Why is this node.js code blocking?

Question

I tried to play a bit with node.js and wrote following code (it doesn't make sense, but that does not matter):

var http = require("http"),
    sys  = require("sys");

sys.puts("Starting...");

var gRes = null;
var cnt = 0;

var srv = http.createServer(function(req, res){
    res.writeHeader(200, {"Content-Type": "text/plain"});
    gRes = res;
    setTimeout(output,1000);
    cnt = 0;
}).listen(81);

function output(){
    gRes.write("Hello World!");
    cnt++;
    if(cnt < 10)
        setTimeout(output,1000);
    else
        gRes.end();
}

I know that there are some bad things in it (like using gRes globally), but my question is, why this code is blocking a second request until the first completed?

if I open the url it starts writing "Hello World" 10 times. But if I open it simultaneous in a second tab, one tab waits connecting until the other tab finished writing "Hello World" ten times.

I found nothing which could explain this behaviour.

Try opening from two different browsers maybe the server handles requests from the same session one at a time. — user447356
– user447356, Commented Jul 21, 2011 at 8:05
hm strange. if I open it from two browsers it works simultaneous but is overwriting gRes to the second page call, but thats ok (i have seen this problem before). But why isn't it working for two tabs from one browser? — Fender
– Fender, Commented Jul 21, 2011 at 8:07
Dunno why as I'm not familiar with node.js however as it proved to be correct, posted it as answer. :) — user447356
– user447356, Commented Jul 21, 2011 at 8:17

Alnitak · Accepted Answer · 2011-07-21 08:24:35Z

5

Surely it's your overwriting of the gRes and cnt variables being used by the first request that's doing it?

[EDIT actually, Chrome won't send two at once, as Shadow Wizard said, but the code as is is seriously broken because each new request will reset the counter, and outstanding requests will never get closed].

Instead of using a global, wrap your output function as a closure within the createServer callback. Then it'll have access to the local res variable at all times.

This code works for me:

var http = require("http"),
    sys  = require("sys");

sys.puts("Starting...");

var srv = http.createServer(function(req, res){
    res.writeHeader(200, {"Content-Type": "text/plain"});

    var cnt = 0;

    var output = function() {
        res.write("Hello World!\n");
        if (++cnt < 10) {
            setTimeout(output,1000);
        } else {
            res.end();
        }
    };

    output();

}).listen(81);

Note however that the browser won't render anything until the connection has closed because the relevant headers that tell it to display as it's downloading aren't there. I tested the above using telnet.

edited Jul 21, 2011 at 8:24

answered Jul 21, 2011 at 8:08

Alnitak

341k72 gold badges420 silver badges503 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Fender Over a year ago

great answer :) much easier than my "workaround" which did not handle all cases

user447356 · Accepted Answer · 2011-07-21 08:16:04Z

0

I'm not familiar with node.js but do familiar with server side languages in general - when browser send request to the server, the server creates a Session for that request and any additional requests from the same browser (within the session life time) are treated as the same Session.

Probably by design, and for good reason, the requests from same session are handled sequentially, one after the other - only after the server finish handling one request it will start handling the next.

answered Jul 21, 2011 at 8:16

user447356

1 Comment

user447356 Over a year ago

Cheers, glad you got real solution! :)

Collectives™ on Stack Overflow

Why is this node.js code blocking?

2 Answers 2

1 Comment

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related