route/vendor/github.com/dgryski/go-failure/failure.go

74 lines
1.8 KiB
Go
Raw Normal View History

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