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- # 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 (
- TYPE_CHECKING,
- Any,
- Awaitable,
- Callable,
- Dict,
- Generic,
- Iterable,
- Optional,
- TypeVar,
- )
- import attr
- from twisted.internet import defer
- from synapse.logging.context import make_deferred_yieldable, run_in_background
- from synapse.logging.opentracing import (
- active_span,
- start_active_span,
- start_active_span_follows_from,
- )
- from synapse.util import Clock
- from synapse.util.async_helpers import AbstractObservableDeferred, ObservableDeferred
- from synapse.util.caches import EvictionReason, register_cache
- logger = logging.getLogger(__name__)
- if TYPE_CHECKING:
- import opentracing
- # 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 ResponseCacheContext(Generic[KV]):
- """Information about a missed ResponseCache hit
- This object can be passed into the callback for additional feedback
- """
- cache_key: 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.
- """
- @attr.s(auto_attribs=True)
- class ResponseCacheEntry:
- result: AbstractObservableDeferred
- """The (possibly incomplete) result of the operation.
- Note that we continue to store an ObservableDeferred even after the operation
- completes (rather than switching to an immediate value), since that makes it
- easier to cache Failure results.
- """
- opentracing_span_context: "Optional[opentracing.SpanContext]"
- """The opentracing span which generated/is generating the result"""
- class ResponseCache(Generic[KV]):
- """
- 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.
- """
- def __init__(self, clock: Clock, name: str, timeout_ms: float = 0):
- self._result_cache: Dict[KV, ResponseCacheEntry] = {}
- self.clock = clock
- self.timeout_sec = timeout_ms / 1000.0
- self._name = name
- self._metrics = register_cache("response_cache", name, self, resizable=False)
- def size(self) -> int:
- return len(self._result_cache)
- def __len__(self) -> int:
- return self.size()
- def keys(self) -> Iterable[KV]:
- """Get the keys currently in the result cache
- Returns both incomplete entries, and (if the timeout on this cache is non-zero),
- complete entries which are still in the cache.
- Note that the returned iterator is not safe in the face of concurrent execution:
- behaviour is undefined if `wrap` is called during iteration.
- """
- return self._result_cache.keys()
- def _get(self, key: KV) -> Optional[ResponseCacheEntry]:
- """Look up the given key.
- Args:
- key: key to get in the cache
- Returns:
- The entry for this key, if any; else None.
- """
- entry = self._result_cache.get(key)
- if entry is not None:
- self._metrics.inc_hits()
- return entry
- else:
- self._metrics.inc_misses()
- return None
- def _set(
- self,
- context: ResponseCacheContext[KV],
- deferred: "defer.Deferred[RV]",
- opentracing_span_context: "Optional[opentracing.SpanContext]",
- ) -> ResponseCacheEntry:
- """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).
- Args:
- context: Information about the cache miss
- deferred: The deferred which resolves to the result.
- opentracing_span_context: An opentracing span wrapping the calculation
- Returns:
- The cache entry object.
- """
- result = ObservableDeferred(deferred, consumeErrors=True)
- key = context.cache_key
- entry = ResponseCacheEntry(result, opentracing_span_context)
- self._result_cache[key] = entry
- def on_complete(r: RV) -> RV:
- # 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:
- self.clock.call_later(self.timeout_sec, self._entry_timeout, key)
- else:
- # otherwise, remove the result immediately.
- self.unset(key)
- return r
- # make sure we do this *after* adding the entry to 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 entry
- def unset(self, key: KV) -> None:
- """Remove the cached value for this key from the cache, if any.
- Args:
- key: key used to remove the cached value
- """
- self._metrics.inc_evictions(EvictionReason.invalidation)
- self._result_cache.pop(key, None)
- def _entry_timeout(self, key: KV) -> None:
- """For the call_later to remove from the cache"""
- self._metrics.inc_evictions(EvictionReason.time)
- self._result_cache.pop(key, None)
- async def wrap(
- self,
- key: KV,
- callback: Callable[..., Awaitable[RV]],
- *args: Any,
- cache_context: bool = False,
- **kwargs: Any,
- ) -> RV:
- """Wrap together a *get* and *set* call, taking care of logcontexts
- First looks up the key in the cache, and if it is present makes it
- follow the synapse logcontext rules and returns it.
- Otherwise, 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:
- key: key to get/set in the cache
- callback: function to call if the key is not found in
- the cache
- *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)
- """
- entry = self._get(key)
- if not entry:
- logger.debug(
- "[%s]: no cached result for [%s], calculating new one", self._name, key
- )
- context = ResponseCacheContext(cache_key=key)
- if cache_context:
- kwargs["cache_context"] = context
- span_context: Optional[opentracing.SpanContext] = None
- async def cb() -> RV:
- # NB it is important that we do not `await` before setting span_context!
- nonlocal span_context
- with start_active_span(f"ResponseCache[{self._name}].calculate"):
- span = active_span()
- if span:
- span_context = span.context
- return await callback(*args, **kwargs)
- d = run_in_background(cb)
- entry = self._set(context, d, span_context)
- return await make_deferred_yieldable(entry.result.observe())
- result = entry.result.observe()
- if result.called:
- logger.info("[%s]: using completed cached result for [%s]", self._name, key)
- else:
- logger.info(
- "[%s]: using incomplete cached result for [%s]", self._name, key
- )
- span_context = entry.opentracing_span_context
- with start_active_span_follows_from(
- f"ResponseCache[{self._name}].wait",
- contexts=(span_context,) if span_context else (),
- ):
- return await make_deferred_yieldable(result)
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