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Add some docstring explaining the snapshot cache does

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This commit is contained in:
Mark Haines 2015-12-23 15:18:11 +00:00
parent 7fa71e3267
commit d12c00bdc3
1 changed files with 23 additions and 1 deletions
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24
synapse/util/caches/snapshot_cache.py
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@ -17,8 +17,28 @@ from synapse.util.async import ObservableDeferred
class SnapshotCache(object):
"""Cache for snapshots like the response of /initialSync.
The response of initialSync only has to be a recent snapshot of the
server state. It shouldn't matter to clients if it is a few minutes out
of date.
DURATION_MS = 5 * 60 * 1000 # Cache results for 2 minutes.
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.
Once the deferred completes it will removed from the cache after 5 minutes.
We delay removing it from the cache because a client retrying its request
could race with us finishing computing the response.
Rather than tracking precisely how long something has been in the cache we
keep two generations of completed responses. Every 5 minutes discard the
old generation, move the new generation to the old generation, and set the
new generation to be empty. This means that a result will be in the cache
somewhere between 5 and 10 minutes.
"""
DURATION_MS = 5 * 60 * 1000 # Cache results for 5 minutes.
def __init__(self):
self.pending_result_cache = {} # Request that haven't finished yet.
@ -51,6 +71,8 @@ class SnapshotCache(object):
result = self.pending_result_cache.get(key, result)
if result is not None:
return result.observe()
else:
return None
def set(self, time_now_ms, key, deferred):
self.rotate(time_now_ms)