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.

7 files changed, 100 insertions, 26 deletions

diff --git a/synapse/storage/databases/__init__.py b/synapse/storage/databases/__init__.py
index 0ac854aee2..c73d54fb67 100644
--- a/synapse/storage/databases/__init__.py
+++ b/synapse/storage/databases/__init__.py
@@ -68,7 +68,7 @@ class Databases(object):
 
                     # 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 0b69aa6a94..4c3c162acf 100644
--- a/synapse/storage/databases/main/event_federation.py
+++ b/synapse/storage/databases/main/event_federation.py
@@ -438,7 +438,7 @@ class EventFederationWorkerStore(EventsWorkerStore, SignatureWorkerStore, SQLBas
         """
 
         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 6313b41eef..46b11e705b 100644
--- a/synapse/storage/databases/main/events.py
+++ b/synapse/storage/databases/main/events.py
@@ -97,6 +97,7 @@ class PersistEventsStore:
         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 @@ class PersistEventsStore:
 
         # 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 @@ class PersistEventsStore:
             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 a7a73cc3d8..17f5997b89 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 import BackfillStream
 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 0000000000..98ff76d709
--- /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 0000000000..97c1e6a0c5
--- /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 9f3d23f0a5..8fd21c2bf8 100644
--- a/synapse/storage/util/id_generators.py
+++ b/synapse/storage/util/id_generators.py
@@ -231,8 +231,12 @@ class MultiWriterIdGenerator:
         # 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 @@ class MultiWriterIdGenerator:
         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.