diff options
Diffstat (limited to 'synapse/util/caches/stream_change_cache.py')
| -rw-r--r-- | synapse/util/caches/stream_change_cache.py | 142 |
1 files changed, 116 insertions, 26 deletions
diff --git a/synapse/util/caches/stream_change_cache.py b/synapse/util/caches/stream_change_cache.py index 666f4b6895..042de8d7c8 100644 --- a/synapse/util/caches/stream_change_cache.py +++ b/synapse/util/caches/stream_change_cache.py @@ -27,13 +27,17 @@ EntityType = str class StreamChangeCache: - """Keeps track of the stream positions of the latest change in a set of entities. + """ + Keeps track of the stream positions of the latest change in a set of entities. + + The entity will is typically a room ID or user ID, but can be any string. - Typically the entity will be a room or user id. + Can be queried for whether a specific entity has changed after a stream position + or for a list of changed entities after a stream position. See the individual + methods for more information. - Given a list of entities and a stream position, it will give a subset of - entities that may have changed since that position. If position key is too - old then the cache will simply return all given entities. + Only tracks to a maximum cache size, any position earlier than the earliest + known stream position must be treated as unknown. """ def __init__( @@ -45,16 +49,20 @@ class StreamChangeCache: ) -> None: self._original_max_size: int = max_size self._max_size = math.floor(max_size) - self._entity_to_key: Dict[EntityType, int] = {} - # map from stream id to the a set of entities which changed at that stream id. + # map from stream id to the set of entities which changed at that stream id. self._cache: SortedDict[int, Set[EntityType]] = SortedDict() + # map from entity to the stream ID of the latest change for that entity. + # + # Must be kept in sync with _cache. + self._entity_to_key: Dict[EntityType, int] = {} # the earliest stream_pos for which we can reliably answer # get_all_entities_changed. In other words, one less than the earliest # stream_pos for which we know _cache is valid. # self._earliest_known_stream_pos = current_stream_pos + self.name = name self.metrics = caches.register_cache( "cache", self.name, self._cache, resize_callback=self.set_cache_factor @@ -82,22 +90,46 @@ class StreamChangeCache: return False def has_entity_changed(self, entity: EntityType, stream_pos: int) -> bool: - """Returns True if the entity may have been updated since stream_pos""" + """ + Returns True if the entity may have been updated after stream_pos. + + Args: + entity: The entity to check for changes. + stream_pos: The stream position to check for changes after. + + Return: + True if the entity may have been updated, this happens if: + * The given stream position is at or earlier than the earliest + known stream position. + * The given stream position is earlier than the latest change for + the entity. + + False otherwise: + * The entity is unknown. + * The given stream position is at or later than the latest change + for the entity. + """ assert isinstance(stream_pos, int) - if stream_pos < self._earliest_known_stream_pos: + # _cache is not valid at or before the earliest known stream position, so + # return that the entity has changed. + if stream_pos <= self._earliest_known_stream_pos: self.metrics.inc_misses() return True + # If the entity is unknown, it hasn't changed. latest_entity_change_pos = self._entity_to_key.get(entity, None) if latest_entity_change_pos is None: self.metrics.inc_hits() return False + # This is a known entity, return true if the stream position is earlier + # than the last change. if stream_pos < latest_entity_change_pos: self.metrics.inc_misses() return True + # Otherwise, the stream position is after the latest change: return false. self.metrics.inc_hits() return False @@ -105,15 +137,27 @@ class StreamChangeCache: self, entities: Collection[EntityType], stream_pos: int ) -> Union[Set[EntityType], FrozenSet[EntityType]]: """ - Returns subset of entities that have had new things since the given - position. Entities unknown to the cache will be returned. If the - position is too old it will just return the given list. + Returns the subset of the given entities that have had changes after the given position. + + Entities unknown to the cache will be returned. + + If the position is too old it will just return the given list. + + Args: + entities: Entities to check for changes. + stream_pos: The stream position to check for changes after. + + Return: + A subset of entities which have changed after the given stream position. + + This will be all entities if the given stream position is at or earlier + than the earliest known stream position. """ changed_entities = self.get_all_entities_changed(stream_pos) if changed_entities is not None: # We now do an intersection, trying to do so in the most efficient # way possible (some of these sets are *large*). First check in the - # given iterable is already set that we can reuse, otherwise we + # given iterable is already a set that we can reuse, otherwise we # create a set of the *smallest* of the two iterables and call # `intersection(..)` on it (this can be twice as fast as the reverse). if isinstance(entities, (set, frozenset)): @@ -130,29 +174,57 @@ class StreamChangeCache: return result def has_any_entity_changed(self, stream_pos: int) -> bool: - """Returns if any entity has changed""" - assert type(stream_pos) is int + """ + Returns true if any entity has changed after the given stream position. + + Args: + stream_pos: The stream position to check for changes after. + + Return: + True if any entity has changed after the given stream position or + if the given stream position is at or earlier than the earliest + known stream position. + + False otherwise. + """ + assert isinstance(stream_pos, int) if not self._cache: # If the cache is empty, nothing can have changed. return False - if stream_pos >= self._earliest_known_stream_pos: - self.metrics.inc_hits() - return self._cache.bisect_right(stream_pos) < len(self._cache) - else: + # _cache is not valid at or before the earliest known stream position, so + # return that an entity has changed. + if stream_pos <= self._earliest_known_stream_pos: self.metrics.inc_misses() return True + self.metrics.inc_hits() + return stream_pos < self._cache.peekitem()[0] + def get_all_entities_changed(self, stream_pos: int) -> Optional[List[EntityType]]: - """Returns all entities that have had new things since the given - position. If the position is too old it will return None. + """ + Returns all entities that have had changes after the given position. + + If the stream change cache does not go far enough back, i.e. the position + is too old, it will return None. Returns the entities in the order that they were changed. + + Args: + stream_pos: The stream position to check for changes after. + + Return: + Entities which have changed after the given stream position. + + None if the given stream position is at or earlier than the earliest + known stream position. """ - assert type(stream_pos) is int + assert isinstance(stream_pos, int) - if stream_pos < self._earliest_known_stream_pos: + # _cache is not valid at or before the earliest known stream position, so + # return None to mark that it is unknown if an entity has changed. + if stream_pos <= self._earliest_known_stream_pos: return None changed_entities: List[EntityType] = [] @@ -162,11 +234,17 @@ class StreamChangeCache: return changed_entities def entity_has_changed(self, entity: EntityType, stream_pos: int) -> None: - """Informs the cache that the entity has been changed at the given - position. """ - assert type(stream_pos) is int + Informs the cache that the entity has been changed at the given position. + + Args: + entity: The entity to mark as changed. + stream_pos: The stream position to update the entity to. + """ + assert isinstance(stream_pos, int) + # For a change before _cache is valid (e.g. at or before the earliest known + # stream position) there's nothing to do. if stream_pos <= self._earliest_known_stream_pos: return @@ -189,6 +267,11 @@ class StreamChangeCache: self._evict() def _evict(self) -> None: + """ + Ensure the cache has not exceeded the maximum size. + + Evicts entries until it is at the maximum size. + """ # if the cache is too big, remove entries while len(self._cache) > self._max_size: k, r = self._cache.popitem(0) @@ -199,5 +282,12 @@ class StreamChangeCache: def get_max_pos_of_last_change(self, entity: EntityType) -> int: """Returns an upper bound of the stream id of the last change to an entity. + + Args: + entity: The entity to check. + + Return: + The stream position of the latest change for the given entity or + the earliest known stream position if the entitiy is unknown. """ return self._entity_to_key.get(entity, self._earliest_known_stream_pos) |