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