Memory efficient message chunk processing using a

I have a XMLHttpRequest with a progress event handler that is requesting a chunked page which continuously sends adds message chunks. If I do not set a responseType, I can access the response property of the XMLHttpRequest in each progress event and handle the additional message chunk. The problem of this approach is that the browser must keep the entire response in memory, and eventually, the browser will crash due to this memory waste.

So, I tried a responseType of arraybuffer in the hope that I can slice the buffer to prevent the previous excessive memory waste. Unfortunately, the progress event handler is no longer capable of reading the response property of the XMLHttpRequest at this point. The event parameter of the progress event does not contain the buffer, either. Here is a short, self-contained example of my attempt at this (this is written for node.js):

var http = require('http');

// -- The server.

http.createServer(function(req, res) {
  if (req.url === '/stream') return serverStream(res);
  serverMain(res);
}).listen(3000);

// -- The server functions to send a HTML page with the client code, or a stream.

function serverMain(res) {
  res.writeHead(200, {'Content-Type': 'text/html'});
  res.write('<html><body>Hello World</body><script>');
  res.end(client.toString() + ';client();</script></html>');
}

function serverStream(res) {
  res.writeHead(200, {'Content-Type': 'text/html'});
  setInterval(function() {
    res.write('Hello World<br />\n');
  }, 1000);
}

// -- The client code which runs in the browser.

function client() {
  var xhr = new XMLHttpRequest();
  xhr.addEventListener('progress', function() {
    if (!xhr.response) return console.log('progress without response :-(');
    console.log('progress: ' + xhr.response.size);
  }, false);
  xhr.open('GET', '/stream', true);
  xhr.responseType = 'arraybuffer';
  xhr.send();
}

The progress event handler has no access to the response I wanted. How can I handle the message chunks in the browser in a memory-efficient way? Please do not suggest a WebSocket. I do not wish to use one just to process a read-only stream of message chunks.

XMLHttpRequest doesn't seem really designed for this kind of usage. The obvious solution is polling, which is a popular use of XMLHttpRequest but I'm guessing you don't want to miss data from your stream that would slip between the calls.

To my question Can the "real" data chunks be identified in some way or is it basically random data ?, you answered With some effort, the chunks could be identified by adding an event-id of sorts to the server-side

Based on this premise, I propose:

The idea: cooperating concurrent listeners

Connect to the stream and set up the progress listener (referred to as listenerA()).
When a chunk arrives, process it and output it. Keep a reference to the ids of both the first and last chunk received by listenerA(). Count how many chunks listenerA() has received.
After listenerA() has received a certain amount of chunks, spawn another "thread" (connection + listener, listenerB()) doing the steps 1 and 2 in parallel to the first one but keep the processed data in a buffer instead of outputting it.
When listenerA() receives the chunk with the same id as the first chunk received by listenerB(), send a signal to listenerB(), drop the first connection and kill listenerA().
When listenerB() receives the termination signal from the listenerA(), dump the buffer to the output and keep processing normally.
Have listenerB() spawn listenerC() on the same conditions as before.
Keep repeating with as many connections + listeners as necessary.

By using two overlapping connections, you can prevent the possible loss of chunks that would result from dropping a single connection and then reconnecting.

Notes

This assumes the data stream is the same for all connections and doesn't introduce some individualized settings.
Depending on the output rate of the stream and the connection delay, the buffer dump during the transition from one connection to another might be noticeable.
You could also measure the total response size rather than the chunks count to decide when to switch to a new connection.
It might be necessary to keep a complete list of chunks ids to compare against rather than just the first and last one because we can't guarantee the timing of the overlap.
The responseType of XMLHttpRequest must be set to its default value of "" or "text", to return text. Other datatypes will not return a partial response. See https://xhr.spec.whatwg.org/#the-response-attribute

Test server in node.js

The following code is a node.js server that outputs a consistent stream of elements for testing purposes. You can open multiple connections to it, the output will be the same accross sessions, minus possible server lag.

http://localhost:5500/stream

will return data where id is an incremented number

http://localhost:5500/streamRandom

will return data where id is a random 40 characters long string. This is meant to test a scenario where the id can not be relied upon for ordering the data.

var crypto = require('crypto');

// init + update nodeId
var nodeId     = 0;
var nodeIdRand = '0000000000000000000000000000000000000000';

setInterval(function() {

    // regular id
    ++nodeId;

    //random id
    nodeIdRand = crypto.createHash('sha1').update(nodeId.toString()).digest('hex');
}, 1000);


// create server  (port 5500)
var http = require('http');
http.createServer(function(req, res) {

  if(req.url === '/stream') {
      return serverStream(res);
  }
  else if(req.url === '/streamRandom') {
      return serverStream(res, true);
  }
}).listen(5500);


// serve nodeId
function serverStream(res, rand) {

    // headers
    res.writeHead(200, {
        'Content-Type'                : 'text/plain',
        'Access-Control-Allow-Origin' : '*',
    });

    // remember last served id
    var last = null;

    // output interval
    setInterval(function() {

        // output on new node
        if(last != nodeId) {
            res.write('[node id="'+(rand ? nodeIdRand : nodeId)+'"]');
            last = nodeId;
        }
    }, 250);
}

Proof of concept, using aforementioned node.js server code

<!DOCTYPE html>
<html>
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
    </head>
    <body>
        <button id="stop">stop</button>
        <div id="output"></div>
        <script>

/*
Listening to a never ending page load (http stream) without running out of
memory by using concurrent overlapping connections to prevent loss of data,
using only xmlHttpRequest, under the condition that the data can be identified.

listen arguments
    url         url of the http stream
    chunkMax    number of chunks to receive before switching to new connection

listen properties
    output           a reference to a DOM element with id "output"
    queue            an array filled with non-duplicate received chunks and metadata
    lastFetcherId    an incrementing number used to assign an id to new fetchers
    fetchers         an array listing all active fetchers

listen methods
    fire        internal use    fire an event
    stop        external use    stop all connections
    fetch       internal use    starts a new connection
    fetchRun    internal use    initialize a new fetcher object

Usage

    var myListen = new listen('http://localhost:5500/streamRandom', 20);
        will listen to url "http://localhost:5500/streamRandom"
        will switch connections every 20 chunks

    myListen.stop()
        will stop all connections in myListen
*/
function listen(url, chunkMax) {

    // main ref
    var that = this;

    // output element
    that.output = document.getElementById('output');

    // main queue
    that.queue = [];

    // last fetcher id
    that.lastFetcherId = 0;

    // list of fetchers
    that.fetchers = [];




    //********************************************************* event dispatcher
    that.fire = function(name, data) {
        document.dispatchEvent(new CustomEvent(name, {'detail':data}));
    }




    //******************************************************** kill all fetchers
    that.stop = function() {
        that.fire('fetch-kill', -1);
    }




    //************************************************************** url fetcher
    that.fetch = function(fetchId, url, fetchRef) {

        //console.log('start fetcher #'+fetchId);
        var len = 0;
        var xhr = new XMLHttpRequest();
        var cb_progress;
        var cb_kill;


        // progress listener
        xhr.addEventListener('progress', cb_progress = function(e) {

            // extract chunk data
            var chunkData = xhr.response.substr(len);

            // chunk id
            var chunkId = chunkData.match(/id="([a-z0-9]+)"/)[1];

            // update response end point
            len = xhr.response.length;

            // signal end of chunk processing
            that.fire('chunk-ready', {
                'fetchId'   : fetchId,
                'fetchRef'  : fetchRef,
                'chunkId'   : chunkId,
                'chunkData' : chunkData,
            });
        }, false);


        // kill switch
        document.addEventListener('fetch-kill', cb_kill = function(e) {

            // kill this fetcher or all fetchers (-1)
            if(e.detail == fetchId || e.detail == -1) {

                //console.log('kill fetcher #'+fetchId);

                xhr.removeEventListener('progress', cb_progress);
                document.removeEventListener('fetch-kill', cb_kill);

                xhr.abort();
                that.fetchers.shift(); // remove oldest fetcher from list
                xhr = null;
                delete xhr;
            }
        }, false);


        // go
        xhr.open('GET', url, true);
        xhr.responseType = 'text';
        xhr.send();
    };




    //****************************************************** start a new fetcher
    that.fetchRun = function() {

        // new id
        var id = ++that.lastFetcherId;

        //console.log('create fetcher #'+id);

        // create fetcher with new id
        var fetchRef = {
            'id'           : id,    // self id
            'queue'        : [],    // internal queue
            'chunksIds'    : [],    // retrieved ids, also used to count
            'hasSuccessor' : false, // keep track of next fetcher spawn
            'ignoreId'     : null,  // when set, ignore chunks until this id is received (this id included)
        };
        that.fetchers.push(fetchRef);

        // run fetcher
        that.fetch(id, url, fetchRef);
    };




    //************************************************ a fetcher returns a chunk
    document.addEventListener('chunk-ready', function(e) {

        // shorthand
        var f = e.detail;

        // ignore flag is not set, process chunk
        if(f.fetchRef.ignoreId == null) {

            // store chunk id
            f.fetchRef.chunksIds.push(f.chunkId);

            // create queue item
            var queueItem = {'id':f.chunkId, 'data':f.chunkData};

            // chunk is received from oldest fetcher
            if(f.fetchId == that.fetchers[0].id) {

                // send to main queue
                that.queue.push(queueItem);

                // signal queue insertion
                that.fire('queue-new');
            }
            // not oldest fetcher
            else {

                // use fetcher internal queue
                f.fetchRef.queue.push(queueItem);
            }
        }
        // ignore flag is set, current chunk id the one to ignore
        else if(f.fetchRef.ignoreId == f.chunkId) {

            // disable ignore flag
            f.fetchRef.ignoreId = null;
        }







        //******************** check chunks count for fetcher, threshold reached
        if(f.fetchRef.chunksIds.length >= chunkMax && !f.fetchRef.hasSuccessor) {

            // remember the spawn
            f.fetchRef.hasSuccessor = true;

            // spawn new fetcher
            that.fetchRun();
        }




        /***********************************************************************
        check if the first chunk of the second oldest fetcher exists in the
        oldest fetcher.
        If true, then they overlap and we can kill the oldest fetcher
        ***********************************************************************/
        if(
            // is this the oldest fetcher ?
            f.fetchId == that.fetchers[0].id
            // is there a successor ?
            && that.fetchers[1]
            // has oldest fetcher received the first chunk of its successor ?
            && that.fetchers[0].chunksIds.indexOf(
                that.fetchers[1].chunksIds[0]
            ) > -1
        ) {

            // get index of last chunk of the oldest fetcher within successor queue
            var lastChunkId    = that.fetchers[0].chunksIds[that.fetchers[0].chunksIds.length-1]
            var lastChunkIndex = that.fetchers[1].chunksIds.indexOf(lastChunkId);

            // successor has not reached its parent last chunk
            if(lastChunkIndex < 0) {

                // discard whole queue
                that.fetchers[1].queue     = [];
                that.fetchers[1].chunksIds = [];

                // set ignore id in successor to future discard duplicates
                that.fetchers[1].ignoreId = lastChunkId;
            }
            // there is overlap
            else {

                /**
                console.log('triming queue start: '+that.fetchers[1].queue.length
                    +"   "+(lastChunkIndex+1)
                    +"   "+(that.fetchers[1].queue.length-1)
                );
                /**/
                var trimStart = lastChunkIndex+1;
                var trimEnd   = that.fetchers[1].queue.length-1;

                // trim queue
                that.fetchers[1].queue = that.fetchers[1].queue.splice(trimStart, trimEnd);
                that.fetchers[1].chunksIds = that.fetchers[1].chunksIds.splice(trimStart, trimEnd);

                //console.log('triming queue end: '+that.fetchers[1].queue.length);
            }

            // kill oldest fetcher
            that.fire('fetch-kill', that.fetchers[0].id);
        }





    }, false);




    //***************************************************** main queue processor
    document.addEventListener('queue-new', function(e) {

        // process chunks in queue
        while(that.queue.length > 0) {

            // get chunk and remove from queue
            var chunk = that.queue.shift();

            // output item to document
            if(that.output) {
                that.output.innerHTML += "<br />"+chunk.data;
            }
        }
    }, false);



    //****************************************************** start first fetcher
    that.fetchRun();
};


// run
var process = new listen('http://localhost:5500/streamRandom', 20);

// bind global kill switch to button
document.getElementById('stop').addEventListener('click', process.stop, false);

        </script>
    </body>
</html>