlation_id = self.destination_for(task_id, request) if not routing_key: return with as producer: producer.publish( self._to_result(task_id, state, result, traceback, request),, routing_key=routing_key, correlation_id=correlation_id, serializer=self.serializer, retry=True, retry_policy=self.retry_policy, declare=self.on_reply_declare(task_id), delivery_mode=self.delivery_mode, ) return result
def _to_result(self, task_id, state, result, traceback, request): return { 'task_id': task_id, 'status': state, 'result': self.encode_result(result, state), 'traceback': traceback, 'children': self.current_task_children(request), }
[docs] def on_out_of_band_result(self, task_id, message): # Callback called when a reply for a task is received, # but we have no idea what to do with it. # Since the result is not pending, we put it in a separate # buffer: probably it will become pending later. if self.result_consumer: self.result_consumer.on_out_of_band_result(message) self._out_of_band[task_id] = message
[docs] def get_task_meta(self, task_id, backlog_limit=1000): buffered = self._out_of_band.pop(task_id, None) if buffered: return self._set_cache_by_message(task_id, buffered) # Polling and using basic_get latest_by_id = {} prev = None for acc in self._slurp_from_queue(task_id, self.accept, backlog_limit): tid = self._get_message_task_id(acc) prev, latest_by_id[tid] = latest_by_id.get(tid), acc if prev: # backends aren't expected to keep history, # so we delete everything except the most recent state. prev.ack() prev = None latest = latest_by_id.pop(task_id, None) for tid, msg in latest_by_id.items(): self.on_out_of_band_result(tid, msg) if latest: latest.requeue() return self._set_cache_by_message(task_id, latest) else: # no new state, use previous try: return self._cache[task_id] except KeyError: # result probably pending. return {'status': states.PENDING, 'result': None}
poll = get_task_meta # XXX compat def _set_cache_by_message(self, task_id, message): payload = self._cache[task_id] = self.meta_from_decoded( message.payload) return payload def _slurp_from_queue(self, task_id, accept, limit=1000, no_ack=False): with as (_, channel): binding = self._create_binding(task_id)(channel) binding.declare() for _ in range(limit): msg = binding.get(accept=accept, no_ack=no_ack) if not msg: break yield msg else: raise self.BacklogLimitExceeded(task_id) def _get_message_task_id(self, message): try: # try property first so we don't have to deserialize # the payload. return['correlation_id'] except (AttributeError, KeyError): # message sent by old Celery version, need to deserialize. return message.payload['task_id']
[docs] def revive(self, channel): pass
[docs] def reload_task_result(self, task_id): raise NotImplementedError( 'reload_task_result is not supported by this backend.')
[docs] def reload_group_result(self, task_id): """Reload group result, even if it has been previously fetched.""" raise NotImplementedError( 'reload_group_result is not supported by this backend.')
[docs] def save_group(self, group_id, result): raise NotImplementedError( 'save_group is not supported by this backend.')
[docs] def restore_group(self, group_id, cache=True): raise NotImplementedError( 'restore_group is not supported by this backend.')
[docs] def delete_group(self, group_id): raise NotImplementedError( 'delete_group is not supported by this backend.')
def __reduce__(self, args=(), kwargs=None): kwargs = {} if not kwargs else kwargs return super().__reduce__(args, dict( kwargs, connection=self._connection,,, persistent=self.persistent, serializer=self.serializer, auto_delete=self.auto_delete, expires=self.expires, )) @property def binding(self): return self.Queue( self.oid,, self.oid, durable=False, auto_delete=True, expires=self.expires, ) @cached_property def oid(self): # cached here is the app thread OID: name of queue we receive results on. return