throttle-stream.js

'use strict';

var Transform = require('stream').Transform,
    inherits = require('util').inherits;

module.exports = ThrottleStream;

function ThrottleStream(opts) {
    if (!(this instanceof ThrottleStream))
        return new ThrottleStream(opts);
    
    opts = opts || { };
    
    Transform.call(this, opts);

    this.penalty = (typeof opts.penalty === 'number' ? opts.penalty : 2) * 1000;
    this.burst = ((typeof opts.burst === 'number' ? opts.burst : 5) - 1) * this.penalty;
    this.time = 0;
}
inherits(ThrottleStream, Transform);

ThrottleStream.prototype._transform = function (chunk, encoding, cb) {
    var now = Date.now(),
        dly = this.time - now - this.burst;

    if (dly > 0) {
        setTimeout(this._transform.bind(this, chunk, encoding, cb), dly);
    } else {
        this.time = Math.max(this.time, now) + this.penalty;
        cb(null, chunk);
    }
};

If you have a rate in lines per second, set the penalty to seconds/(lines-burst). Ex: 20 lines in 30 seconds: penalty should be 30/15 or 0.5. This will result in the 20th chunk being emitted exactly 30 seconds from the 1st chunk even though the first five chunks were sent instantly. Subsequent chunks after 20 will actually come out slower, but if you give it a rest to 'catch its breath' it will come back towards 30 again.