throttle requests in Node.js

Question

throttle requests in Node.js

I have an array. I can loop over it with the foreach method.

data.forEach(function (result, i) {

     url = data[i].url;

     request(url);

});

The request function is making a http request to the given url. However making all these requests at the same time leads to all sorts of problems.

So I thought I should slow things down by introducing some-sort of timer.

But I have no idea how will be able to combine a forach loop with setTimeOut/setInterval

Please note am doing this on the server (nodejs) rather on the browser.

Thanks for you help.

javascript
node.js
settimeout
throttling

Answer 1

As your problem is global, you should adjust your request function to have only 5 request running at a time - using a global, static counter. If your request was before something like

function request(url, callback) {
    ajax(url, callback);
}

now use something like

var count = 0;
var waiting = [];
function request(url, callback) {
    if (count < 5) {
        count++;
        ajax(url, function() {
            count--;
            if (waiting.length)
                request.apply(null, waiting.shift());
            callback.apply(this, arguments);
        });
    } else
        waiting.push(arguments);
}

Answer 2

you can delay call using setTimeout. following code will insure that each request get called after timerMultiPlier milliseconds from its previous request.

var timerMultiPlier = 1000;
data.forEach(function (result, i) {
     setTimeout(function(){
           url = data[i].url;         
           request(url);
   }, timerMultiPlier*i );

});

Answer 3

data.forEach(function (result, i) {

     url = data[i].url;

     setTimeout(
         function () {
              request(url);
         }, 
         1000 * (i + 1) // where they will each progressively wait 1 sec more each
     );

 });

Answer 4

Instead of setTimeout could have them run in sequence. I assume there's a callback parameter to your request() function.

function makeRequest(arr, i) {
    if (i < arr.length) {
        request(arr[i].url, function() { 
                                i++; 
                                makeRequest(arr, i); 
                            });
    }
}

makeRequest(data, 0);

If you need a little more time between requests, then add the setTimeout to the callback.

function makeRequest(arr, i) {
    if (i < arr.length) {
        request(arr[i].url, function() { 
                                i++; 
                                setTimeout(makeRequest, 1000, arr, i); 
                            });
    }
}

makeRequest(data, 0);

Answer 5

You can the offset the execution delay of each item by the index, like this:

data.forEach(function (result, i) { 
  setTimeout(function() {
    url = data[i].url; 
    request(url);
  }, i * 100);
});

This will make each iteration execute about 100 milliseconds after the previous one. You can change 100 to whatever number you like to change the delay.