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78 lines
2.0 KiB
78 lines
2.0 KiB
"Stampede barrier implementation."
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import functools as ft
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import math
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import random
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import tempfile
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import time
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from .core import Cache, ENOVAL
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class StampedeBarrier(object):
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"""Stampede barrier mitigates cache stampedes.
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Cache stampedes are also known as dog-piling, cache miss storm, cache
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choking, or the thundering herd problem.
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Based on research by Vattani, A.; Chierichetti, F.; Lowenstein, K. (2015),
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Optimal Probabilistic Cache Stampede Prevention,
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VLDB, pp. 886?897, ISSN 2150-8097
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Example:
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```python
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stampede_barrier = StampedeBarrier('/tmp/user_data', expire=3)
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@stampede_barrier
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def load_user_info(user_id):
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return database.lookup_user_info_by_id(user_id)
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```
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"""
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# pylint: disable=too-few-public-methods
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def __init__(self, cache=None, expire=None):
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if isinstance(cache, Cache):
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pass
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elif cache is None:
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cache = Cache(tempfile.mkdtemp())
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else:
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cache = Cache(cache)
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self._cache = cache
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self._expire = expire
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def __call__(self, func):
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cache = self._cache
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expire = self._expire
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@ft.wraps(func)
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def wrapper(*args, **kwargs):
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"Wrapper function to cache function result."
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key = (args, kwargs)
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try:
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result, expire_time, delta = cache.get(
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key, default=ENOVAL, expire_time=True, tag=True
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)
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if result is ENOVAL:
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raise KeyError
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now = time.time()
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ttl = expire_time - now
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if (-delta * math.log(random.random())) < ttl:
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return result
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except KeyError:
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pass
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now = time.time()
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result = func(*args, **kwargs)
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delta = time.time() - now
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cache.set(key, result, expire=expire, tag=delta)
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return result
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return wrapper
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