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import random
class ExponentialBackoff:
"""
Keeps track of a backoff delay that staggers down and staggers up
exponentially.
"""
def __init__(self, start_delay, max_delay):
self._error_count = 0 # A tally of server errors
self._start_delay = start_delay
self._max_delay = max_delay
def decrease(self):
"""Decreases error count with zero being the lowest"""
self._error_count -= 1
self._error_count = max(self._error_count, 0)
def increase(self):
"""Increases error count but not higher than gives the max delay"""
if self.get_delay() < self._max_delay:
self._error_count += 1
def get_delay(self):
"""
Calculates the delay using formula that gives this chart. In this
specific example start is 5 seconds and max is 60 seconds
Count Delay
0 0
1 5
2 10
3 20
4 40
5 60 (max)
"""
if self._error_count:
delay = (2 ** (self._error_count - 1)) * self._start_delay
else:
delay = 0
return min(int(delay), self._max_delay)
def get_random_delay(self, stagger_fraction=0.25):
"""
Adds randomness to the specified stagger of the delay. For example
for a delay of 12 and 25% stagger, it works out to 9 + rand(0,3)
"""
delay = self.get_delay()
non_random_part = delay * (1-stagger_fraction)
random_part = delay * stagger_fraction * random.random()
return int(non_random_part + random_part)