asenchi / afd.py

Created May 16, 2011


    
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            import math
import time
 
__all__ = ['AccrualFailureDetector']
 
class AccrualFailureDetector(object):
 
    SAMPLE_SIZE = 1000
 
    def __init__(self):
        self.last_time = None
        self.intervals = []
 
    def add(self, arrival=None):
        if not arrival:
            raise AttributeError("You must supply an arrival time")
        else:
            arrival = int(arrival)
 
        if not self.last_time:
            # Set arbitrary interval, allow suspicion to be lowered by peer.
            interval = 500
        else:
            interval = arrival - self.last_time
 
        self.last_time = arrival
        self.intervals.append(interval)
        if len(self.intervals) > self.SAMPLE_SIZE:
            self.intervals.pop(0)
 
    def phi(self, current_time=None):
        if not current_time:
            current_time = int(time.time())
 
        current_interval = current_time - self.last_time
        exp = -1.0 * current_interval / self._interval_mean()
        prob = 1 - (1.0 - math.pow(math.e, exp))
 
        return -1 * math.log10(prob)
 
    def _interval_mean(self):
        return sum(self.intervals) / len(self.intervals)