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Extend StreamChangeCache to support multiple entities per stream ID (#7303)

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First some background: StreamChangeCache is used to keep track of what "entities" have 
changed since a given stream ID. So for example, we might use it to keep track of when the last
to-device message for a given user was received [1], and hence whether we need to pull any to-device messages from the database on a sync [2].

Now, it turns out that StreamChangeCache didn't support more than one thing being changed at
a given stream_id (this was part of the problem with #7206). However, it's entirely valid to send
to-device messages to more than one user at a time.

As it turns out, this did in fact work, because *some* methods of StreamChangeCache coped
ok with having multiple things changing on the same stream ID, and it seems we never actually
use the methods which don't work on the stream change caches where we allow multiple
changes at the same stream ID. But that feels horribly fragile, hence: let's update
StreamChangeCache to properly support this, and add some typing and some more tests while
we're at it.

[1]: https://github.com/matrix-org/synapse/blob/release-v1.12.3/synapse/storage/data_stores/main/deviceinbox.py#L301
[2]: https://github.com/matrix-org/synapse/blob/release-v1.12.3/synapse/storage/data_stores/main/deviceinbox.py#L47-L51
This commit is contained in:
Richard van der Hoff 2020-04-22 13:45:40 +01:00 • committed by GitHub
parent 6b6685db9f
commit 13683a3a22
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6 changed files with 269 additions and 53 deletions
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111
synapse/util/caches/stream_change_cache.py
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@ -14,6 +14,7 @@
# limitations under the License.
import logging
from typing import Dict, Iterable, List, Mapping, Optional, Set
from six import integer_types
@ -23,8 +24,11 @@ from synapse.util import caches
logger = logging.getLogger(__name__)
# for now, assume all entities in the cache are strings
EntityType = str
class StreamChangeCache(object):
class StreamChangeCache:
"""Keeps track of the stream positions of the latest change in a set of entities.
Typically the entity will be a room or user id.
@ -34,10 +38,23 @@ class StreamChangeCache(object):
old then the cache will simply return all given entities.
"""
def __init__(self, name, current_stream_pos, max_size=10000, prefilled_cache=None):
def __init__(
self,
name: str,
current_stream_pos: int,
max_size=10000,
prefilled_cache: Optional[Mapping[EntityType, int]] = None,
):
self._max_size = int(max_size * caches.CACHE_SIZE_FACTOR)
self._entity_to_key = {}
self._cache = SortedDict()
self._entity_to_key = {} # type: Dict[EntityType, int]
# map from stream id to the a set of entities which changed at that stream id.
self._cache = SortedDict() # type: SortedDict[int, Set[EntityType]]
# 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)
@ -46,7 +63,7 @@ class StreamChangeCache(object):
for entity, stream_pos in prefilled_cache.items():
self.entity_has_changed(entity, stream_pos)
def has_entity_changed(self, entity, stream_pos):
def has_entity_changed(self, entity: EntityType, stream_pos: int) -> bool:
"""Returns True if the entity may have been updated since stream_pos
"""
assert type(stream_pos) in integer_types
@ -67,22 +84,17 @@ class StreamChangeCache(object):
self.metrics.inc_hits()
return False
def get_entities_changed(self, entities, stream_pos):
def get_entities_changed(
self, entities: Iterable[EntityType], stream_pos: int
) -> Set[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.
"""
assert type(stream_pos) is int
if stream_pos >= self._earliest_known_stream_pos:
changed_entities = {
self._cache[k]
for k in self._cache.islice(start=self._cache.bisect_right(stream_pos))
}
result = changed_entities.intersection(entities)
changed_entities = self.get_all_entities_changed(stream_pos)
if changed_entities is not None:
result = set(changed_entities).intersection(entities)
self.metrics.inc_hits()
else:
result = set(entities)
@ -90,13 +102,13 @@ class StreamChangeCache(object):
return result
def has_any_entity_changed(self, stream_pos):
def has_any_entity_changed(self, stream_pos: int) -> bool:
"""Returns if any entity has changed
"""
assert type(stream_pos) is int
if not self._cache:
# If we have no cache, nothing can have changed.
# If the cache is empty, nothing can have changed.
return False
if stream_pos >= self._earliest_known_stream_pos:
@ -106,45 +118,58 @@ class StreamChangeCache(object):
self.metrics.inc_misses()
return True
def get_all_entities_changed(self, stream_pos):
"""Returns all entites that have had new things since the given
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 the entities in the order that they were changed.
"""
assert type(stream_pos) is int
if stream_pos >= self._earliest_known_stream_pos:
return [
self._cache[k]
for k in self._cache.islice(start=self._cache.bisect_right(stream_pos))
]
else:
if stream_pos < self._earliest_known_stream_pos:
return None
def entity_has_changed(self, entity, stream_pos):
changed_entities = [] # type: List[EntityType]
for k in self._cache.islice(start=self._cache.bisect_right(stream_pos)):
changed_entities.extend(self._cache[k])
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
# FIXME: add a sanity check here that we are not overwriting existing
# data in self._cache
if stream_pos <= self._earliest_known_stream_pos:
return
if stream_pos > self._earliest_known_stream_pos:
old_pos = self._entity_to_key.get(entity, None)
if old_pos is not None:
stream_pos = max(stream_pos, old_pos)
self._cache.pop(old_pos, None)
self._cache[stream_pos] = entity
self._entity_to_key[entity] = stream_pos
old_pos = self._entity_to_key.get(entity, None)
if old_pos is not None:
if old_pos >= stream_pos:
# nothing to do
return
e = self._cache[old_pos]
e.remove(entity)
if not e:
# cache at this point is now empty
del self._cache[old_pos]
while len(self._cache) > self._max_size:
k, r = self._cache.popitem(0)
self._earliest_known_stream_pos = max(
k, self._earliest_known_stream_pos
)
self._entity_to_key.pop(r, None)
e1 = self._cache.get(stream_pos)
if e1 is None:
e1 = self._cache[stream_pos] = set()
e1.add(entity)
self._entity_to_key[entity] = stream_pos
# if the cache is too big, remove entries
while len(self._cache) > self._max_size:
k, r = self._cache.popitem(0)
self._earliest_known_stream_pos = max(k, self._earliest_known_stream_pos)
for entity in r:
del self._entity_to_key[entity]
def get_max_pos_of_last_change(self, entity: EntityType) -> int:
def get_max_pos_of_last_change(self, entity):
"""Returns an upper bound of the stream id of the last change to an
entity.
"""