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// Based on http://bit.ly/215MBZ
function FailureDetector(ws) {
// ws stands for window size.
// How many heartbeat intervals we keep track of.
this.ws = ws || 100;
this.window = [];
this.last = null;
}
exports.FailureDetector = FailureDetector;
// Call this every time a heartbeat is received.
FailureDetector.prototype.report = function(when) {
if (!when) when = Date.now();
if (this.last) {
var interval = when - this.last;
this.window.push(interval);
if (this.window.length > this.ws) {
this.window.shift();
}
}
this.last = when;
if (this.window.length) {
// calculate mean and variance of intervals in our window.
// This could be done more efficiently. Whatever.
var sum = 0;
for (var i = 0; i < this.window.length; i++) {
sum += this.window[i];
}
this.mean = sum / this.window.length;
sum = 0;
for (var i = 0; i < this.window.length; i++) {
var x = this.window[i] - this.mean;
sum += x * x;
}
this.variance = sum / this.window.length;
}
};
function log10(x) {
return Math.log(x) / Math.LN10;
}
// Use this function to determine failure.
// Using 1 as a threshold means the likeliness of mistaken failure is 10%
// Using 2 as a threshold means the likeliness of mistaken failure is 1%
// Using 3 as a threshold means the likeliness of mistaken failure is 0.1%
FailureDetector.prototype.phi = function(when) {
if (!this.last) return 0;
var t = (when || Date.now()) - this.last;
// Took this approximation from casandra. Don't know where they got it.
// suspect.
var probability = Math.pow(Math.E, -1 * t / this.mean);
var log = (-1) * log10(probability);
return log;
};
var FailureDetector = require('./failure-detector').FailureDetector;
var assert = require('assert');
var d = new FailureDetector();
for (var i = 0; i < 100; i ++ ) {
d.report(Date.now() + i * 1000);
}
// mean should be almost exactly one second
console.log("mean = ", d.mean);
assert.ok(999 <= d.mean && d.mean <= 1001);
// should be pretty certain everything is fine.
console.log("phi = ", d.phi());
assert.ok(d.phi() <= 0.01);
// test stolen from casandra
var d = new FailureDetector(4);
d.report(111);
d.report(222);
d.report(333);
d.report(444);
d.report(555);
var expected = 0.4342;
var actual = d.phi(666);
console.log("mean = ", d.mean);
console.log("phi = ", actual);
assert.ok(expected - 0.01 <= actual && actual <= expected + 0.01);
//oh noes, a much higher timestamp, something went wrong!
expected = 9.566;
actual = d.phi(3000);
console.log("mean = ", d.mean);
console.log("phi = ", actual);
assert.ok(expected - 0.01 <= actual && actual <= expected + 0.01);
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