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c3e38b88f2
This adds some opentracing annotations to ResponseCache, to make it easier to see what's going on; in particular, it adds a link back to the initial trace which is actually doing the work of generating the response.
266 lines
8.6 KiB
Python
266 lines
8.6 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 (
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TYPE_CHECKING,
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Any,
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Awaitable,
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Callable,
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Dict,
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Generic,
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Iterable,
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Optional,
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TypeVar,
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)
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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.logging.opentracing import (
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active_span,
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start_active_span,
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start_active_span_follows_from,
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)
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from synapse.util import Clock
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from synapse.util.async_helpers import AbstractObservableDeferred, ObservableDeferred
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from synapse.util.caches import register_cache
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logger = logging.getLogger(__name__)
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if TYPE_CHECKING:
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import opentracing
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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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@attr.s(auto_attribs=True)
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class ResponseCacheEntry:
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result: AbstractObservableDeferred
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"""The (possibly incomplete) result of the operation.
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Note that we continue to store an ObservableDeferred even after the operation
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completes (rather than switching to an immediate value), since that makes it
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easier to cache Failure results.
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"""
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opentracing_span_context: "Optional[opentracing.SpanContext]"
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"""The opentracing span which generated/is generating the result"""
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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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self._result_cache: Dict[KV, ResponseCacheEntry] = {}
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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._result_cache)
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def __len__(self) -> int:
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return self.size()
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def keys(self) -> Iterable[KV]:
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"""Get the keys currently in the result cache
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Returns both incomplete entries, and (if the timeout on this cache is non-zero),
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complete entries which are still in the cache.
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Note that the returned iterator is not safe in the face of concurrent execution:
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behaviour is undefined if `wrap` is called during iteration.
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"""
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return self._result_cache.keys()
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def _get(self, key: KV) -> Optional[ResponseCacheEntry]:
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"""Look up the given key.
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Args:
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key: key to get in the cache
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Returns:
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The entry for this key, if any; else None.
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"""
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entry = self._result_cache.get(key)
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if entry is not None:
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self._metrics.inc_hits()
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return entry
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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,
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context: ResponseCacheContext[KV],
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deferred: "defer.Deferred[RV]",
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opentracing_span_context: "Optional[opentracing.SpanContext]",
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) -> ResponseCacheEntry:
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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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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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opentracing_span_context: An opentracing span wrapping the calculation
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Returns:
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The cache entry object.
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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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entry = ResponseCacheEntry(result, opentracing_span_context)
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self._result_cache[key] = entry
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def on_complete(r: RV) -> RV:
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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._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._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 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 entry
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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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entry = self._get(key)
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if not entry:
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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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span_context: Optional[opentracing.SpanContext] = None
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async def cb() -> RV:
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# NB it is important that we do not `await` before setting span_context!
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nonlocal span_context
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with start_active_span(f"ResponseCache[{self._name}].calculate"):
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span = active_span()
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if span:
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span_context = span.context
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return await callback(*args, **kwargs)
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d = run_in_background(cb)
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entry = self._set(context, d, span_context)
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return await make_deferred_yieldable(entry.result.observe())
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result = entry.result.observe()
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if 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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span_context = entry.opentracing_span_context
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with start_active_span_follows_from(
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f"ResponseCache[{self._name}].wait",
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contexts=(span_context,) if span_context else (),
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):
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return await make_deferred_yieldable(result)
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