94 lines
3.9 KiB
Python
94 lines
3.9 KiB
Python
# -*- coding: utf-8 -*-
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# Copyright 2015, 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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from synapse.util.async import ObservableDeferred
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class SnapshotCache(object):
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"""Cache for snapshots like the response of /initialSync.
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The response of initialSync only has to be a recent snapshot of the
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server state. It shouldn't matter to clients if it is a few minutes out
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of date.
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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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Once the deferred completes it will removed from the cache after 5 minutes.
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We delay removing it from the cache because a client retrying its request
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could race with us finishing computing the response.
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Rather than tracking precisely how long something has been in the cache we
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keep two generations of completed responses. Every 5 minutes discard the
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old generation, move the new generation to the old generation, and set the
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new generation to be empty. This means that a result will be in the cache
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somewhere between 5 and 10 minutes.
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"""
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DURATION_MS = 5 * 60 * 1000 # Cache results for 5 minutes.
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def __init__(self):
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self.pending_result_cache = {} # Request that haven't finished yet.
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self.prev_result_cache = {} # The older requests that have finished.
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self.next_result_cache = {} # The newer requests that have finished.
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self.time_last_rotated_ms = 0
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def rotate(self, time_now_ms):
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# Rotate once if the cache duration has passed since the last rotation.
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if time_now_ms - self.time_last_rotated_ms >= self.DURATION_MS:
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self.prev_result_cache = self.next_result_cache
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self.next_result_cache = {}
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self.time_last_rotated_ms += self.DURATION_MS
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# Rotate again if the cache duration has passed twice since the last
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# rotation.
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if time_now_ms - self.time_last_rotated_ms >= self.DURATION_MS:
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self.prev_result_cache = self.next_result_cache
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self.next_result_cache = {}
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self.time_last_rotated_ms = time_now_ms
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def get(self, time_now_ms, key):
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self.rotate(time_now_ms)
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# This cache is intended to deduplicate requests, so we expect it to be
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# missed most of the time. So we just lookup the key in all of the
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# dictionaries rather than trying to short circuit the lookup if the
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# key is found.
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result = self.prev_result_cache.get(key)
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result = self.next_result_cache.get(key, result)
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result = self.pending_result_cache.get(key, result)
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if result is not None:
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return result.observe()
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else:
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return None
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def set(self, time_now_ms, key, deferred):
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self.rotate(time_now_ms)
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result = ObservableDeferred(deferred)
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self.pending_result_cache[key] = result
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def shuffle_along(r):
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# When the deferred completes we shuffle it along to the first
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# generation of the result cache. So that it will eventually
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# expire from the rotation of that cache.
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self.next_result_cache[key] = result
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self.pending_result_cache.pop(key, None)
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result.observe().addBoth(shuffle_along)
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return result.observe()
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