diff --git a/synapse/federation/sender/per_destination_queue.py b/synapse/federation/sender/per_destination_queue.py
new file mode 100644
index 0000000000..385039add4
--- /dev/null
+++ b/synapse/federation/sender/per_destination_queue.py
@@ -0,0 +1,318 @@
+# -*- coding: utf-8 -*-
+# Copyright 2014-2016 OpenMarket Ltd
+# Copyright 2019 New Vector Ltd
+#
+# 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 datetime
+import logging
+
+from prometheus_client import Counter
+
+from twisted.internet import defer
+
+from synapse.api.errors import (
+ FederationDeniedError,
+ HttpResponseException,
+ RequestSendFailed,
+)
+from synapse.events import EventBase
+from synapse.federation.units import Edu
+from synapse.handlers.presence import format_user_presence_state
+from synapse.metrics import sent_transactions_counter
+from synapse.metrics.background_process_metrics import run_as_background_process
+from synapse.storage import UserPresenceState
+from synapse.util.retryutils import NotRetryingDestination, get_retry_limiter
+
+logger = logging.getLogger(__name__)
+
+
+sent_edus_counter = Counter(
+ "synapse_federation_client_sent_edus",
+ "Total number of EDUs successfully sent",
+)
+
+sent_edus_by_type = Counter(
+ "synapse_federation_client_sent_edus_by_type",
+ "Number of sent EDUs successfully sent, by event type",
+ ["type"],
+)
+
+
+class PerDestinationQueue(object):
+ """
+ Manages the per-destination transmission queues.
+
+ Args:
+ hs (synapse.HomeServer):
+ transaction_sender (TransactionManager):
+ destination (str): the server_name of the destination that we are managing
+ transmission for.
+ """
+ def __init__(self, hs, transaction_manager, destination):
+ self._server_name = hs.hostname
+ self._clock = hs.get_clock()
+ self._store = hs.get_datastore()
+ self._transaction_manager = transaction_manager
+
+ self._destination = destination
+ self.transmission_loop_running = False
+
+ # a list of tuples of (pending pdu, order)
+ self._pending_pdus = [] # type: list[tuple[EventBase, int]]
+ self._pending_edus = [] # type: list[Edu]
+
+ # Pending EDUs by their "key". Keyed EDUs are EDUs that get clobbered
+ # based on their key (e.g. typing events by room_id)
+ # Map of (edu_type, key) -> Edu
+ self._pending_edus_keyed = {} # type: dict[tuple[str, str], Edu]
+
+ # Map of user_id -> UserPresenceState of pending presence to be sent to this
+ # destination
+ self._pending_presence = {} # type: dict[str, UserPresenceState]
+
+ # stream_id of last successfully sent to-device message.
+ # NB: may be a long or an int.
+ self._last_device_stream_id = 0
+
+ # stream_id of last successfully sent device list update.
+ self._last_device_list_stream_id = 0
+
+ def pending_pdu_count(self):
+ return len(self._pending_pdus)
+
+ def pending_edu_count(self):
+ return (
+ len(self._pending_edus)
+ + len(self._pending_presence)
+ + len(self._pending_edus_keyed)
+ )
+
+ def send_pdu(self, pdu, order):
+ """Add a PDU to the queue, and start the transmission loop if neccessary
+
+ Args:
+ pdu (EventBase): pdu to send
+ order (int):
+ """
+ self._pending_pdus.append((pdu, order))
+ self.attempt_new_transaction()
+
+ def send_presence(self, states):
+ """Add presence updates to the queue. Start the transmission loop if neccessary.
+
+ Args:
+ states (iterable[UserPresenceState]): presence to send
+ """
+ self._pending_presence.update({
+ state.user_id: state for state in states
+ })
+ self.attempt_new_transaction()
+
+ def send_keyed_edu(self, edu, key):
+ self._pending_edus_keyed[(edu.edu_type, key)] = edu
+ self.attempt_new_transaction()
+
+ def send_edu(self, edu):
+ self._pending_edus.append(edu)
+ self.attempt_new_transaction()
+
+ def attempt_new_transaction(self):
+ """Try to start a new transaction to this destination
+
+ If there is already a transaction in progress to this destination,
+ returns immediately. Otherwise kicks off the process of sending a
+ transaction in the background.
+ """
+ # list of (pending_pdu, deferred, order)
+ if self.transmission_loop_running:
+ # XXX: this can get stuck on by a never-ending
+ # request at which point pending_pdus just keeps growing.
+ # we need application-layer timeouts of some flavour of these
+ # requests
+ logger.debug(
+ "TX [%s] Transaction already in progress",
+ self._destination
+ )
+ return
+
+ logger.debug("TX [%s] Starting transaction loop", self._destination)
+
+ run_as_background_process(
+ "federation_transaction_transmission_loop",
+ self._transaction_transmission_loop,
+ )
+
+ @defer.inlineCallbacks
+ def _transaction_transmission_loop(self):
+ pending_pdus = []
+ try:
+ self.transmission_loop_running = True
+
+ # This will throw if we wouldn't retry. We do this here so we fail
+ # quickly, but we will later check this again in the http client,
+ # hence why we throw the result away.
+ yield get_retry_limiter(self._destination, self._clock, self._store)
+
+ pending_pdus = []
+ while True:
+ device_message_edus, device_stream_id, dev_list_id = (
+ yield self._get_new_device_messages()
+ )
+
+ # BEGIN CRITICAL SECTION
+ #
+ # In order to avoid a race condition, we need to make sure that
+ # the following code (from popping the queues up to the point
+ # where we decide if we actually have any pending messages) is
+ # atomic - otherwise new PDUs or EDUs might arrive in the
+ # meantime, but not get sent because we hold the
+ # transmission_loop_running flag.
+
+ pending_pdus = self._pending_pdus
+
+ # We can only include at most 50 PDUs per transactions
+ pending_pdus, self._pending_pdus = pending_pdus[:50], pending_pdus[50:]
+
+ pending_edus = self._pending_edus
+
+ # We can only include at most 100 EDUs per transactions
+ pending_edus, self._pending_edus = pending_edus[:100], pending_edus[100:]
+
+ pending_edus.extend(
+ self._pending_edus_keyed.values()
+ )
+
+ self._pending_edus_keyed = {}
+
+ pending_edus.extend(device_message_edus)
+
+ pending_presence = self._pending_presence
+ self._pending_presence = {}
+ if pending_presence:
+ pending_edus.append(
+ Edu(
+ origin=self._server_name,
+ destination=self._destination,
+ edu_type="m.presence",
+ content={
+ "push": [
+ format_user_presence_state(
+ presence, self._clock.time_msec()
+ )
+ for presence in pending_presence.values()
+ ]
+ },
+ )
+ )
+
+ if pending_pdus:
+ logger.debug("TX [%s] len(pending_pdus_by_dest[dest]) = %d",
+ self._destination, len(pending_pdus))
+
+ if not pending_pdus and not pending_edus:
+ logger.debug("TX [%s] Nothing to send", self._destination)
+ self._last_device_stream_id = device_stream_id
+ return
+
+ # END CRITICAL SECTION
+
+ success = yield self._transaction_manager.send_new_transaction(
+ self._destination, pending_pdus, pending_edus
+ )
+ if success:
+ sent_transactions_counter.inc()
+ sent_edus_counter.inc(len(pending_edus))
+ for edu in pending_edus:
+ sent_edus_by_type.labels(edu.edu_type).inc()
+ # Remove the acknowledged device messages from the database
+ # Only bother if we actually sent some device messages
+ if device_message_edus:
+ yield self._store.delete_device_msgs_for_remote(
+ self._destination, device_stream_id
+ )
+ logger.info(
+ "Marking as sent %r %r", self._destination, dev_list_id
+ )
+ yield self._store.mark_as_sent_devices_by_remote(
+ self._destination, dev_list_id
+ )
+
+ self._last_device_stream_id = device_stream_id
+ self._last_device_list_stream_id = dev_list_id
+ else:
+ break
+ except NotRetryingDestination as e:
+ logger.debug(
+ "TX [%s] not ready for retry yet (next retry at %s) - "
+ "dropping transaction for now",
+ self._destination,
+ datetime.datetime.fromtimestamp(
+ (e.retry_last_ts + e.retry_interval) / 1000.0
+ ),
+ )
+ except FederationDeniedError as e:
+ logger.info(e)
+ except HttpResponseException as e:
+ logger.warning(
+ "TX [%s] Received %d response to transaction: %s",
+ self._destination, e.code, e,
+ )
+ except RequestSendFailed as e:
+ logger.warning("TX [%s] Failed to send transaction: %s", self._destination, e)
+
+ for p, _ in pending_pdus:
+ logger.info("Failed to send event %s to %s", p.event_id,
+ self._destination)
+ except Exception:
+ logger.exception(
+ "TX [%s] Failed to send transaction",
+ self._destination,
+ )
+ for p, _ in pending_pdus:
+ logger.info("Failed to send event %s to %s", p.event_id,
+ self._destination)
+ finally:
+ # We want to be *very* sure we clear this after we stop processing
+ self.transmission_loop_running = False
+
+ @defer.inlineCallbacks
+ def _get_new_device_messages(self):
+ last_device_stream_id = self._last_device_stream_id
+ to_device_stream_id = self._store.get_to_device_stream_token()
+ contents, stream_id = yield self._store.get_new_device_msgs_for_remote(
+ self._destination, last_device_stream_id, to_device_stream_id
+ )
+ edus = [
+ Edu(
+ origin=self._server_name,
+ destination=self._destination,
+ edu_type="m.direct_to_device",
+ content=content,
+ )
+ for content in contents
+ ]
+
+ last_device_list = self._last_device_list_stream_id
+ now_stream_id, results = yield self._store.get_devices_by_remote(
+ self._destination, last_device_list
+ )
+ edus.extend(
+ Edu(
+ origin=self._server_name,
+ destination=self._destination,
+ edu_type="m.device_list_update",
+ content=content,
+ )
+ for content in results
+ )
+ defer.returnValue((edus, stream_id, now_stream_id))
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