diff --git a/synapse/federation/transaction_queue.py b/synapse/federation/transaction_queue.py
new file mode 100644
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+++ b/synapse/federation/transaction_queue.py
@@ -0,0 +1,317 @@
+# -*- coding: utf-8 -*-
+# Copyright 2014, 2015 OpenMarket 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.
+
+
+from twisted.internet import defer
+
+from .persistence import TransactionActions
+from .units import Transaction
+
+from synapse.util.logutils import log_function
+from synapse.util.logcontext import PreserveLoggingContext
+
+import logging
+
+
+logger = logging.getLogger(__name__)
+
+
+class TransactionQueue(object):
+ """This class makes sure we only have one transaction in flight at
+ a time for a given destination.
+
+ It batches pending PDUs into single transactions.
+ """
+
+ def __init__(self, hs, transport_layer):
+ self.server_name = hs.hostname
+
+ self.store = hs.get_datastore()
+ self.transaction_actions = TransactionActions(self.store)
+
+ self.transport_layer = transport_layer
+
+ self._clock = hs.get_clock()
+
+ # Is a mapping from destinations -> deferreds. Used to keep track
+ # of which destinations have transactions in flight and when they are
+ # done
+ self.pending_transactions = {}
+
+ # Is a mapping from destination -> list of
+ # tuple(pending pdus, deferred, order)
+ self.pending_pdus_by_dest = {}
+ # destination -> list of tuple(edu, deferred)
+ self.pending_edus_by_dest = {}
+
+ # destination -> list of tuple(failure, deferred)
+ self.pending_failures_by_dest = {}
+
+ # HACK to get unique tx id
+ self._next_txn_id = int(self._clock.time_msec())
+
+ @defer.inlineCallbacks
+ @log_function
+ def enqueue_pdu(self, pdu, destinations, order):
+ # We loop through all destinations to see whether we already have
+ # a transaction in progress. If we do, stick it in the pending_pdus
+ # table and we'll get back to it later.
+
+ destinations = set(destinations)
+ destinations.discard(self.server_name)
+ destinations.discard("localhost")
+
+ logger.debug("Sending to: %s", str(destinations))
+
+ if not destinations:
+ return
+
+ deferreds = []
+
+ for destination in destinations:
+ deferred = defer.Deferred()
+ self.pending_pdus_by_dest.setdefault(destination, []).append(
+ (pdu, deferred, order)
+ )
+
+ def eb(failure):
+ if not deferred.called:
+ deferred.errback(failure)
+ else:
+ logger.warn("Failed to send pdu", failure)
+
+ with PreserveLoggingContext():
+ self._attempt_new_transaction(destination).addErrback(eb)
+
+ deferreds.append(deferred)
+
+ yield defer.DeferredList(deferreds)
+
+ # NO inlineCallbacks
+ def enqueue_edu(self, edu):
+ destination = edu.destination
+
+ if destination == self.server_name:
+ return
+
+ deferred = defer.Deferred()
+ self.pending_edus_by_dest.setdefault(destination, []).append(
+ (edu, deferred)
+ )
+
+ def eb(failure):
+ if not deferred.called:
+ deferred.errback(failure)
+ else:
+ logger.warn("Failed to send edu", failure)
+
+ with PreserveLoggingContext():
+ self._attempt_new_transaction(destination).addErrback(eb)
+
+ return deferred
+
+ @defer.inlineCallbacks
+ def enqueue_failure(self, failure, destination):
+ deferred = defer.Deferred()
+
+ self.pending_failures_by_dest.setdefault(
+ destination, []
+ ).append(
+ (failure, deferred)
+ )
+
+ yield deferred
+
+ @defer.inlineCallbacks
+ @log_function
+ def _attempt_new_transaction(self, destination):
+
+ (retry_last_ts, retry_interval) = (0, 0)
+ retry_timings = yield self.store.get_destination_retry_timings(
+ destination
+ )
+ if retry_timings:
+ (retry_last_ts, retry_interval) = (
+ retry_timings.retry_last_ts, retry_timings.retry_interval
+ )
+ if retry_last_ts + retry_interval > int(self._clock.time_msec()):
+ logger.info(
+ "TX [%s] not ready for retry yet - "
+ "dropping transaction for now",
+ destination,
+ )
+ return
+ else:
+ logger.info("TX [%s] is ready for retry", destination)
+
+ logger.info("TX [%s] _attempt_new_transaction", destination)
+
+ if destination in self.pending_transactions:
+ # XXX: pending_transactions can get stuck on by a never-ending
+ # request at which point pending_pdus_by_dest just keeps growing.
+ # we need application-layer timeouts of some flavour of these
+ # requests
+ return
+
+ # list of (pending_pdu, deferred, order)
+ pending_pdus = self.pending_pdus_by_dest.pop(destination, [])
+ pending_edus = self.pending_edus_by_dest.pop(destination, [])
+ pending_failures = self.pending_failures_by_dest.pop(destination, [])
+
+ if pending_pdus:
+ logger.info("TX [%s] len(pending_pdus_by_dest[dest]) = %d",
+ destination, len(pending_pdus))
+
+ if not pending_pdus and not pending_edus and not pending_failures:
+ return
+
+ logger.debug(
+ "TX [%s] Attempting new transaction"
+ " (pdus: %d, edus: %d, failures: %d)",
+ destination,
+ len(pending_pdus),
+ len(pending_edus),
+ len(pending_failures)
+ )
+
+ # Sort based on the order field
+ pending_pdus.sort(key=lambda t: t[2])
+
+ pdus = [x[0] for x in pending_pdus]
+ edus = [x[0] for x in pending_edus]
+ failures = [x[0].get_dict() for x in pending_failures]
+ deferreds = [
+ x[1]
+ for x in pending_pdus + pending_edus + pending_failures
+ ]
+
+ try:
+ self.pending_transactions[destination] = 1
+
+ logger.debug("TX [%s] Persisting transaction...", destination)
+
+ transaction = Transaction.create_new(
+ origin_server_ts=int(self._clock.time_msec()),
+ transaction_id=str(self._next_txn_id),
+ origin=self.server_name,
+ destination=destination,
+ pdus=pdus,
+ edus=edus,
+ pdu_failures=failures,
+ )
+
+ self._next_txn_id += 1
+
+ yield self.transaction_actions.prepare_to_send(transaction)
+
+ logger.debug("TX [%s] Persisted transaction", destination)
+ logger.info(
+ "TX [%s] Sending transaction [%s]",
+ destination,
+ transaction.transaction_id,
+ )
+
+ # Actually send the transaction
+
+ # FIXME (erikj): This is a bit of a hack to make the Pdu age
+ # keys work
+ def json_data_cb():
+ data = transaction.get_dict()
+ now = int(self._clock.time_msec())
+ if "pdus" in data:
+ for p in data["pdus"]:
+ if "age_ts" in p:
+ unsigned = p.setdefault("unsigned", {})
+ unsigned["age"] = now - int(p["age_ts"])
+ del p["age_ts"]
+ return data
+
+ code, response = yield self.transport_layer.send_transaction(
+ transaction, json_data_cb
+ )
+
+ logger.info("TX [%s] got %d response", destination, code)
+
+ logger.debug("TX [%s] Sent transaction", destination)
+ logger.debug("TX [%s] Marking as delivered...", destination)
+
+ yield self.transaction_actions.delivered(
+ transaction, code, response
+ )
+
+ logger.debug("TX [%s] Marked as delivered", destination)
+ logger.debug("TX [%s] Yielding to callbacks...", destination)
+
+ for deferred in deferreds:
+ if code == 200:
+ if retry_last_ts:
+ # this host is alive! reset retry schedule
+ yield self.store.set_destination_retry_timings(
+ destination, 0, 0
+ )
+ deferred.callback(None)
+ else:
+ self.set_retrying(destination, retry_interval)
+ deferred.errback(RuntimeError("Got status %d" % code))
+
+ # Ensures we don't continue until all callbacks on that
+ # deferred have fired
+ try:
+ yield deferred
+ except:
+ pass
+
+ logger.debug("TX [%s] Yielded to callbacks", destination)
+
+ except Exception as e:
+ # We capture this here as there as nothing actually listens
+ # for this finishing functions deferred.
+ logger.warn(
+ "TX [%s] Problem in _attempt_transaction: %s",
+ destination,
+ e,
+ )
+
+ self.set_retrying(destination, retry_interval)
+
+ for deferred in deferreds:
+ if not deferred.called:
+ deferred.errback(e)
+
+ finally:
+ # We want to be *very* sure we delete this after we stop processing
+ self.pending_transactions.pop(destination, None)
+
+ # Check to see if there is anything else to send.
+ self._attempt_new_transaction(destination)
+
+ @defer.inlineCallbacks
+ def set_retrying(self, destination, retry_interval):
+ # track that this destination is having problems and we should
+ # give it a chance to recover before trying it again
+
+ if retry_interval:
+ retry_interval *= 2
+ # plateau at hourly retries for now
+ if retry_interval >= 60 * 60 * 1000:
+ retry_interval = 60 * 60 * 1000
+ else:
+ retry_interval = 2000 # try again at first after 2 seconds
+
+ yield self.store.set_destination_retry_timings(
+ destination,
+ int(self._clock.time_msec()),
+ retry_interval
+ )
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