Why does Node.js not execute the callback function?

Question

I have the following code with http module:

function PostCode(stream) {
    // Build the post string from an object
    console.log("Start upload: " + stream);
    console.log("A");
    var post_data = JSON.stringify({
        'stream' : stream,
    });
    console.log("B");
    // An object of options to indicate where to post to
    var post_options = {
        'host': 'myserver',
        'port': '5000',
        'path': '/upload',
        'method': 'POST',
        'headers': {
            'Content-Type': 'application/json'
        }
    };
    // Set up the request
    console.log("C");
    var post_req = http.request(post_options, function(res) {
        console.log("D");
        res.setEncoding('utf8');
        console.log(post_options);
        res.on('data', function (chunk) {
            console.log('Response: ' + chunk);
        });
    });

    // post the data
    post_req.write(post_data,'utf8');
    post_req.end();

}

I execute the postCode function 1000~ times in order to backup my filesystem.

The probelm is that the callback isn't executed, I see a sequence output of:

A
B
C
A
B
C

and so on.. without D.

Just when all the postCode were executed, the callback is starting to run.

How I exceute the callback parallel? So that also D will be printed?

EDIT:

this is the new question, hope it clear. I still don't understand how to fix this issue:

The problem is that I have a loop which call to function A.

In this function, there is a codeblock with execute a function with callback, let say that the callback is B.

Now, B call to other function C.

Meaning:

`function main(){
   for (int i = 0; i<5; i++){
        console.log("main" + i);
        A();
   }
}

function A(){
   // do some stuff
   var u = http.request(o, function (res){
        console.log("A" + i);
        B();
   })
}

function B(){
   //do some stuff
      var u = http.request(o, function (res){
        console.log("B" + i);
        C();
   })
}


function C(){
        console.log("C" + i);

}

I see that C() callback is waiting until all A loop is finish, and just then execute. In this case, that what I would see:

main 0
A 0
B 0 
main 1
A 1
B 1 
main 2
A 2
B 2 
main 3
A 3
B 3 
main 4
A 4
B 4
C 0
C 1
C 2
C 3
C 4

How can I fix it, so C would be printed after main, a, and b?

That's the whole point of callback functions: they're executed only when the asynchronous task has been completed. Why would you want to wait for each upload to finish before starting the next? It would just make the process last much longer. — JJJ
– JJJ, Commented May 25, 2014 at 11:56
@Juhana: I want that http.request(post_options, function(res) {...} will be executed, so each upload shouldn't wait to the other. but this doesn't happen: http.request doesn't executed (The server doesn't get the request), and for that the callback doesn't executed. — Or Smith
– Or Smith, Commented May 25, 2014 at 12:01
@Juhana: My question is how I execute http.request(post_options, function(res) {}? Why it doesn't execute? Why it waits to other upload processes? — Or Smith
– Or Smith, Commented May 25, 2014 at 12:03
@Juhana: Please consider read again my question, maybe I was not clear. thank you. — Or Smith
– Or Smith, Commented May 25, 2014 at 16:15

mthierer · Accepted Answer · 2014-05-25 14:25:35Z

1

Your code misses the part where PostCode() is called, but it's probably something like a for-loop. As you already found out, that only fills the event queue which only gets executed when no other code is running.

You might try something like this:

function postNextStream() {
  if (streams.length > 0)
    PostCode(streams.shift());
}

var streams = [];

for (*however you find the stuff you'd like to post*)
  streams.push(stream);

postNextStream();

Then you set an listener for the "end" event on the ClientRequset returned from http.request() and call postNextStream() from there.

This makes all the requests sequentially. If you want some kind of parallelism, you have to implement a more sophisticated queue management in postNextStream().

answered May 25, 2014 at 14:25

mthierer

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2 Comments

Or Smith Over a year ago

Thank you, but I don't understand - how the event loop simulate threads?

mthierer Over a year ago

If you really want to know how the event loop in node.js works, I suggest you read about libuv, but that won't solve your problem. As I said: Stop calling http.request() from a for loop instead trigger requests from the callback to the "end" event.

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