window-reduce

circular-buffer-reduce

apply a reduce function to a circular buffer.

I created this for performance monitoring, I need to make performance measurements, but do not require high precision measurements to be stored forever. For example, I want to keep a measure of bandwidth used per second, but I don't really need to track the bandwidth per second for the entire time the app has been running - the last 60 seconds is enough! It is useful to know the history of performance, but instead we could track the averages over the last hour at the precision of minutes, not seconds, and then the past few days at the precision of hours!

This means we can measure performance characteristics over a wide range of time spans without using much resources at all. This means we can add performance monitoring to our app without adversely effecting performance.

Example

var stats = require('stats/mutable')
function id (item) { return item.ts }
function reduce (acc, item) {
  return stats(acc, item.value)
}
var circular_reduce = require('circular-buffer-reduce')(id, reduce, 1000, 60)


var acc
//whenever something happens, apply the reduce function.
acc = circular_reduce(acc, {ts: Date.now(), value: Math.random()})

api - createCircularReduce(getTs, reduce, width, max) => circular_reduce

createCircularReduce returns a reduce function that aggregates incoming items into max aggregations that are each width units wide. If you want to track the last minute worth of seconds, and each item has a javascript timestamp (in milliseconds) then width should be 1000, and max should be 60.

getTs is a function that returns the timestamp for the current item passed to circular_reduce. reduce is a standard reduce function that takes the current accumulator and the data item. since there is no option for a initial item, reduce must accept reduce(null, item) as the first item in a new accumulation.