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https://github.com/element-hq/synapse.git
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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.
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4 changed files with 158 additions and 43 deletions
1
changelog.d/10119.misc
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1
changelog.d/10119.misc
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@ -0,0 +1 @@
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Improve event caching mechanism to avoid having multiple copies of an event in memory at a time.
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@ -14,7 +14,6 @@
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import logging
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import threading
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from collections import namedtuple
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from typing import (
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Collection,
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Container,
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@ -27,6 +26,7 @@ from typing import (
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overload,
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)
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import attr
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from constantly import NamedConstant, Names
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from typing_extensions import Literal
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@ -42,7 +42,11 @@ from synapse.api.room_versions import (
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from synapse.events import EventBase, make_event_from_dict
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from synapse.events.snapshot import EventContext
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from synapse.events.utils import prune_event
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from synapse.logging.context import PreserveLoggingContext, current_context
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from synapse.logging.context import (
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PreserveLoggingContext,
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current_context,
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make_deferred_yieldable,
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)
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from synapse.metrics.background_process_metrics import (
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run_as_background_process,
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wrap_as_background_process,
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@ -56,6 +60,8 @@ from synapse.storage.engines import PostgresEngine
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from synapse.storage.util.id_generators import MultiWriterIdGenerator, StreamIdGenerator
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from synapse.storage.util.sequence import build_sequence_generator
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from synapse.types import JsonDict, get_domain_from_id
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from synapse.util import unwrapFirstError
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from synapse.util.async_helpers import ObservableDeferred
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from synapse.util.caches.descriptors import cached, cachedList
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from synapse.util.caches.lrucache import LruCache
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from synapse.util.iterutils import batch_iter
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@ -74,7 +80,10 @@ EVENT_QUEUE_ITERATIONS = 3 # No. times we block waiting for requests for events
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EVENT_QUEUE_TIMEOUT_S = 0.1 # Timeout when waiting for requests for events
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_EventCacheEntry = namedtuple("_EventCacheEntry", ("event", "redacted_event"))
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@attr.s(slots=True, auto_attribs=True)
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class _EventCacheEntry:
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event: EventBase
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redacted_event: Optional[EventBase]
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class EventRedactBehaviour(Names):
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@ -161,6 +170,13 @@ class EventsWorkerStore(SQLBaseStore):
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max_size=hs.config.caches.event_cache_size,
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)
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# Map from event ID to a deferred that will result in a map from event
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# ID to cache entry. Note that the returned dict may not have the
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# requested event in it if the event isn't in the DB.
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self._current_event_fetches: Dict[
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str, ObservableDeferred[Dict[str, _EventCacheEntry]]
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] = {}
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self._event_fetch_lock = threading.Condition()
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self._event_fetch_list = []
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self._event_fetch_ongoing = 0
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@ -476,7 +492,9 @@ class EventsWorkerStore(SQLBaseStore):
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return events
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async def _get_events_from_cache_or_db(self, event_ids, allow_rejected=False):
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async def _get_events_from_cache_or_db(
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self, event_ids: Iterable[str], allow_rejected: bool = False
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) -> Dict[str, _EventCacheEntry]:
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"""Fetch a bunch of events from the cache or the database.
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If events are pulled from the database, they will be cached for future lookups.
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@ -485,53 +503,107 @@ class EventsWorkerStore(SQLBaseStore):
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Args:
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event_ids (Iterable[str]): The event_ids of the events to fetch
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event_ids: The event_ids of the events to fetch
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allow_rejected (bool): Whether to include rejected events. If False,
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allow_rejected: Whether to include rejected events. If False,
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rejected events are omitted from the response.
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Returns:
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Dict[str, _EventCacheEntry]:
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map from event id to result
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map from event id to result
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"""
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event_entry_map = self._get_events_from_cache(
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event_ids, allow_rejected=allow_rejected
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event_ids,
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)
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missing_events_ids = [e for e in event_ids if e not in event_entry_map]
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missing_events_ids = {e for e in event_ids if e not in event_entry_map}
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# We now look up if we're already fetching some of the events in the DB,
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# if so we wait for those lookups to finish instead of pulling the same
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# events out of the DB multiple times.
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already_fetching: Dict[str, defer.Deferred] = {}
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for event_id in missing_events_ids:
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deferred = self._current_event_fetches.get(event_id)
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if deferred is not None:
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# We're already pulling the event out of the DB. Add the deferred
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# to the collection of deferreds to wait on.
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already_fetching[event_id] = deferred.observe()
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missing_events_ids.difference_update(already_fetching)
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if missing_events_ids:
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log_ctx = current_context()
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log_ctx.record_event_fetch(len(missing_events_ids))
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# Add entries to `self._current_event_fetches` for each event we're
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# going to pull from the DB. We use a single deferred that resolves
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# to all the events we pulled from the DB (this will result in this
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# function returning more events than requested, but that can happen
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# already due to `_get_events_from_db`).
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fetching_deferred: ObservableDeferred[
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Dict[str, _EventCacheEntry]
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] = ObservableDeferred(defer.Deferred())
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for event_id in missing_events_ids:
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self._current_event_fetches[event_id] = fetching_deferred
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# Note that _get_events_from_db is also responsible for turning db rows
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# into FrozenEvents (via _get_event_from_row), which involves seeing if
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# the events have been redacted, and if so pulling the redaction event out
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# of the database to check it.
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#
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missing_events = await self._get_events_from_db(
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missing_events_ids, allow_rejected=allow_rejected
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)
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try:
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missing_events = await self._get_events_from_db(
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missing_events_ids,
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)
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event_entry_map.update(missing_events)
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event_entry_map.update(missing_events)
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except Exception as e:
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with PreserveLoggingContext():
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fetching_deferred.errback(e)
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raise e
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finally:
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# Ensure that we mark these events as no longer being fetched.
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for event_id in missing_events_ids:
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self._current_event_fetches.pop(event_id, None)
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with PreserveLoggingContext():
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fetching_deferred.callback(missing_events)
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if already_fetching:
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# Wait for the other event requests to finish and add their results
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# to ours.
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results = await make_deferred_yieldable(
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defer.gatherResults(
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already_fetching.values(),
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consumeErrors=True,
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)
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).addErrback(unwrapFirstError)
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for result in results:
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event_entry_map.update(result)
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if not allow_rejected:
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event_entry_map = {
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event_id: entry
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for event_id, entry in event_entry_map.items()
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if not entry.event.rejected_reason
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}
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return event_entry_map
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def _invalidate_get_event_cache(self, event_id):
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self._get_event_cache.invalidate((event_id,))
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def _get_events_from_cache(self, events, allow_rejected, update_metrics=True):
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"""Fetch events from the caches
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def _get_events_from_cache(
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self, events: Iterable[str], update_metrics: bool = True
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) -> Dict[str, _EventCacheEntry]:
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"""Fetch events from the caches.
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May return rejected events.
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Args:
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events (Iterable[str]): list of event_ids to fetch
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allow_rejected (bool): Whether to return events that were rejected
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update_metrics (bool): Whether to update the cache hit ratio metrics
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Returns:
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dict of event_id -> _EventCacheEntry for each event_id in cache. If
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allow_rejected is `False` then there will still be an entry but it
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will be `None`
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events: list of event_ids to fetch
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update_metrics: Whether to update the cache hit ratio metrics
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"""
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event_map = {}
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@ -542,10 +614,7 @@ class EventsWorkerStore(SQLBaseStore):
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if not ret:
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continue
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if allow_rejected or not ret.event.rejected_reason:
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event_map[event_id] = ret
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else:
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event_map[event_id] = None
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event_map[event_id] = ret
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return event_map
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@ -672,23 +741,23 @@ class EventsWorkerStore(SQLBaseStore):
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with PreserveLoggingContext():
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self.hs.get_reactor().callFromThread(fire, event_list, e)
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async def _get_events_from_db(self, event_ids, allow_rejected=False):
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async def _get_events_from_db(
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self, event_ids: Iterable[str]
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) -> Dict[str, _EventCacheEntry]:
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"""Fetch a bunch of events from the database.
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May return rejected events.
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Returned events will be added to the cache for future lookups.
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Unknown events are omitted from the response.
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Args:
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event_ids (Iterable[str]): The event_ids of the events to fetch
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allow_rejected (bool): Whether to include rejected events. If False,
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rejected events are omitted from the response.
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event_ids: The event_ids of the events to fetch
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Returns:
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Dict[str, _EventCacheEntry]:
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map from event id to result. May return extra events which
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weren't asked for.
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map from event id to result. May return extra events which
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weren't asked for.
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"""
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fetched_events = {}
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events_to_fetch = event_ids
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rejected_reason = row["rejected_reason"]
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if not allow_rejected and rejected_reason:
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continue
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# If the event or metadata cannot be parsed, log the error and act
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# as if the event is unknown.
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try:
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@ -629,14 +629,12 @@ class RoomMemberWorkerStore(EventsWorkerStore):
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# We don't update the event cache hit ratio as it completely throws off
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# the hit ratio counts. After all, we don't populate the cache if we
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# miss it here
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event_map = self._get_events_from_cache(
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member_event_ids, allow_rejected=False, update_metrics=False
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)
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event_map = self._get_events_from_cache(member_event_ids, update_metrics=False)
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missing_member_event_ids = []
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for event_id in member_event_ids:
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ev_entry = event_map.get(event_id)
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if ev_entry:
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if ev_entry and not ev_entry.event.rejected_reason:
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if ev_entry.event.membership == Membership.JOIN:
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users_in_room[ev_entry.event.state_key] = ProfileInfo(
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display_name=ev_entry.event.content.get("displayname", None),
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@ -14,7 +14,10 @@
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import json
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from synapse.logging.context import LoggingContext
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from synapse.rest import admin
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from synapse.rest.client.v1 import login, room
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from synapse.storage.databases.main.events_worker import EventsWorkerStore
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from synapse.util.async_helpers import yieldable_gather_results
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from tests import unittest
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@ -94,3 +97,50 @@ class HaveSeenEventsTestCase(unittest.HomeserverTestCase):
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res = self.get_success(self.store.have_seen_events("room1", ["event10"]))
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self.assertEquals(res, {"event10"})
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self.assertEquals(ctx.get_resource_usage().db_txn_count, 0)
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class EventCacheTestCase(unittest.HomeserverTestCase):
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"""Test that the various layers of event cache works."""
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servlets = [
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admin.register_servlets,
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room.register_servlets,
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login.register_servlets,
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]
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def prepare(self, reactor, clock, hs):
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self.store: EventsWorkerStore = hs.get_datastore()
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self.user = self.register_user("user", "pass")
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self.token = self.login(self.user, "pass")
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self.room = self.helper.create_room_as(self.user, tok=self.token)
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res = self.helper.send(self.room, tok=self.token)
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self.event_id = res["event_id"]
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# Reset the event cache so the tests start with it empty
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self.store._get_event_cache.clear()
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def test_simple(self):
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"""Test that we cache events that we pull from the DB."""
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with LoggingContext("test") as ctx:
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self.get_success(self.store.get_event(self.event_id))
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# We should have fetched the event from the DB
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self.assertEqual(ctx.get_resource_usage().evt_db_fetch_count, 1)
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def test_dedupe(self):
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"""Test that if we request the same event multiple times we only pull it
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out once.
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"""
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with LoggingContext("test") as ctx:
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d = yieldable_gather_results(
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self.store.get_event, [self.event_id, self.event_id]
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)
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self.get_success(d)
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# We should have fetched the event from the DB
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self.assertEqual(ctx.get_resource_usage().evt_db_fetch_count, 1)
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