diff --git a/changelog.d/5183.misc b/changelog.d/5183.misc
new file mode 100644
index 000000000..a8970f29e
--- /dev/null
+++ b/changelog.d/5183.misc
@@ -0,0 +1 @@
+Allow client event serialization to be async.
diff --git a/synapse/events/utils.py b/synapse/events/utils.py
index 07fccdd8f..a5454556c 100644
--- a/synapse/events/utils.py
+++ b/synapse/events/utils.py
@@ -19,7 +19,10 @@ from six import string_types
 
 from frozendict import frozendict
 
+from twisted.internet import defer
+
 from synapse.api.constants import EventTypes
+from synapse.util.async_helpers import yieldable_gather_results
 
 from . import EventBase
 
@@ -311,3 +314,44 @@ def serialize_event(e, time_now_ms, as_client_event=True,
         d = only_fields(d, only_event_fields)
 
     return d
+
+
+class EventClientSerializer(object):
+    """Serializes events that are to be sent to clients.
+
+    This is used for bundling extra information with any events to be sent to
+    clients.
+    """
+
+    def __init__(self, hs):
+        pass
+
+    def serialize_event(self, event, time_now, **kwargs):
+        """Serializes a single event.
+
+        Args:
+            event (EventBase)
+            time_now (int): The current time in milliseconds
+            **kwargs: Arguments to pass to `serialize_event`
+
+        Returns:
+            Deferred[dict]: The serialized event
+        """
+        event = serialize_event(event, time_now, **kwargs)
+        return defer.succeed(event)
+
+    def serialize_events(self, events, time_now, **kwargs):
+        """Serializes multiple events.
+
+        Args:
+            event (iter[EventBase])
+            time_now (int): The current time in milliseconds
+            **kwargs: Arguments to pass to `serialize_event`
+
+        Returns:
+            Deferred[list[dict]]: The list of serialized events
+        """
+        return yieldable_gather_results(
+            self.serialize_event, events,
+            time_now=time_now, **kwargs
+        )
diff --git a/synapse/handlers/events.py b/synapse/handlers/events.py
index 1b4d8c74a..6003ad9cc 100644
--- a/synapse/handlers/events.py
+++ b/synapse/handlers/events.py
@@ -21,7 +21,6 @@ from twisted.internet import defer
 from synapse.api.constants import EventTypes, Membership
 from synapse.api.errors import AuthError, SynapseError
 from synapse.events import EventBase
-from synapse.events.utils import serialize_event
 from synapse.types import UserID
 from synapse.util.logutils import log_function
 from synapse.visibility import filter_events_for_client
@@ -50,6 +49,7 @@ class EventStreamHandler(BaseHandler):
         self.notifier = hs.get_notifier()
         self.state = hs.get_state_handler()
         self._server_notices_sender = hs.get_server_notices_sender()
+        self._event_serializer = hs.get_event_client_serializer()
 
     @defer.inlineCallbacks
     @log_function
@@ -120,9 +120,9 @@ class EventStreamHandler(BaseHandler):
 
             time_now = self.clock.time_msec()
 
-            chunks = [
-                serialize_event(e, time_now, as_client_event) for e in events
-            ]
+            chunks = yield self._event_serializer.serialize_events(
+                events, time_now, as_client_event=as_client_event,
+            )
 
             chunk = {
                 "chunk": chunks,
diff --git a/synapse/handlers/initial_sync.py b/synapse/handlers/initial_sync.py
index 7dfae78db..aaee5db0b 100644
--- a/synapse/handlers/initial_sync.py
+++ b/synapse/handlers/initial_sync.py
@@ -19,7 +19,6 @@ from twisted.internet import defer
 
 from synapse.api.constants import EventTypes, Membership
 from synapse.api.errors import AuthError, Codes, SynapseError
-from synapse.events.utils import serialize_event
 from synapse.events.validator import EventValidator
 from synapse.handlers.presence import format_user_presence_state
 from synapse.streams.config import PaginationConfig
@@ -43,6 +42,7 @@ class InitialSyncHandler(BaseHandler):
         self.clock = hs.get_clock()
         self.validator = EventValidator()
         self.snapshot_cache = SnapshotCache()
+        self._event_serializer = hs.get_event_client_serializer()
 
     def snapshot_all_rooms(self, user_id=None, pagin_config=None,
                            as_client_event=True, include_archived=False):
@@ -138,7 +138,9 @@ class InitialSyncHandler(BaseHandler):
                 d["inviter"] = event.sender
 
                 invite_event = yield self.store.get_event(event.event_id)
-                d["invite"] = serialize_event(invite_event, time_now, as_client_event)
+                d["invite"] = yield self._event_serializer.serialize_event(
+                    invite_event, time_now, as_client_event,
+                )
 
             rooms_ret.append(d)
 
@@ -185,18 +187,21 @@ class InitialSyncHandler(BaseHandler):
                 time_now = self.clock.time_msec()
 
                 d["messages"] = {
-                    "chunk": [
-                        serialize_event(m, time_now, as_client_event)
-                        for m in messages
-                    ],
+                    "chunk": (
+                        yield self._event_serializer.serialize_events(
+                            messages, time_now=time_now,
+                            as_client_event=as_client_event,
+                        )
+                    ),
                     "start": start_token.to_string(),
                     "end": end_token.to_string(),
                 }
 
-                d["state"] = [
-                    serialize_event(c, time_now, as_client_event)
-                    for c in current_state.values()
-                ]
+                d["state"] = yield self._event_serializer.serialize_events(
+                    current_state.values(),
+                    time_now=time_now,
+                    as_client_event=as_client_event
+                )
 
                 account_data_events = []
                 tags = tags_by_room.get(event.room_id)
@@ -337,11 +342,15 @@ class InitialSyncHandler(BaseHandler):
             "membership": membership,
             "room_id": room_id,
             "messages": {
-                "chunk": [serialize_event(m, time_now) for m in messages],
+                "chunk": (yield self._event_serializer.serialize_events(
+                    messages, time_now,
+                )),
                 "start": start_token.to_string(),
                 "end": end_token.to_string(),
             },
-            "state": [serialize_event(s, time_now) for s in room_state.values()],
+            "state": (yield self._event_serializer.serialize_events(
+                room_state.values(), time_now,
+            )),
             "presence": [],
             "receipts": [],
         })
@@ -355,10 +364,9 @@ class InitialSyncHandler(BaseHandler):
 
         # TODO: These concurrently
         time_now = self.clock.time_msec()
-        state = [
-            serialize_event(x, time_now)
-            for x in current_state.values()
-        ]
+        state = yield self._event_serializer.serialize_events(
+            current_state.values(), time_now,
+        )
 
         now_token = yield self.hs.get_event_sources().get_current_token()
 
@@ -425,7 +433,9 @@ class InitialSyncHandler(BaseHandler):
         ret = {
             "room_id": room_id,
             "messages": {
-                "chunk": [serialize_event(m, time_now) for m in messages],
+                "chunk": (yield self._event_serializer.serialize_events(
+                    messages, time_now,
+                )),
                 "start": start_token.to_string(),
                 "end": end_token.to_string(),
             },
diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index e5afeadf6..7b2c33a92 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -32,7 +32,6 @@ from synapse.api.errors import (
 )
 from synapse.api.room_versions import RoomVersions
 from synapse.api.urls import ConsentURIBuilder
-from synapse.events.utils import serialize_event
 from synapse.events.validator import EventValidator
 from synapse.replication.http.send_event import ReplicationSendEventRestServlet
 from synapse.storage.state import StateFilter
@@ -57,6 +56,7 @@ class MessageHandler(object):
         self.clock = hs.get_clock()
         self.state = hs.get_state_handler()
         self.store = hs.get_datastore()
+        self._event_serializer = hs.get_event_client_serializer()
 
     @defer.inlineCallbacks
     def get_room_data(self, user_id=None, room_id=None,
@@ -164,9 +164,10 @@ class MessageHandler(object):
                 room_state = room_state[membership_event_id]
 
         now = self.clock.time_msec()
-        defer.returnValue(
-            [serialize_event(c, now) for c in room_state.values()]
+        events = yield self._event_serializer.serialize_events(
+            room_state.values(), now,
         )
+        defer.returnValue(events)
 
     @defer.inlineCallbacks
     def get_joined_members(self, requester, room_id):
diff --git a/synapse/handlers/pagination.py b/synapse/handlers/pagination.py
index e4fdae926..8f811e24f 100644
--- a/synapse/handlers/pagination.py
+++ b/synapse/handlers/pagination.py
@@ -20,7 +20,6 @@ from twisted.python.failure import Failure
 
 from synapse.api.constants import EventTypes, Membership
 from synapse.api.errors import SynapseError
-from synapse.events.utils import serialize_event
 from synapse.storage.state import StateFilter
 from synapse.types import RoomStreamToken
 from synapse.util.async_helpers import ReadWriteLock
@@ -78,6 +77,7 @@ class PaginationHandler(object):
         self._purges_in_progress_by_room = set()
         # map from purge id to PurgeStatus
         self._purges_by_id = {}
+        self._event_serializer = hs.get_event_client_serializer()
 
     def start_purge_history(self, room_id, token,
                             delete_local_events=False):
@@ -278,18 +278,22 @@ class PaginationHandler(object):
         time_now = self.clock.time_msec()
 
         chunk = {
-            "chunk": [
-                serialize_event(e, time_now, as_client_event)
-                for e in events
-            ],
+            "chunk": (
+                yield self._event_serializer.serialize_events(
+                    events, time_now,
+                    as_client_event=as_client_event,
+                )
+            ),
             "start": pagin_config.from_token.to_string(),
             "end": next_token.to_string(),
         }
 
         if state:
-            chunk["state"] = [
-                serialize_event(e, time_now, as_client_event)
-                for e in state
-            ]
+            chunk["state"] = (
+                yield self._event_serializer.serialize_events(
+                    state, time_now,
+                    as_client_event=as_client_event,
+                )
+            )
 
         defer.returnValue(chunk)
diff --git a/synapse/handlers/search.py b/synapse/handlers/search.py
index 49c439313..9bba74d6c 100644
--- a/synapse/handlers/search.py
+++ b/synapse/handlers/search.py
@@ -23,7 +23,6 @@ from twisted.internet import defer
 from synapse.api.constants import EventTypes, Membership
 from synapse.api.errors import SynapseError
 from synapse.api.filtering import Filter
-from synapse.events.utils import serialize_event
 from synapse.storage.state import StateFilter
 from synapse.visibility import filter_events_for_client
 
@@ -36,6 +35,7 @@ class SearchHandler(BaseHandler):
 
     def __init__(self, hs):
         super(SearchHandler, self).__init__(hs)
+        self._event_serializer = hs.get_event_client_serializer()
 
     @defer.inlineCallbacks
     def get_old_rooms_from_upgraded_room(self, room_id):
@@ -401,14 +401,16 @@ class SearchHandler(BaseHandler):
         time_now = self.clock.time_msec()
 
         for context in contexts.values():
-            context["events_before"] = [
-                serialize_event(e, time_now)
-                for e in context["events_before"]
-            ]
-            context["events_after"] = [
-                serialize_event(e, time_now)
-                for e in context["events_after"]
-            ]
+            context["events_before"] = (
+                yield self._event_serializer.serialize_events(
+                    context["events_before"], time_now,
+                )
+            )
+            context["events_after"] = (
+                yield self._event_serializer.serialize_events(
+                    context["events_after"], time_now,
+                )
+            )
 
         state_results = {}
         if include_state:
@@ -422,14 +424,13 @@ class SearchHandler(BaseHandler):
         # We're now about to serialize the events. We should not make any
         # blocking calls after this. Otherwise the 'age' will be wrong
 
-        results = [
-            {
+        results = []
+        for e in allowed_events:
+            results.append({
                 "rank": rank_map[e.event_id],
-                "result": serialize_event(e, time_now),
+                "result": (yield self._event_serializer.serialize_event(e, time_now)),
                 "context": contexts.get(e.event_id, {}),
-            }
-            for e in allowed_events
-        ]
+            })
 
         rooms_cat_res = {
             "results": results,
@@ -438,10 +439,13 @@ class SearchHandler(BaseHandler):
         }
 
         if state_results:
-            rooms_cat_res["state"] = {
-                room_id: [serialize_event(e, time_now) for e in state]
-                for room_id, state in state_results.items()
-            }
+            s = {}
+            for room_id, state in state_results.items():
+                s[room_id] = yield self._event_serializer.serialize_events(
+                    state, time_now,
+                )
+
+            rooms_cat_res["state"] = s
 
         if room_groups and "room_id" in group_keys:
             rooms_cat_res.setdefault("groups", {})["room_id"] = room_groups
diff --git a/synapse/rest/client/v1/events.py b/synapse/rest/client/v1/events.py
index cd9b3bdbd..c3b0a39ab 100644
--- a/synapse/rest/client/v1/events.py
+++ b/synapse/rest/client/v1/events.py
@@ -19,7 +19,6 @@ import logging
 from twisted.internet import defer
 
 from synapse.api.errors import SynapseError
-from synapse.events.utils import serialize_event
 from synapse.streams.config import PaginationConfig
 
 from .base import ClientV1RestServlet, client_path_patterns
@@ -84,6 +83,7 @@ class EventRestServlet(ClientV1RestServlet):
         super(EventRestServlet, self).__init__(hs)
         self.clock = hs.get_clock()
         self.event_handler = hs.get_event_handler()
+        self._event_serializer = hs.get_event_client_serializer()
 
     @defer.inlineCallbacks
     def on_GET(self, request, event_id):
@@ -92,7 +92,8 @@ class EventRestServlet(ClientV1RestServlet):
 
         time_now = self.clock.time_msec()
         if event:
-            defer.returnValue((200, serialize_event(event, time_now)))
+            event = yield self._event_serializer.serialize_event(event, time_now)
+            defer.returnValue((200, event))
         else:
             defer.returnValue((404, "Event not found."))
 
diff --git a/synapse/rest/client/v1/room.py b/synapse/rest/client/v1/room.py
index fab04965c..255a85c58 100644
--- a/synapse/rest/client/v1/room.py
+++ b/synapse/rest/client/v1/room.py
@@ -26,7 +26,7 @@ from twisted.internet import defer
 from synapse.api.constants import EventTypes, Membership
 from synapse.api.errors import AuthError, Codes, SynapseError
 from synapse.api.filtering import Filter
-from synapse.events.utils import format_event_for_client_v2, serialize_event
+from synapse.events.utils import format_event_for_client_v2
 from synapse.http.servlet import (
     assert_params_in_dict,
     parse_integer,
@@ -537,6 +537,7 @@ class RoomEventServlet(ClientV1RestServlet):
         super(RoomEventServlet, self).__init__(hs)
         self.clock = hs.get_clock()
         self.event_handler = hs.get_event_handler()
+        self._event_serializer = hs.get_event_client_serializer()
 
     @defer.inlineCallbacks
     def on_GET(self, request, room_id, event_id):
@@ -545,7 +546,8 @@ class RoomEventServlet(ClientV1RestServlet):
 
         time_now = self.clock.time_msec()
         if event:
-            defer.returnValue((200, serialize_event(event, time_now)))
+            event = yield self._event_serializer.serialize_event(event, time_now)
+            defer.returnValue((200, event))
         else:
             defer.returnValue((404, "Event not found."))
 
@@ -559,6 +561,7 @@ class RoomEventContextServlet(ClientV1RestServlet):
         super(RoomEventContextServlet, self).__init__(hs)
         self.clock = hs.get_clock()
         self.room_context_handler = hs.get_room_context_handler()
+        self._event_serializer = hs.get_event_client_serializer()
 
     @defer.inlineCallbacks
     def on_GET(self, request, room_id, event_id):
@@ -588,16 +591,18 @@ class RoomEventContextServlet(ClientV1RestServlet):
             )
 
         time_now = self.clock.time_msec()
-        results["events_before"] = [
-            serialize_event(event, time_now) for event in results["events_before"]
-        ]
-        results["event"] = serialize_event(results["event"], time_now)
-        results["events_after"] = [
-            serialize_event(event, time_now) for event in results["events_after"]
-        ]
-        results["state"] = [
-            serialize_event(event, time_now) for event in results["state"]
-        ]
+        results["events_before"] = yield self._event_serializer.serialize_events(
+            results["events_before"], time_now,
+        )
+        results["event"] = yield self._event_serializer.serialize_event(
+            results["event"], time_now,
+        )
+        results["events_after"] = yield self._event_serializer.serialize_events(
+            results["events_after"], time_now,
+        )
+        results["state"] = yield self._event_serializer.serialize_events(
+            results["state"], time_now,
+        )
 
         defer.returnValue((200, results))
 
diff --git a/synapse/rest/client/v2_alpha/notifications.py b/synapse/rest/client/v2_alpha/notifications.py
index 2a6ea3df5..0a1eb0ae4 100644
--- a/synapse/rest/client/v2_alpha/notifications.py
+++ b/synapse/rest/client/v2_alpha/notifications.py
@@ -17,10 +17,7 @@ import logging
 
 from twisted.internet import defer
 
-from synapse.events.utils import (
-    format_event_for_client_v2_without_room_id,
-    serialize_event,
-)
+from synapse.events.utils import format_event_for_client_v2_without_room_id
 from synapse.http.servlet import RestServlet, parse_integer, parse_string
 
 from ._base import client_v2_patterns
@@ -36,6 +33,7 @@ class NotificationsServlet(RestServlet):
         self.store = hs.get_datastore()
         self.auth = hs.get_auth()
         self.clock = hs.get_clock()
+        self._event_serializer = hs.get_event_client_serializer()
 
     @defer.inlineCallbacks
     def on_GET(self, request):
@@ -69,11 +67,11 @@ class NotificationsServlet(RestServlet):
                 "profile_tag": pa["profile_tag"],
                 "actions": pa["actions"],
                 "ts": pa["received_ts"],
-                "event": serialize_event(
+                "event": (yield self._event_serializer.serialize_event(
                     notif_events[pa["event_id"]],
                     self.clock.time_msec(),
                     event_format=format_event_for_client_v2_without_room_id,
-                ),
+                )),
             }
 
             if pa["room_id"] not in receipts_by_room:
diff --git a/synapse/rest/client/v2_alpha/sync.py b/synapse/rest/client/v2_alpha/sync.py
index 39d157a44..c701e534e 100644
--- a/synapse/rest/client/v2_alpha/sync.py
+++ b/synapse/rest/client/v2_alpha/sync.py
@@ -26,7 +26,6 @@ from synapse.api.filtering import DEFAULT_FILTER_COLLECTION, FilterCollection
 from synapse.events.utils import (
     format_event_for_client_v2_without_room_id,
     format_event_raw,
-    serialize_event,
 )
 from synapse.handlers.presence import format_user_presence_state
 from synapse.handlers.sync import SyncConfig
@@ -86,6 +85,7 @@ class SyncRestServlet(RestServlet):
         self.filtering = hs.get_filtering()
         self.presence_handler = hs.get_presence_handler()
         self._server_notices_sender = hs.get_server_notices_sender()
+        self._event_serializer = hs.get_event_client_serializer()
 
     @defer.inlineCallbacks
     def on_GET(self, request):
@@ -168,14 +168,14 @@ class SyncRestServlet(RestServlet):
             )
 
         time_now = self.clock.time_msec()
-        response_content = self.encode_response(
+        response_content = yield self.encode_response(
             time_now, sync_result, requester.access_token_id, filter
         )
 
         defer.returnValue((200, response_content))
 
-    @staticmethod
-    def encode_response(time_now, sync_result, access_token_id, filter):
+    @defer.inlineCallbacks
+    def encode_response(self, time_now, sync_result, access_token_id, filter):
         if filter.event_format == 'client':
             event_formatter = format_event_for_client_v2_without_room_id
         elif filter.event_format == 'federation':
@@ -183,24 +183,24 @@ class SyncRestServlet(RestServlet):
         else:
             raise Exception("Unknown event format %s" % (filter.event_format, ))
 
-        joined = SyncRestServlet.encode_joined(
+        joined = yield self.encode_joined(
             sync_result.joined, time_now, access_token_id,
             filter.event_fields,
             event_formatter,
         )
 
-        invited = SyncRestServlet.encode_invited(
+        invited = yield self.encode_invited(
             sync_result.invited, time_now, access_token_id,
             event_formatter,
         )
 
-        archived = SyncRestServlet.encode_archived(
+        archived = yield self.encode_archived(
             sync_result.archived, time_now, access_token_id,
             filter.event_fields,
             event_formatter,
         )
 
-        return {
+        defer.returnValue({
             "account_data": {"events": sync_result.account_data},
             "to_device": {"events": sync_result.to_device},
             "device_lists": {
@@ -222,7 +222,7 @@ class SyncRestServlet(RestServlet):
             },
             "device_one_time_keys_count": sync_result.device_one_time_keys_count,
             "next_batch": sync_result.next_batch.to_string(),
-        }
+        })
 
     @staticmethod
     def encode_presence(events, time_now):
@@ -239,8 +239,8 @@ class SyncRestServlet(RestServlet):
             ]
         }
 
-    @staticmethod
-    def encode_joined(rooms, time_now, token_id, event_fields, event_formatter):
+    @defer.inlineCallbacks
+    def encode_joined(self, rooms, time_now, token_id, event_fields, event_formatter):
         """
         Encode the joined rooms in a sync result
 
@@ -261,15 +261,15 @@ class SyncRestServlet(RestServlet):
         """
         joined = {}
         for room in rooms:
-            joined[room.room_id] = SyncRestServlet.encode_room(
+            joined[room.room_id] = yield self.encode_room(
                 room, time_now, token_id, joined=True, only_fields=event_fields,
                 event_formatter=event_formatter,
             )
 
-        return joined
+        defer.returnValue(joined)
 
-    @staticmethod
-    def encode_invited(rooms, time_now, token_id, event_formatter):
+    @defer.inlineCallbacks
+    def encode_invited(self, rooms, time_now, token_id, event_formatter):
         """
         Encode the invited rooms in a sync result
 
@@ -289,7 +289,7 @@ class SyncRestServlet(RestServlet):
         """
         invited = {}
         for room in rooms:
-            invite = serialize_event(
+            invite = yield self._event_serializer.serialize_event(
                 room.invite, time_now, token_id=token_id,
                 event_format=event_formatter,
                 is_invite=True,
@@ -302,10 +302,10 @@ class SyncRestServlet(RestServlet):
                 "invite_state": {"events": invited_state}
             }
 
-        return invited
+        defer.returnValue(invited)
 
-    @staticmethod
-    def encode_archived(rooms, time_now, token_id, event_fields, event_formatter):
+    @defer.inlineCallbacks
+    def encode_archived(self, rooms, time_now, token_id, event_fields, event_formatter):
         """
         Encode the archived rooms in a sync result
 
@@ -326,17 +326,17 @@ class SyncRestServlet(RestServlet):
         """
         joined = {}
         for room in rooms:
-            joined[room.room_id] = SyncRestServlet.encode_room(
+            joined[room.room_id] = yield self.encode_room(
                 room, time_now, token_id, joined=False,
                 only_fields=event_fields,
                 event_formatter=event_formatter,
             )
 
-        return joined
+        defer.returnValue(joined)
 
-    @staticmethod
+    @defer.inlineCallbacks
     def encode_room(
-            room, time_now, token_id, joined,
+            self, room, time_now, token_id, joined,
             only_fields, event_formatter,
     ):
         """
@@ -355,9 +355,10 @@ class SyncRestServlet(RestServlet):
         Returns:
             dict[str, object]: the room, encoded in our response format
         """
-        def serialize(event):
-            return serialize_event(
-                event, time_now, token_id=token_id,
+        def serialize(events):
+            return self._event_serializer.serialize_events(
+                events, time_now=time_now,
+                token_id=token_id,
                 event_format=event_formatter,
                 only_event_fields=only_fields,
             )
@@ -376,8 +377,8 @@ class SyncRestServlet(RestServlet):
                     event.event_id, room.room_id, event.room_id,
                 )
 
-        serialized_state = [serialize(e) for e in state_events]
-        serialized_timeline = [serialize(e) for e in timeline_events]
+        serialized_state = yield serialize(state_events)
+        serialized_timeline = yield serialize(timeline_events)
 
         account_data = room.account_data
 
@@ -397,7 +398,7 @@ class SyncRestServlet(RestServlet):
             result["unread_notifications"] = room.unread_notifications
             result["summary"] = room.summary
 
-        return result
+        defer.returnValue(result)
 
 
 def register_servlets(hs, http_server):
diff --git a/synapse/server.py b/synapse/server.py
index 8c30ac2fa..80d40b927 100644
--- a/synapse/server.py
+++ b/synapse/server.py
@@ -35,6 +35,7 @@ from synapse.crypto import context_factory
 from synapse.crypto.keyring import Keyring
 from synapse.events.builder import EventBuilderFactory
 from synapse.events.spamcheck import SpamChecker
+from synapse.events.utils import EventClientSerializer
 from synapse.federation.federation_client import FederationClient
 from synapse.federation.federation_server import (
     FederationHandlerRegistry,
@@ -185,6 +186,7 @@ class HomeServer(object):
         'sendmail',
         'registration_handler',
         'account_validity_handler',
+        'event_client_serializer',
     ]
 
     REQUIRED_ON_MASTER_STARTUP = [
@@ -511,6 +513,9 @@ class HomeServer(object):
     def build_account_validity_handler(self):
         return AccountValidityHandler(self)
 
+    def build_event_client_serializer(self):
+        return EventClientSerializer(self)
+
     def remove_pusher(self, app_id, push_key, user_id):
         return self.get_pusherpool().remove_pusher(app_id, push_key, user_id)
 
diff --git a/synapse/util/async_helpers.py b/synapse/util/async_helpers.py
index 2f16f23d9..7253ba120 100644
--- a/synapse/util/async_helpers.py
+++ b/synapse/util/async_helpers.py
@@ -156,6 +156,25 @@ def concurrently_execute(func, args, limit):
     ], consumeErrors=True)).addErrback(unwrapFirstError)
 
 
+def yieldable_gather_results(func, iter, *args, **kwargs):
+    """Executes the function with each argument concurrently.
+
+    Args:
+        func (func): Function to execute that returns a Deferred
+        iter (iter): An iterable that yields items that get passed as the first
+            argument to the function
+        *args: Arguments to be passed to each call to func
+
+    Returns
+        Deferred[list]: Resolved when all functions have been invoked, or errors if
+        one of the function calls fails.
+    """
+    return logcontext.make_deferred_yieldable(defer.gatherResults([
+        run_in_background(func, item, *args, **kwargs)
+        for item in iter
+    ], consumeErrors=True)).addErrback(unwrapFirstError)
+
+
 class Linearizer(object):
     """Limits concurrent access to resources based on a key. Useful to ensure
     only a few things happen at a time on a given resource.