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d7808a2dde
This is the first of two PRs which seek to address #8518. This first PR lays the groundwork by extending ResponseCache; a second PR (#10158) will update the SyncHandler to actually use it, and fix the bug. The idea here is that we allow the callback given to ResponseCache.wrap to decide whether its result should be cached or not. We do that by (optionally) passing a ResponseCacheContext into it, which it can modify.
209 lines
7.2 KiB
Python
209 lines
7.2 KiB
Python
# Copyright 2016 OpenMarket Ltd
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#
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# Licensed under the Apache License, Version 2.0 (the "License");
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# you may not use this file except in compliance with the License.
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# You may obtain a copy of the License at
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#
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# http://www.apache.org/licenses/LICENSE-2.0
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#
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# Unless required by applicable law or agreed to in writing, software
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# distributed under the License is distributed on an "AS IS" BASIS,
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# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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# See the License for the specific language governing permissions and
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# limitations under the License.
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import logging
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from typing import Any, Awaitable, Callable, Dict, Generic, Optional, TypeVar
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import attr
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from twisted.internet import defer
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from synapse.logging.context import make_deferred_yieldable, run_in_background
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from synapse.util import Clock
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from synapse.util.async_helpers import ObservableDeferred
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from synapse.util.caches import register_cache
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logger = logging.getLogger(__name__)
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# the type of the key in the cache
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KV = TypeVar("KV")
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# the type of the result from the operation
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RV = TypeVar("RV")
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@attr.s(auto_attribs=True)
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class ResponseCacheContext(Generic[KV]):
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"""Information about a missed ResponseCache hit
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This object can be passed into the callback for additional feedback
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"""
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cache_key: KV
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"""The cache key that caused the cache miss
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This should be considered read-only.
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TODO: in attrs 20.1, make it frozen with an on_setattr.
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"""
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should_cache: bool = True
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"""Whether the result should be cached once the request completes.
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This can be modified by the callback if it decides its result should not be cached.
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"""
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class ResponseCache(Generic[KV]):
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"""
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This caches a deferred response. Until the deferred completes it will be
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returned from the cache. This means that if the client retries the request
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while the response is still being computed, that original response will be
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used rather than trying to compute a new response.
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"""
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def __init__(self, clock: Clock, name: str, timeout_ms: float = 0):
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# This is poorly-named: it includes both complete and incomplete results.
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# We keep complete results rather than switching to absolute values because
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# that makes it easier to cache Failure results.
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self.pending_result_cache = {} # type: Dict[KV, ObservableDeferred]
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self.clock = clock
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self.timeout_sec = timeout_ms / 1000.0
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self._name = name
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self._metrics = register_cache("response_cache", name, self, resizable=False)
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def size(self) -> int:
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return len(self.pending_result_cache)
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def __len__(self) -> int:
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return self.size()
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def get(self, key: KV) -> Optional[defer.Deferred]:
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"""Look up the given key.
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Returns a new Deferred (which also doesn't follow the synapse
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logcontext rules). You will probably want to make_deferred_yieldable the result.
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If there is no entry for the key, returns None.
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Args:
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key: key to get/set in the cache
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Returns:
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None if there is no entry for this key; otherwise a deferred which
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resolves to the result.
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"""
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result = self.pending_result_cache.get(key)
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if result is not None:
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self._metrics.inc_hits()
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return result.observe()
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else:
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self._metrics.inc_misses()
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return None
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def _set(
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self, context: ResponseCacheContext[KV], deferred: defer.Deferred
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) -> defer.Deferred:
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"""Set the entry for the given key to the given deferred.
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*deferred* should run its callbacks in the sentinel logcontext (ie,
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you should wrap normal synapse deferreds with
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synapse.logging.context.run_in_background).
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Returns a new Deferred (which also doesn't follow the synapse logcontext rules).
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You will probably want to make_deferred_yieldable the result.
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Args:
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context: Information about the cache miss
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deferred: The deferred which resolves to the result.
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Returns:
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A new deferred which resolves to the actual result.
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"""
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result = ObservableDeferred(deferred, consumeErrors=True)
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key = context.cache_key
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self.pending_result_cache[key] = result
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def on_complete(r):
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# if this cache has a non-zero timeout, and the callback has not cleared
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# the should_cache bit, we leave it in the cache for now and schedule
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# its removal later.
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if self.timeout_sec and context.should_cache:
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self.clock.call_later(
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self.timeout_sec, self.pending_result_cache.pop, key, None
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)
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else:
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# otherwise, remove the result immediately.
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self.pending_result_cache.pop(key, None)
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return r
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# make sure we do this *after* adding the entry to pending_result_cache,
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# in case the result is already complete (in which case flipping the order would
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# leave us with a stuck entry in the cache).
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result.addBoth(on_complete)
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return result.observe()
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async def wrap(
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self,
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key: KV,
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callback: Callable[..., Awaitable[RV]],
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*args: Any,
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cache_context: bool = False,
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**kwargs: Any,
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) -> RV:
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"""Wrap together a *get* and *set* call, taking care of logcontexts
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First looks up the key in the cache, and if it is present makes it
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follow the synapse logcontext rules and returns it.
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Otherwise, makes a call to *callback(*args, **kwargs)*, which should
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follow the synapse logcontext rules, and adds the result to the cache.
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Example usage:
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async def handle_request(request):
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# etc
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return result
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result = await response_cache.wrap(
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key,
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handle_request,
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request,
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)
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Args:
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key: key to get/set in the cache
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callback: function to call if the key is not found in
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the cache
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*args: positional parameters to pass to the callback, if it is used
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cache_context: if set, the callback will be given a `cache_context` kw arg,
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which will be a ResponseCacheContext object.
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**kwargs: named parameters to pass to the callback, if it is used
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Returns:
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The result of the callback (from the cache, or otherwise)
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"""
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result = self.get(key)
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if not result:
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logger.debug(
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"[%s]: no cached result for [%s], calculating new one", self._name, key
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)
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context = ResponseCacheContext(cache_key=key)
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if cache_context:
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kwargs["cache_context"] = context
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d = run_in_background(callback, *args, **kwargs)
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result = self._set(context, d)
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elif not isinstance(result, defer.Deferred) or result.called:
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logger.info("[%s]: using completed cached result for [%s]", self._name, key)
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else:
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logger.info(
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"[%s]: using incomplete cached result for [%s]", self._name, key
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)
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return await make_deferred_yieldable(result)
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