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Add some docstring explaining the snapshot cache does
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@ -17,8 +17,28 @@ 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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DURATION_MS = 5 * 60 * 1000 # Cache results for 2 minutes.
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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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@ -51,6 +71,8 @@ class SnapshotCache(object):
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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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