74 lines
1.8 KiB
Go
74 lines
1.8 KiB
Go
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// Package failure implements the Phi Accrual Failure Detector
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/*
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This package implements the paper "The φ Accrual Failure Detector" (2004)
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(available at http://hdl.handle.net/10119/4784).
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To use the failure detection algorithm, you need a heartbeat loop that will
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call Ping() at regular intervals. At any point, you can call Phi() which will
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report how suspicious it is that a heartbeat has not been heard since the last
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time Ping() was called.
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*/
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package failure
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import (
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"math"
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"sync"
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"time"
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"github.com/dgryski/go-onlinestats"
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)
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// Detector is a failure detector
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type Detector struct {
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w *onlinestats.Windowed
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last time.Time
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minSamples int
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mu sync.Mutex
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}
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// New returns a new failure detector that considers the last windowSize
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// samples, and ensures there are at least minSamples in the window before
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// returning an answer
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func New(windowSize, minSamples int) *Detector {
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d := &Detector{
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w: onlinestats.NewWindowed(windowSize),
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minSamples: minSamples,
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}
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return d
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}
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// Ping registers a heart-beat at time now
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func (d *Detector) Ping(now time.Time) {
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d.mu.Lock()
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defer d.mu.Unlock()
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if !d.last.IsZero() {
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d.w.Push(now.Sub(d.last).Seconds())
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}
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d.last = now
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}
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// Phi calculates the suspicion level at time 'now' that the remote end has failed
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func (d *Detector) Phi(now time.Time) float64 {
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d.mu.Lock()
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defer d.mu.Unlock()
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if d.w.Len() < d.minSamples {
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return 0
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}
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t := now.Sub(d.last).Seconds()
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pLater := 1 - cdf(d.w.Mean(), d.w.Stddev(), t)
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phi := -math.Log10(pLater)
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return phi
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}
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// cdf is the cumulative distribution function of a normally distributed random
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// variable with the given mean and standard deviation
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func cdf(mean, stddev, x float64) float64 {
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return 0.5 + 0.5*math.Erf((x-mean)/(stddev*math.Sqrt2))
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}
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