ewlr

Stability: 1 - Experimental

Exponentially Weighted Loss Rate.

Contents

• Installation
• Overview
• Usage
• Tests
• Documentation
  • EWLR

Installation

npm install ewlr

Overview

EWLR is an Exponentially Weighted Loss Rate calculation module to estimate event loss rate.

Usage

To run the below example run:

npm run readme

"use strict";

var EWLR = require('../index.js');

// keep track of last tick and tick interval
var lastTick = EWLR.getTime();
var tickInterval = 1000; // 1 second (in milliseconds)

var ewlr = new EWLR();

// update with success
ewlr.update();

// update with failure
ewlr.update(1, 1);

// update with 20% loss rate
ewlr.update(1000, 200);

// or a less precise 20% loss rate
for (var i = 0; i < 1000; i++) {
    ewlr.update(1, (Math.random() < 0.2 ? 1 : 0));
}

// tick the required number of times in order to get an accurate loss rate
var shouldTick = EWLR.tickIfNecessary(lastTick, tickInterval);
if (shouldTick) {
    lastTick = shouldTick.newLastTick;
    for (var i = 0; i < shouldTick.requiredTicks; i++) {
        ewlr.tick();
    }
}

console.dir(ewlr.rate()); // may be 0 if we haven't tick()'ed yet

setTimeout(function () {
    shouldTick = EWLR.tickIfNecessary(lastTick, tickInterval);
    if (shouldTick) {
        lastTick = shouldTick.newLastTick;
        for (var i = 0; i < shouldTick.requiredTicks; i++) {
            ewlr.tick();
        }
    }

    console.dir(ewlr.rate());
}, 3000); // will get 3 ticks if we wait 3 seconds

Tests

npm test

Documentation

• EWLR

EWLR

Public API

• EWLR.getTime()
• EWLR.tickIfNecessary(lastTick, tickInterval)
• new EWLR(config)
• ewlr.rate()
• ewlr.tick()
• ewlr.update([n], [lost])

EWLR.getTime()

• Return: Number Time in milliseconds (for example: 109.372263)

EWLR.tickIfNecessary(lastTick, tickInterval)

• lastTick: Number Time of last tick in milliseconds.
• tickInterval: Number Tick interval in milliseconds.
• Return: undefined If no tick necessary.
• Return: Object If tick is necessary
  • lastTick: Number Time of new last tick to set in milliseconds
  • requiredTicks: Number The number of ticks to execute to catch up.

Determines if and how many ticks are necessary.

new EWLR(config)

• config: Object
  • timePeriod: Number (Default: 1000) Time period in milliseconds.
  • tickInterval: Number (Default: 1000) Tick inteval in milliseconds.

Creates a new EWLR instance.

ewlr.rate()

• Return: Object
  • rate: Number Rate of events. Rate is returned so that it can be determined if lossRate is meaningful. For example, if rate is 2.504763981949714e-32, the loss rate (even if 20%), may not be meaningful since the tiny rate may mean that no updates have happened for a long time.
  • lossRate: Number Normalized ([0..1]) loss rate of events
  • lossRate: Number Normalized ([0..1]) loss rate of events

Returns the event rate and the normalized loss rate. The rate is only meant as a liveness/recency measure to see if lossRate is meaningful.

ewlr.tick()

Update the rate and loss rate estimates in accordance with time period and tick interval.

ewlr.update([n], [lost])

• n: Integer (Default: 1) Number of events to update with.
• lost: Integer (Default: 0) Number of lost events to update with (0 < lost <= n)