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3e6ee8ff88
When there have been lots of changes compared with the number of entities, we can do a fast(er) path. Locally I ran some benchmarking, and the comparison seems to give the best determination of which method we use.
342 lines
12 KiB
Python
342 lines
12 KiB
Python
#
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# This file is licensed under the Affero General Public License (AGPL) version 3.
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#
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# Copyright 2016 OpenMarket Ltd
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# Copyright (C) 2023 New Vector, Ltd
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#
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# This program is free software: you can redistribute it and/or modify
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# it under the terms of the GNU Affero General Public License as
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# published by the Free Software Foundation, either version 3 of the
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# License, or (at your option) any later version.
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#
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# See the GNU Affero General Public License for more details:
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# <https://www.gnu.org/licenses/agpl-3.0.html>.
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#
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# Originally licensed under the Apache License, Version 2.0:
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# <http://www.apache.org/licenses/LICENSE-2.0>.
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#
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# [This file includes modifications made by New Vector Limited]
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#
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#
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import logging
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import math
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from typing import Collection, Dict, FrozenSet, List, Mapping, Optional, Set, Union
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import attr
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from sortedcontainers import SortedDict
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from synapse.util import caches
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logger = logging.getLogger(__name__)
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# for now, assume all entities in the cache are strings
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EntityType = str
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@attr.s(auto_attribs=True, frozen=True, slots=True)
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class AllEntitiesChangedResult:
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"""Return type of `get_all_entities_changed`.
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Callers must check that there was a cache hit, via `result.hit`, before
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using the entities in `result.entities`.
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This specifically does *not* implement helpers such as `__bool__` to ensure
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that callers do the correct checks.
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"""
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_entities: Optional[List[EntityType]]
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@property
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def hit(self) -> bool:
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return self._entities is not None
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@property
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def entities(self) -> List[EntityType]:
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assert self._entities is not None
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return self._entities
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class StreamChangeCache:
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"""
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Keeps track of the stream positions of the latest change in a set of entities.
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The entity will is typically a room ID or user ID, but can be any string.
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Can be queried for whether a specific entity has changed after a stream position
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or for a list of changed entities after a stream position. See the individual
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methods for more information.
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Only tracks to a maximum cache size, any position earlier than the earliest
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known stream position must be treated as unknown.
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"""
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def __init__(
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self,
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name: str,
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current_stream_pos: int,
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max_size: int = 10000,
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prefilled_cache: Optional[Mapping[EntityType, int]] = None,
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) -> None:
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self._original_max_size: int = max_size
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self._max_size = math.floor(max_size)
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# map from stream id to the set of entities which changed at that stream id.
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self._cache: SortedDict[int, Set[EntityType]] = SortedDict()
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# map from entity to the stream ID of the latest change for that entity.
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#
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# Must be kept in sync with _cache.
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self._entity_to_key: Dict[EntityType, int] = {}
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# the earliest stream_pos for which we can reliably answer
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# get_all_entities_changed. In other words, one less than the earliest
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# stream_pos for which we know _cache is valid.
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#
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self._earliest_known_stream_pos = current_stream_pos
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self.name = name
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self.metrics = caches.register_cache(
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"cache", self.name, self._cache, resize_callback=self.set_cache_factor
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)
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if prefilled_cache:
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for entity, stream_pos in prefilled_cache.items():
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self.entity_has_changed(entity, stream_pos)
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def set_cache_factor(self, factor: float) -> bool:
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"""
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Set the cache factor for this individual cache.
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This will trigger a resize if it changes, which may require evicting
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items from the cache.
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Returns:
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Whether the cache changed size or not.
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"""
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new_size = math.floor(self._original_max_size * factor)
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if new_size != self._max_size:
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self._max_size = new_size
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self._evict()
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return True
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return False
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def has_entity_changed(self, entity: EntityType, stream_pos: int) -> bool:
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"""
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Returns True if the entity may have been updated after stream_pos.
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Args:
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entity: The entity to check for changes.
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stream_pos: The stream position to check for changes after.
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Return:
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True if the entity may have been updated, this happens if:
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* The given stream position is at or earlier than the earliest
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known stream position.
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* The given stream position is earlier than the latest change for
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the entity.
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False otherwise:
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* The entity is unknown.
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* The given stream position is at or later than the latest change
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for the entity.
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"""
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assert isinstance(stream_pos, int)
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# _cache is not valid at or before the earliest known stream position, so
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# return that the entity has changed.
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if stream_pos <= self._earliest_known_stream_pos:
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self.metrics.inc_misses()
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return True
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# If the entity is unknown, it hasn't changed.
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latest_entity_change_pos = self._entity_to_key.get(entity, None)
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if latest_entity_change_pos is None:
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self.metrics.inc_hits()
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return False
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# This is a known entity, return true if the stream position is earlier
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# than the last change.
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if stream_pos < latest_entity_change_pos:
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self.metrics.inc_misses()
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return True
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# Otherwise, the stream position is after the latest change: return false.
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self.metrics.inc_hits()
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return False
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def get_entities_changed(
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self, entities: Collection[EntityType], stream_pos: int, _perf_factor: int = 1
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) -> Union[Set[EntityType], FrozenSet[EntityType]]:
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"""
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Returns the subset of the given entities that have had changes after the given position.
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Entities unknown to the cache will be returned.
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If the position is too old it will just return the given list.
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Args:
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entities: Entities to check for changes.
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stream_pos: The stream position to check for changes after.
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_perf_factor: Used by unit tests to choose when to use each
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optimisation.
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Return:
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A subset of entities which have changed after the given stream position.
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This will be all entities if the given stream position is at or earlier
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than the earliest known stream position.
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"""
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if not self._cache or stream_pos <= self._earliest_known_stream_pos:
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self.metrics.inc_misses()
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return set(entities)
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# If there have been tonnes of changes compared with the number of
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# entities, it is faster to check each entities stream ordering
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# one-by-one.
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max_stream_pos, _ = self._cache.peekitem()
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if max_stream_pos - stream_pos > _perf_factor * len(entities):
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self.metrics.inc_hits()
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return {
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entity
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for entity in entities
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if self._entity_to_key.get(entity, -1) > stream_pos
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}
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cache_result = self.get_all_entities_changed(stream_pos)
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if cache_result.hit:
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# We now do an intersection, trying to do so in the most efficient
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# way possible (some of these sets are *large*). First check in the
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# given iterable is already a set that we can reuse, otherwise we
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# create a set of the *smallest* of the two iterables and call
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# `intersection(..)` on it (this can be twice as fast as the reverse).
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if isinstance(entities, (set, frozenset)):
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result = entities.intersection(cache_result.entities)
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elif len(cache_result.entities) < len(entities):
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result = set(cache_result.entities).intersection(entities)
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else:
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result = set(entities).intersection(cache_result.entities)
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self.metrics.inc_hits()
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else:
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result = set(entities)
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self.metrics.inc_misses()
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return result
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def has_any_entity_changed(self, stream_pos: int) -> bool:
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"""
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Returns true if any entity has changed after the given stream position.
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Args:
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stream_pos: The stream position to check for changes after.
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Return:
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True if any entity has changed after the given stream position or
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if the given stream position is at or earlier than the earliest
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known stream position.
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False otherwise.
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"""
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assert isinstance(stream_pos, int)
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# _cache is not valid at or before the earliest known stream position, so
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# return that an entity has changed.
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if stream_pos <= self._earliest_known_stream_pos:
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self.metrics.inc_misses()
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return True
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# If the cache is empty, nothing can have changed.
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if not self._cache:
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self.metrics.inc_misses()
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return False
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self.metrics.inc_hits()
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return stream_pos < self._cache.peekitem()[0]
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def get_all_entities_changed(self, stream_pos: int) -> AllEntitiesChangedResult:
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"""
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Returns all entities that have had changes after the given position.
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If the stream change cache does not go far enough back, i.e. the
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position is too old, it will return None.
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Returns the entities in the order that they were changed.
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Args:
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stream_pos: The stream position to check for changes after.
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Return:
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A class indicating if we have the requested data cached, and if so
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includes the entities in the order they were changed.
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"""
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assert isinstance(stream_pos, int)
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# _cache is not valid at or before the earliest known stream position, so
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# return None to mark that it is unknown if an entity has changed.
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if stream_pos <= self._earliest_known_stream_pos:
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return AllEntitiesChangedResult(None)
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changed_entities: List[EntityType] = []
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for k in self._cache.islice(start=self._cache.bisect_right(stream_pos)):
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changed_entities.extend(self._cache[k])
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return AllEntitiesChangedResult(changed_entities)
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def entity_has_changed(self, entity: EntityType, stream_pos: int) -> None:
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"""
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Informs the cache that the entity has been changed at the given position.
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Args:
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entity: The entity to mark as changed.
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stream_pos: The stream position to update the entity to.
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"""
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assert isinstance(stream_pos, int)
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# For a change before _cache is valid (e.g. at or before the earliest known
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# stream position) there's nothing to do.
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if stream_pos <= self._earliest_known_stream_pos:
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return
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old_pos = self._entity_to_key.get(entity, None)
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if old_pos is not None:
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if old_pos >= stream_pos:
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# nothing to do
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return
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e = self._cache[old_pos]
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e.remove(entity)
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if not e:
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# cache at this point is now empty
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del self._cache[old_pos]
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e1 = self._cache.get(stream_pos)
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if e1 is None:
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e1 = self._cache[stream_pos] = set()
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e1.add(entity)
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self._entity_to_key[entity] = stream_pos
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self._evict()
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def _evict(self) -> None:
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"""
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Ensure the cache has not exceeded the maximum size.
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Evicts entries until it is at the maximum size.
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"""
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# if the cache is too big, remove entries
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while len(self._cache) > self._max_size:
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k, r = self._cache.popitem(0)
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self._earliest_known_stream_pos = max(k, self._earliest_known_stream_pos)
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for entity in r:
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self._entity_to_key.pop(entity, None)
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def get_max_pos_of_last_change(self, entity: EntityType) -> int:
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"""Returns an upper bound of the stream id of the last change to an
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entity.
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Args:
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entity: The entity to check.
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Return:
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The stream position of the latest change for the given entity or
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the earliest known stream position if the entitiy is unknown.
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"""
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return self._entity_to_key.get(entity, self._earliest_known_stream_pos)
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