From 82c1ee1c22a87b9e6e3179947014b0f11c0a1ac3 Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Wed, 2 Sep 2020 15:48:37 +0100
Subject: [PATCH] Add experimental support for sharding event persister.
 (#8170)

This is *not* ready for production yet. Caveats:

1. We should write some tests...
2. The stream token that we use for events can get stalled at the minimum position of all writers. This means that new events may not be processed and e.g. sent down sync streams if a writer isn't writing or is slow.
---
 changelog.d/8170.feature                      |  1 +
 synapse/config/_base.py                       | 21 +++++-
 synapse/config/_base.pyi                      |  1 +
 synapse/config/workers.py                     | 37 ++++++++---
 synapse/handlers/federation.py                | 44 +++++++++----
 synapse/handlers/message.py                   | 14 ++--
 synapse/handlers/room.py                      | 14 ++--
 synapse/handlers/room_member.py               |  7 --
 synapse/replication/http/federation.py        | 12 +++-
 synapse/replication/tcp/handler.py            |  2 +-
 synapse/replication/tcp/streams/events.py     |  4 +-
 synapse/storage/databases/__init__.py         |  2 +-
 .../databases/main/event_federation.py        |  2 +-
 synapse/storage/databases/main/events.py      |  4 +-
 .../storage/databases/main/events_worker.py   | 66 +++++++++++++------
 .../delta/58/14events_instance_name.sql       | 16 +++++
 .../58/14events_instance_name.sql.postgres    | 26 ++++++++
 synapse/storage/util/id_generators.py         | 10 +--
 18 files changed, 206 insertions(+), 77 deletions(-)
 create mode 100644 changelog.d/8170.feature
 create mode 100644 synapse/storage/databases/main/schema/delta/58/14events_instance_name.sql
 create mode 100644 synapse/storage/databases/main/schema/delta/58/14events_instance_name.sql.postgres

diff --git a/changelog.d/8170.feature b/changelog.d/8170.feature
new file mode 100644
index 000000000000..b363e929ea8c
--- /dev/null
+++ b/changelog.d/8170.feature
@@ -0,0 +1 @@
+Add experimental support for sharding event persister.
diff --git a/synapse/config/_base.py b/synapse/config/_base.py
index 1417487427ea..73f0717b0d0f 100644
--- a/synapse/config/_base.py
+++ b/synapse/config/_base.py
@@ -832,11 +832,26 @@ class ShardedWorkerHandlingConfig:
     def should_handle(self, instance_name: str, key: str) -> bool:
         """Whether this instance is responsible for handling the given key.
         """
-
-        # If multiple instances are not defined we always return true.
+        # If multiple instances are not defined we always return true
         if not self.instances or len(self.instances) == 1:
             return True
 
+        return self.get_instance(key) == instance_name
+
+    def get_instance(self, key: str) -> str:
+        """Get the instance responsible for handling the given key.
+
+        Note: For things like federation sending the config for which instance
+        is sending is known only to the sender instance if there is only one.
+        Therefore `should_handle` should be used where possible.
+        """
+
+        if not self.instances:
+            return "master"
+
+        if len(self.instances) == 1:
+            return self.instances[0]
+
         # We shard by taking the hash, modulo it by the number of instances and
         # then checking whether this instance matches the instance at that
         # index.
@@ -846,7 +861,7 @@ def should_handle(self, instance_name: str, key: str) -> bool:
         dest_hash = sha256(key.encode("utf8")).digest()
         dest_int = int.from_bytes(dest_hash, byteorder="little")
         remainder = dest_int % (len(self.instances))
-        return self.instances[remainder] == instance_name
+        return self.instances[remainder]
 
 
 __all__ = ["Config", "RootConfig", "ShardedWorkerHandlingConfig"]
diff --git a/synapse/config/_base.pyi b/synapse/config/_base.pyi
index eb911e8f9f45..b8faafa9bdce 100644
--- a/synapse/config/_base.pyi
+++ b/synapse/config/_base.pyi
@@ -142,3 +142,4 @@ class ShardedWorkerHandlingConfig:
     instances: List[str]
     def __init__(self, instances: List[str]) -> None: ...
     def should_handle(self, instance_name: str, key: str) -> bool: ...
+    def get_instance(self, key: str) -> str: ...
diff --git a/synapse/config/workers.py b/synapse/config/workers.py
index c784a7150897..f23e42cdf98c 100644
--- a/synapse/config/workers.py
+++ b/synapse/config/workers.py
@@ -13,12 +13,24 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from typing import List, Union
+
 import attr
 
 from ._base import Config, ConfigError, ShardedWorkerHandlingConfig
 from .server import ListenerConfig, parse_listener_def
 
 
+def _instance_to_list_converter(obj: Union[str, List[str]]) -> List[str]:
+    """Helper for allowing parsing a string or list of strings to a config
+    option expecting a list of strings.
+    """
+
+    if isinstance(obj, str):
+        return [obj]
+    return obj
+
+
 @attr.s
 class InstanceLocationConfig:
     """The host and port to talk to an instance via HTTP replication.
@@ -33,11 +45,13 @@ class WriterLocations:
     """Specifies the instances that write various streams.
 
     Attributes:
-        events: The instance that writes to the event and backfill streams.
-        events: The instance that writes to the typing stream.
+        events: The instances that write to the event and backfill streams.
+        typing: The instance that writes to the typing stream.
     """
 
-    events = attr.ib(default="master", type=str)
+    events = attr.ib(
+        default=["master"], type=List[str], converter=_instance_to_list_converter
+    )
     typing = attr.ib(default="master", type=str)
 
 
@@ -105,15 +119,18 @@ def read_config(self, config, **kwargs):
         writers = config.get("stream_writers") or {}
         self.writers = WriterLocations(**writers)
 
-        # Check that the configured writer for events and typing also appears in
+        # Check that the configured writers for events and typing also appears in
         # `instance_map`.
         for stream in ("events", "typing"):
-            instance = getattr(self.writers, stream)
-            if instance != "master" and instance not in self.instance_map:
-                raise ConfigError(
-                    "Instance %r is configured to write %s but does not appear in `instance_map` config."
-                    % (instance, stream)
-                )
+            instances = _instance_to_list_converter(getattr(self.writers, stream))
+            for instance in instances:
+                if instance != "master" and instance not in self.instance_map:
+                    raise ConfigError(
+                        "Instance %r is configured to write %s but does not appear in `instance_map` config."
+                        % (instance, stream)
+                    )
+
+        self.events_shard_config = ShardedWorkerHandlingConfig(self.writers.events)
 
     def generate_config_section(self, config_dir_path, server_name, **kwargs):
         return """\
diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index 16389a0dca4c..bd8efbb768db 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -923,7 +923,8 @@ async def backfill(self, dest, room_id, limit, extremities):
                 )
             )
 
-        await self._handle_new_events(dest, ev_infos, backfilled=True)
+        if ev_infos:
+            await self._handle_new_events(dest, room_id, ev_infos, backfilled=True)
 
         # Step 2: Persist the rest of the events in the chunk one by one
         events.sort(key=lambda e: e.depth)
@@ -1216,7 +1217,7 @@ async def get_event(event_id: str):
             event_infos.append(_NewEventInfo(event, None, auth))
 
         await self._handle_new_events(
-            destination, event_infos,
+            destination, room_id, event_infos,
         )
 
     def _sanity_check_event(self, ev):
@@ -1363,15 +1364,15 @@ async def do_invite_join(
             )
 
             max_stream_id = await self._persist_auth_tree(
-                origin, auth_chain, state, event, room_version_obj
+                origin, room_id, auth_chain, state, event, room_version_obj
             )
 
             # We wait here until this instance has seen the events come down
             # replication (if we're using replication) as the below uses caches.
-            #
-            # TODO: Currently the events stream is written to from master
             await self._replication.wait_for_stream_position(
-                self.config.worker.writers.events, "events", max_stream_id
+                self.config.worker.events_shard_config.get_instance(room_id),
+                "events",
+                max_stream_id,
             )
 
             # Check whether this room is the result of an upgrade of a room we already know
@@ -1625,7 +1626,7 @@ async def on_invite_request(
         )
 
         context = await self.state_handler.compute_event_context(event)
-        await self.persist_events_and_notify([(event, context)])
+        await self.persist_events_and_notify(event.room_id, [(event, context)])
 
         return event
 
@@ -1652,7 +1653,9 @@ async def do_remotely_reject_invite(
         await self.federation_client.send_leave(host_list, event)
 
         context = await self.state_handler.compute_event_context(event)
-        stream_id = await self.persist_events_and_notify([(event, context)])
+        stream_id = await self.persist_events_and_notify(
+            event.room_id, [(event, context)]
+        )
 
         return event, stream_id
 
@@ -1900,7 +1903,7 @@ async def _handle_new_event(
                 )
 
             await self.persist_events_and_notify(
-                [(event, context)], backfilled=backfilled
+                event.room_id, [(event, context)], backfilled=backfilled
             )
         except Exception:
             run_in_background(
@@ -1913,6 +1916,7 @@ async def _handle_new_event(
     async def _handle_new_events(
         self,
         origin: str,
+        room_id: str,
         event_infos: Iterable[_NewEventInfo],
         backfilled: bool = False,
     ) -> None:
@@ -1944,6 +1948,7 @@ async def prep(ev_info: _NewEventInfo):
         )
 
         await self.persist_events_and_notify(
+            room_id,
             [
                 (ev_info.event, context)
                 for ev_info, context in zip(event_infos, contexts)
@@ -1954,6 +1959,7 @@ async def prep(ev_info: _NewEventInfo):
     async def _persist_auth_tree(
         self,
         origin: str,
+        room_id: str,
         auth_events: List[EventBase],
         state: List[EventBase],
         event: EventBase,
@@ -1968,6 +1974,7 @@ async def _persist_auth_tree(
 
         Args:
             origin: Where the events came from
+            room_id,
             auth_events
             state
             event
@@ -2042,17 +2049,20 @@ async def _persist_auth_tree(
                 events_to_context[e.event_id].rejected = RejectedReason.AUTH_ERROR
 
         await self.persist_events_and_notify(
+            room_id,
             [
                 (e, events_to_context[e.event_id])
                 for e in itertools.chain(auth_events, state)
-            ]
+            ],
         )
 
         new_event_context = await self.state_handler.compute_event_context(
             event, old_state=state
         )
 
-        return await self.persist_events_and_notify([(event, new_event_context)])
+        return await self.persist_events_and_notify(
+            room_id, [(event, new_event_context)]
+        )
 
     async def _prep_event(
         self,
@@ -2903,6 +2913,7 @@ async def _check_key_revocation(self, public_key, url):
 
     async def persist_events_and_notify(
         self,
+        room_id: str,
         event_and_contexts: Sequence[Tuple[EventBase, EventContext]],
         backfilled: bool = False,
     ) -> int:
@@ -2910,14 +2921,19 @@ async def persist_events_and_notify(
         necessary.
 
         Args:
-            event_and_contexts:
+            room_id: The room ID of events being persisted.
+            event_and_contexts: Sequence of events with their associated
+                context that should be persisted. All events must belong to
+                the same room.
             backfilled: Whether these events are a result of
                 backfilling or not
         """
-        if self.config.worker.writers.events != self._instance_name:
+        instance = self.config.worker.events_shard_config.get_instance(room_id)
+        if instance != self._instance_name:
             result = await self._send_events(
-                instance_name=self.config.worker.writers.events,
+                instance_name=instance,
                 store=self.store,
+                room_id=room_id,
                 event_and_contexts=event_and_contexts,
                 backfilled=backfilled,
             )
diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index 72bb638167e2..6398774c026e 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -376,9 +376,8 @@ def __init__(self, hs: "HomeServer"):
         self.notifier = hs.get_notifier()
         self.config = hs.config
         self.require_membership_for_aliases = hs.config.require_membership_for_aliases
-        self._is_event_writer = (
-            self.config.worker.writers.events == hs.get_instance_name()
-        )
+        self._events_shard_config = self.config.worker.events_shard_config
+        self._instance_name = hs.get_instance_name()
 
         self.room_invite_state_types = self.hs.config.room_invite_state_types
 
@@ -904,9 +903,10 @@ async def handle_new_client_event(
 
         try:
             # If we're a worker we need to hit out to the master.
-            if not self._is_event_writer:
+            writer_instance = self._events_shard_config.get_instance(event.room_id)
+            if writer_instance != self._instance_name:
                 result = await self.send_event(
-                    instance_name=self.config.worker.writers.events,
+                    instance_name=writer_instance,
                     event_id=event.event_id,
                     store=self.store,
                     requester=requester,
@@ -974,7 +974,9 @@ async def persist_and_notify_client_event(
 
         This should only be run on the instance in charge of persisting events.
         """
-        assert self._is_event_writer
+        assert self._events_shard_config.should_handle(
+            self._instance_name, event.room_id
+        )
 
         if ratelimit:
             # We check if this is a room admin redacting an event so that we
diff --git a/synapse/handlers/room.py b/synapse/handlers/room.py
index 9d5b1828df00..55794c305794 100644
--- a/synapse/handlers/room.py
+++ b/synapse/handlers/room.py
@@ -804,7 +804,9 @@ async def create_room(
 
         # Always wait for room creation to progate before returning
         await self._replication.wait_for_stream_position(
-            self.hs.config.worker.writers.events, "events", last_stream_id
+            self.hs.config.worker.events_shard_config.get_instance(room_id),
+            "events",
+            last_stream_id,
         )
 
         return result, last_stream_id
@@ -1260,10 +1262,10 @@ async def shutdown_room(
             # We now wait for the create room to come back in via replication so
             # that we can assume that all the joins/invites have propogated before
             # we try and auto join below.
-            #
-            # TODO: Currently the events stream is written to from master
             await self._replication.wait_for_stream_position(
-                self.hs.config.worker.writers.events, "events", stream_id
+                self.hs.config.worker.events_shard_config.get_instance(new_room_id),
+                "events",
+                stream_id,
             )
         else:
             new_room_id = None
@@ -1293,7 +1295,9 @@ async def shutdown_room(
 
                 # Wait for leave to come in over replication before trying to forget.
                 await self._replication.wait_for_stream_position(
-                    self.hs.config.worker.writers.events, "events", stream_id
+                    self.hs.config.worker.events_shard_config.get_instance(room_id),
+                    "events",
+                    stream_id,
                 )
 
                 await self.room_member_handler.forget(target_requester.user, room_id)
diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
index a7962b0ada39..e5cb3c2d5c4e 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -82,13 +82,6 @@ def __init__(self, hs: "HomeServer"):
         self._enable_lookup = hs.config.enable_3pid_lookup
         self.allow_per_room_profiles = self.config.allow_per_room_profiles
 
-        self._event_stream_writer_instance = hs.config.worker.writers.events
-        self._is_on_event_persistence_instance = (
-            self._event_stream_writer_instance == hs.get_instance_name()
-        )
-        if self._is_on_event_persistence_instance:
-            self.persist_event_storage = hs.get_storage().persistence
-
         self._join_rate_limiter_local = Ratelimiter(
             clock=self.clock,
             rate_hz=hs.config.ratelimiting.rc_joins_local.per_second,
diff --git a/synapse/replication/http/federation.py b/synapse/replication/http/federation.py
index 6b563151487b..5c8be747e140 100644
--- a/synapse/replication/http/federation.py
+++ b/synapse/replication/http/federation.py
@@ -65,10 +65,11 @@ def __init__(self, hs):
         self.federation_handler = hs.get_handlers().federation_handler
 
     @staticmethod
-    async def _serialize_payload(store, event_and_contexts, backfilled):
+    async def _serialize_payload(store, room_id, event_and_contexts, backfilled):
         """
         Args:
             store
+            room_id (str)
             event_and_contexts (list[tuple[FrozenEvent, EventContext]])
             backfilled (bool): Whether or not the events are the result of
                 backfilling
@@ -88,7 +89,11 @@ async def _serialize_payload(store, event_and_contexts, backfilled):
                 }
             )
 
-        payload = {"events": event_payloads, "backfilled": backfilled}
+        payload = {
+            "events": event_payloads,
+            "backfilled": backfilled,
+            "room_id": room_id,
+        }
 
         return payload
 
@@ -96,6 +101,7 @@ async def _handle_request(self, request):
         with Measure(self.clock, "repl_fed_send_events_parse"):
             content = parse_json_object_from_request(request)
 
+            room_id = content["room_id"]
             backfilled = content["backfilled"]
 
             event_payloads = content["events"]
@@ -120,7 +126,7 @@ async def _handle_request(self, request):
         logger.info("Got %d events from federation", len(event_and_contexts))
 
         max_stream_id = await self.federation_handler.persist_events_and_notify(
-            event_and_contexts, backfilled
+            room_id, event_and_contexts, backfilled
         )
 
         return 200, {"max_stream_id": max_stream_id}
diff --git a/synapse/replication/tcp/handler.py b/synapse/replication/tcp/handler.py
index 1c303f3a46d8..b323841f73cb 100644
--- a/synapse/replication/tcp/handler.py
+++ b/synapse/replication/tcp/handler.py
@@ -109,7 +109,7 @@ def __init__(self, hs):
             if isinstance(stream, (EventsStream, BackfillStream)):
                 # Only add EventStream and BackfillStream as a source on the
                 # instance in charge of event persistence.
-                if hs.config.worker.writers.events == hs.get_instance_name():
+                if hs.get_instance_name() in hs.config.worker.writers.events:
                     self._streams_to_replicate.append(stream)
 
                 continue
diff --git a/synapse/replication/tcp/streams/events.py b/synapse/replication/tcp/streams/events.py
index 16c63ff4eca8..3705618b4f72 100644
--- a/synapse/replication/tcp/streams/events.py
+++ b/synapse/replication/tcp/streams/events.py
@@ -19,7 +19,7 @@
 
 import attr
 
-from ._base import Stream, StreamUpdateResult, Token, current_token_without_instance
+from ._base import Stream, StreamUpdateResult, Token
 
 """Handling of the 'events' replication stream
 
@@ -117,7 +117,7 @@ def __init__(self, hs):
         self._store = hs.get_datastore()
         super().__init__(
             hs.get_instance_name(),
-            current_token_without_instance(self._store.get_current_events_token),
+            self._store._stream_id_gen.get_current_token_for_writer,
             self._update_function,
         )
 
diff --git a/synapse/storage/databases/__init__.py b/synapse/storage/databases/__init__.py
index 0ac854aee2df..c73d54fb6750 100644
--- a/synapse/storage/databases/__init__.py
+++ b/synapse/storage/databases/__init__.py
@@ -68,7 +68,7 @@ def __init__(self, main_store_class, hs):
 
                     # If we're on a process that can persist events also
                     # instantiate a `PersistEventsStore`
-                    if hs.config.worker.writers.events == hs.get_instance_name():
+                    if hs.get_instance_name() in hs.config.worker.writers.events:
                         persist_events = PersistEventsStore(hs, database, main)
 
                 if "state" in database_config.databases:
diff --git a/synapse/storage/databases/main/event_federation.py b/synapse/storage/databases/main/event_federation.py
index 0b69aa6a940a..4c3c162acf54 100644
--- a/synapse/storage/databases/main/event_federation.py
+++ b/synapse/storage/databases/main/event_federation.py
@@ -438,7 +438,7 @@ async def _get_forward_extremeties_for_room(self, room_id, stream_ordering):
         """
 
         if stream_ordering <= self.stream_ordering_month_ago:
-            raise StoreError(400, "stream_ordering too old")
+            raise StoreError(400, "stream_ordering too old %s" % (stream_ordering,))
 
         sql = """
                 SELECT event_id FROM stream_ordering_to_exterm
diff --git a/synapse/storage/databases/main/events.py b/synapse/storage/databases/main/events.py
index 6313b41eeffc..46b11e705bad 100644
--- a/synapse/storage/databases/main/events.py
+++ b/synapse/storage/databases/main/events.py
@@ -97,6 +97,7 @@ def __init__(
         self.store = main_data_store
         self.database_engine = db.engine
         self._clock = hs.get_clock()
+        self._instance_name = hs.get_instance_name()
 
         self._ephemeral_messages_enabled = hs.config.enable_ephemeral_messages
         self.is_mine_id = hs.is_mine_id
@@ -108,7 +109,7 @@ def __init__(
 
         # This should only exist on instances that are configured to write
         assert (
-            hs.config.worker.writers.events == hs.get_instance_name()
+            hs.get_instance_name() in hs.config.worker.writers.events
         ), "Can only instantiate EventsStore on master"
 
     async def _persist_events_and_state_updates(
@@ -800,6 +801,7 @@ def event_dict(event):
             table="events",
             values=[
                 {
+                    "instance_name": self._instance_name,
                     "stream_ordering": event.internal_metadata.stream_ordering,
                     "topological_ordering": event.depth,
                     "depth": event.depth,
diff --git a/synapse/storage/databases/main/events_worker.py b/synapse/storage/databases/main/events_worker.py
index a7a73cc3d8bb..17f5997b89f0 100644
--- a/synapse/storage/databases/main/events_worker.py
+++ b/synapse/storage/databases/main/events_worker.py
@@ -42,7 +42,8 @@
 from synapse.replication.tcp.streams.events import EventsStream
 from synapse.storage._base import SQLBaseStore, db_to_json, make_in_list_sql_clause
 from synapse.storage.database import DatabasePool
-from synapse.storage.util.id_generators import StreamIdGenerator
+from synapse.storage.engines import PostgresEngine
+from synapse.storage.util.id_generators import MultiWriterIdGenerator, StreamIdGenerator
 from synapse.types import Collection, get_domain_from_id
 from synapse.util.caches.descriptors import Cache, cached
 from synapse.util.iterutils import batch_iter
@@ -78,27 +79,54 @@ class EventsWorkerStore(SQLBaseStore):
     def __init__(self, database: DatabasePool, db_conn, hs):
         super(EventsWorkerStore, self).__init__(database, db_conn, hs)
 
-        if hs.config.worker.writers.events == hs.get_instance_name():
-            # We are the process in charge of generating stream ids for events,
-            # so instantiate ID generators based on the database
-            self._stream_id_gen = StreamIdGenerator(
-                db_conn, "events", "stream_ordering",
+        if isinstance(database.engine, PostgresEngine):
+            # If we're using Postgres than we can use `MultiWriterIdGenerator`
+            # regardless of whether this process writes to the streams or not.
+            self._stream_id_gen = MultiWriterIdGenerator(
+                db_conn=db_conn,
+                db=database,
+                instance_name=hs.get_instance_name(),
+                table="events",
+                instance_column="instance_name",
+                id_column="stream_ordering",
+                sequence_name="events_stream_seq",
             )
-            self._backfill_id_gen = StreamIdGenerator(
-                db_conn,
-                "events",
-                "stream_ordering",
-                step=-1,
-                extra_tables=[("ex_outlier_stream", "event_stream_ordering")],
+            self._backfill_id_gen = MultiWriterIdGenerator(
+                db_conn=db_conn,
+                db=database,
+                instance_name=hs.get_instance_name(),
+                table="events",
+                instance_column="instance_name",
+                id_column="stream_ordering",
+                sequence_name="events_backfill_stream_seq",
+                positive=False,
             )
         else:
-            # Another process is in charge of persisting events and generating
-            # stream IDs: rely on the replication streams to let us know which
-            # IDs we can process.
-            self._stream_id_gen = SlavedIdTracker(db_conn, "events", "stream_ordering")
-            self._backfill_id_gen = SlavedIdTracker(
-                db_conn, "events", "stream_ordering", step=-1
-            )
+            # We shouldn't be running in worker mode with SQLite, but its useful
+            # to support it for unit tests.
+            #
+            # If this process is the writer than we need to use
+            # `StreamIdGenerator`, otherwise we use `SlavedIdTracker` which gets
+            # updated over replication. (Multiple writers are not supported for
+            # SQLite).
+            if hs.get_instance_name() in hs.config.worker.writers.events:
+                self._stream_id_gen = StreamIdGenerator(
+                    db_conn, "events", "stream_ordering",
+                )
+                self._backfill_id_gen = StreamIdGenerator(
+                    db_conn,
+                    "events",
+                    "stream_ordering",
+                    step=-1,
+                    extra_tables=[("ex_outlier_stream", "event_stream_ordering")],
+                )
+            else:
+                self._stream_id_gen = SlavedIdTracker(
+                    db_conn, "events", "stream_ordering"
+                )
+                self._backfill_id_gen = SlavedIdTracker(
+                    db_conn, "events", "stream_ordering", step=-1
+                )
 
         self._get_event_cache = Cache(
             "*getEvent*",
diff --git a/synapse/storage/databases/main/schema/delta/58/14events_instance_name.sql b/synapse/storage/databases/main/schema/delta/58/14events_instance_name.sql
new file mode 100644
index 000000000000..98ff76d7091c
--- /dev/null
+++ b/synapse/storage/databases/main/schema/delta/58/14events_instance_name.sql
@@ -0,0 +1,16 @@
+/* Copyright 2020 The Matrix.org Foundation C.I.C.
+ *
+ * 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.
+ */
+
+ALTER TABLE events ADD COLUMN instance_name TEXT;
diff --git a/synapse/storage/databases/main/schema/delta/58/14events_instance_name.sql.postgres b/synapse/storage/databases/main/schema/delta/58/14events_instance_name.sql.postgres
new file mode 100644
index 000000000000..97c1e6a0c5d7
--- /dev/null
+++ b/synapse/storage/databases/main/schema/delta/58/14events_instance_name.sql.postgres
@@ -0,0 +1,26 @@
+/* Copyright 2020 The Matrix.org Foundation C.I.C.
+ *
+ * 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.
+ */
+
+CREATE SEQUENCE IF NOT EXISTS events_stream_seq;
+
+SELECT setval('events_stream_seq', (
+    SELECT COALESCE(MAX(stream_ordering), 1) FROM events
+));
+
+CREATE SEQUENCE IF NOT EXISTS events_backfill_stream_seq;
+
+SELECT setval('events_backfill_stream_seq', (
+    SELECT COALESCE(-MIN(stream_ordering), 1) FROM events
+));
diff --git a/synapse/storage/util/id_generators.py b/synapse/storage/util/id_generators.py
index 9f3d23f0a524..8fd21c2bf863 100644
--- a/synapse/storage/util/id_generators.py
+++ b/synapse/storage/util/id_generators.py
@@ -231,8 +231,12 @@ def __init__(
         # gaps should be relatively rare it's still worth doing the book keeping
         # that allows us to skip forwards when there are gapless runs of
         # positions.
+        #
+        # We start at 1 here as a) the first generated stream ID will be 2, and
+        # b) other parts of the code assume that stream IDs are strictly greater
+        # than 0.
         self._persisted_upto_position = (
-            min(self._current_positions.values()) if self._current_positions else 0
+            min(self._current_positions.values()) if self._current_positions else 1
         )
         self._known_persisted_positions = []  # type: List[int]
 
@@ -362,9 +366,7 @@ def get_current_token(self) -> int:
         equal to it have been successfully persisted.
         """
 
-        # Currently we don't support this operation, as it's not obvious how to
-        # condense the stream positions of multiple writers into a single int.
-        raise NotImplementedError()
+        return self.get_persisted_upto_position()
 
     def get_current_token_for_writer(self, instance_name: str) -> int:
         """Returns the position of the given writer.