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Improve event caching code (#10119) 

Ensure we only load an event from the DB once when the same event is requested multiple times at once.
This commit is contained in:
Erik Johnston 2021-08-04 13:54:51 +01:00 committed by GitHub
parent 11540be55e
commit c37dad67ab
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4 changed files with 158 additions and 43 deletions
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1
changelog.d/10119.misc Normal file
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@ -0,0 +1 @@
Improve event caching mechanism to avoid having multiple copies of an event in memory at a time.

130
synapse/storage/databases/main/events_worker.py
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@ -14,7 +14,6 @@
import logging
import threading
from collections import namedtuple
from typing import (
Collection,
Container,
@ -27,6 +26,7 @@ from typing import (
overload,
)
import attr
from constantly import NamedConstant, Names
from typing_extensions import Literal
@ -42,7 +42,11 @@ from synapse.api.room_versions import (
from synapse.events import EventBase, make_event_from_dict
from synapse.events.snapshot import EventContext
from synapse.events.utils import prune_event
from synapse.logging.context import PreserveLoggingContext, current_context
from synapse.logging.context import (
PreserveLoggingContext,
current_context,
make_deferred_yieldable,
)
from synapse.metrics.background_process_metrics import (
run_as_background_process,
wrap_as_background_process,
@ -56,6 +60,8 @@ from synapse.storage.engines import PostgresEngine
from synapse.storage.util.id_generators import MultiWriterIdGenerator, StreamIdGenerator
from synapse.storage.util.sequence import build_sequence_generator
from synapse.types import JsonDict, get_domain_from_id
from synapse.util import unwrapFirstError
from synapse.util.async_helpers import ObservableDeferred
from synapse.util.caches.descriptors import cached, cachedList
from synapse.util.caches.lrucache import LruCache
from synapse.util.iterutils import batch_iter
@ -74,7 +80,10 @@ EVENT_QUEUE_ITERATIONS = 3 # No. times we block waiting for requests for events
EVENT_QUEUE_TIMEOUT_S = 0.1 # Timeout when waiting for requests for events
_EventCacheEntry = namedtuple("_EventCacheEntry", ("event", "redacted_event"))
@attr.s(slots=True, auto_attribs=True)
class _EventCacheEntry:
event: EventBase
redacted_event: Optional[EventBase]
class EventRedactBehaviour(Names):
@ -161,6 +170,13 @@ class EventsWorkerStore(SQLBaseStore):
max_size=hs.config.caches.event_cache_size,
)
# Map from event ID to a deferred that will result in a map from event
# ID to cache entry. Note that the returned dict may not have the
# requested event in it if the event isn't in the DB.
self._current_event_fetches: Dict[
str, ObservableDeferred[Dict[str, _EventCacheEntry]]
] = {}
self._event_fetch_lock = threading.Condition()
self._event_fetch_list = []
self._event_fetch_ongoing = 0
@ -476,7 +492,9 @@ class EventsWorkerStore(SQLBaseStore):
return events
async def _get_events_from_cache_or_db(self, event_ids, allow_rejected=False):
async def _get_events_from_cache_or_db(
self, event_ids: Iterable[str], allow_rejected: bool = False
) -> Dict[str, _EventCacheEntry]:
"""Fetch a bunch of events from the cache or the database.
If events are pulled from the database, they will be cached for future lookups.
@ -485,53 +503,107 @@ class EventsWorkerStore(SQLBaseStore):
Args:
event_ids (Iterable[str]): The event_ids of the events to fetch
event_ids: The event_ids of the events to fetch
allow_rejected (bool): Whether to include rejected events. If False,
allow_rejected: Whether to include rejected events. If False,
rejected events are omitted from the response.
Returns:
Dict[str, _EventCacheEntry]:
map from event id to result
"""
event_entry_map = self._get_events_from_cache(
event_ids, allow_rejected=allow_rejected
event_ids,
)
missing_events_ids = [e for e in event_ids if e not in event_entry_map]
missing_events_ids = {e for e in event_ids if e not in event_entry_map}
# We now look up if we're already fetching some of the events in the DB,
# if so we wait for those lookups to finish instead of pulling the same
# events out of the DB multiple times.
already_fetching: Dict[str, defer.Deferred] = {}
for event_id in missing_events_ids:
deferred = self._current_event_fetches.get(event_id)
if deferred is not None:
# We're already pulling the event out of the DB. Add the deferred
# to the collection of deferreds to wait on.
already_fetching[event_id] = deferred.observe()
missing_events_ids.difference_update(already_fetching)
if missing_events_ids:
log_ctx = current_context()
log_ctx.record_event_fetch(len(missing_events_ids))
# Add entries to `self._current_event_fetches` for each event we're
# going to pull from the DB. We use a single deferred that resolves
# to all the events we pulled from the DB (this will result in this
# function returning more events than requested, but that can happen
# already due to `_get_events_from_db`).
fetching_deferred: ObservableDeferred[
Dict[str, _EventCacheEntry]
] = ObservableDeferred(defer.Deferred())
for event_id in missing_events_ids:
self._current_event_fetches[event_id] = fetching_deferred
# Note that _get_events_from_db is also responsible for turning db rows
# into FrozenEvents (via _get_event_from_row), which involves seeing if
# the events have been redacted, and if so pulling the redaction event out
# of the database to check it.
#
try:
missing_events = await self._get_events_from_db(
missing_events_ids, allow_rejected=allow_rejected
missing_events_ids,
)
event_entry_map.update(missing_events)
except Exception as e:
with PreserveLoggingContext():
fetching_deferred.errback(e)
raise e
finally:
# Ensure that we mark these events as no longer being fetched.
for event_id in missing_events_ids:
self._current_event_fetches.pop(event_id, None)
with PreserveLoggingContext():
fetching_deferred.callback(missing_events)
if already_fetching:
# Wait for the other event requests to finish and add their results
# to ours.
results = await make_deferred_yieldable(
defer.gatherResults(
already_fetching.values(),
consumeErrors=True,
)
).addErrback(unwrapFirstError)
for result in results:
event_entry_map.update(result)
if not allow_rejected:
event_entry_map = {
event_id: entry
for event_id, entry in event_entry_map.items()
if not entry.event.rejected_reason
}
return event_entry_map
def _invalidate_get_event_cache(self, event_id):
self._get_event_cache.invalidate((event_id,))
def _get_events_from_cache(self, events, allow_rejected, update_metrics=True):
"""Fetch events from the caches
def _get_events_from_cache(
self, events: Iterable[str], update_metrics: bool = True
) -> Dict[str, _EventCacheEntry]:
"""Fetch events from the caches.
May return rejected events.
Args:
events (Iterable[str]): list of event_ids to fetch
allow_rejected (bool): Whether to return events that were rejected
update_metrics (bool): Whether to update the cache hit ratio metrics
Returns:
dict of event_id -> _EventCacheEntry for each event_id in cache. If
allow_rejected is `False` then there will still be an entry but it
will be `None`
events: list of event_ids to fetch
update_metrics: Whether to update the cache hit ratio metrics
"""
event_map = {}
@ -542,10 +614,7 @@ class EventsWorkerStore(SQLBaseStore):
if not ret:
continue
if allow_rejected or not ret.event.rejected_reason:
event_map[event_id] = ret
else:
event_map[event_id] = None
return event_map
@ -672,21 +741,21 @@ class EventsWorkerStore(SQLBaseStore):
with PreserveLoggingContext():
self.hs.get_reactor().callFromThread(fire, event_list, e)
async def _get_events_from_db(self, event_ids, allow_rejected=False):
async def _get_events_from_db(
self, event_ids: Iterable[str]
) -> Dict[str, _EventCacheEntry]:
"""Fetch a bunch of events from the database.
May return rejected events.
Returned events will be added to the cache for future lookups.
Unknown events are omitted from the response.
Args:
event_ids (Iterable[str]): The event_ids of the events to fetch
allow_rejected (bool): Whether to include rejected events. If False,
rejected events are omitted from the response.
event_ids: The event_ids of the events to fetch
Returns:
Dict[str, _EventCacheEntry]:
map from event id to result. May return extra events which
weren't asked for.
"""
@ -717,9 +786,6 @@ class EventsWorkerStore(SQLBaseStore):
rejected_reason = row["rejected_reason"]
if not allow_rejected and rejected_reason:
continue
# If the event or metadata cannot be parsed, log the error and act
# as if the event is unknown.
try:

6
synapse/storage/databases/main/roommember.py
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@ -629,14 +629,12 @@ class RoomMemberWorkerStore(EventsWorkerStore):
# We don't update the event cache hit ratio as it completely throws off
# the hit ratio counts. After all, we don't populate the cache if we
# miss it here
event_map = self._get_events_from_cache(
member_event_ids, allow_rejected=False, update_metrics=False
)
event_map = self._get_events_from_cache(member_event_ids, update_metrics=False)
missing_member_event_ids = []
for event_id in member_event_ids:
ev_entry = event_map.get(event_id)
if ev_entry:
if ev_entry and not ev_entry.event.rejected_reason:
if ev_entry.event.membership == Membership.JOIN:
users_in_room[ev_entry.event.state_key] = ProfileInfo(
display_name=ev_entry.event.content.get("displayname", None),

50
tests/storage/databases/main/test_events_worker.py
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@ -14,7 +14,10 @@
import json
from synapse.logging.context import LoggingContext
from synapse.rest import admin
from synapse.rest.client.v1 import login, room
from synapse.storage.databases.main.events_worker import EventsWorkerStore
from synapse.util.async_helpers import yieldable_gather_results
from tests import unittest
@ -94,3 +97,50 @@ class HaveSeenEventsTestCase(unittest.HomeserverTestCase):
res = self.get_success(self.store.have_seen_events("room1", ["event10"]))
self.assertEquals(res, {"event10"})
self.assertEquals(ctx.get_resource_usage().db_txn_count, 0)
class EventCacheTestCase(unittest.HomeserverTestCase):
"""Test that the various layers of event cache works."""
servlets = [
admin.register_servlets,
room.register_servlets,
login.register_servlets,
]
def prepare(self, reactor, clock, hs):
self.store: EventsWorkerStore = hs.get_datastore()
self.user = self.register_user("user", "pass")
self.token = self.login(self.user, "pass")
self.room = self.helper.create_room_as(self.user, tok=self.token)
res = self.helper.send(self.room, tok=self.token)
self.event_id = res["event_id"]
# Reset the event cache so the tests start with it empty
self.store._get_event_cache.clear()
def test_simple(self):
"""Test that we cache events that we pull from the DB."""
with LoggingContext("test") as ctx:
self.get_success(self.store.get_event(self.event_id))
# We should have fetched the event from the DB
self.assertEqual(ctx.get_resource_usage().evt_db_fetch_count, 1)
def test_dedupe(self):
"""Test that if we request the same event multiple times we only pull it
out once.
"""
with LoggingContext("test") as ctx:
d = yieldable_gather_results(
self.store.get_event, [self.event_id, self.event_id]
)
self.get_success(d)
# We should have fetched the event from the DB
self.assertEqual(ctx.get_resource_usage().evt_db_fetch_count, 1)