For a web application i develop we had a problem with the performance. After a small investigation we found out that it had relations with the amount of requests to the server that were done.

The application is running in a browser (currently IE7) and browsers are generally limited to do not more then 2 parallel request to the same domain.(this has improved a bit in later versions of the browsers). In this post i will describe the quest for solutions.

A simplified description of the application will help to understand the problem better i think. The application is used to track movements of cars around the country so a lot of data is gathered every N seconds with the help of XMLHttpRequest. For every type of car there is a separate request done to the server. Currently about 5 requests are issued every 5 seconds.

There wouldn’t have been a noticeable problem if all the queries had performed well and every request would take less then say 1 second, but one of the queries was performing very slow with a response time of over 20 seconds. This query was used in 2 out of the 5 requests. Also the app was build in a way that if a new request was triggered the current request was aborted. So the problem was that at least one of the 2 parallel requests was never able to deliver an answer and was holding up other requests.

Using sub domains

So even after improving the query performance there was a need to do more then the 2 parallel request at a time to speed things up even more. For normal http requests a widely used solution is to set up sub domains in your DNS server pointing to the same domain which lets the browser believe the request is made to a different domain. This technique is used with Google Maps for example to get the images from multiple domains for a faster user experience.

I wanted to implement the same kind of functionality for my XMLHttpRequests so i went off and implemented a solution to select an sub domain from an array and send the request there. Unfortunately this didn’t work because of the security restrictions on cross domain XMLHttpRequest. Even if i know better then the browser that this sub domain is essentially the same domain the browser was restricting me here. That’s what you get for tricking the browser to do more in parallel right?

Searching a bit more i found all kind of workaround-ish solutions using frames and stuff to do cross domain XMLHttpRequests but the most useful solution was limited to the newer generation of browsers (Firefox 3.5+, IE8 etc.)

Using response header Access-Control-Allow-Origin

Setting the response header Access-control-Allow-Origin to the exact (including http://) value of the origin your web application is loaded from made it possible to do cross domain XMLHttpRequests.The easiest way i implemented this was to load the module headers_module (modules/mod_headers.so) in my Apache http.conf and add the following line to the configuration file:

Header always set Access-Control-Allow-Origin: http://maindomain.sample.org

this made the browser send an OPTION request on the XMLHttpRequest.open call to check if the call was allowed before sending the data with the POST request.

And what for the older browsers?

But he! I’m still running on IE7 right? Right so back to the drawing board.

So currently I’m trying to combine as much of the requests into one request and filter out the data on return. In the meantime I found that for IE you can influence the amount of parallel requests done to the same domain with an adjustment of your registry settings. Since the app is running in a closed and controlled environment this might be the way to go if my programming attempts are unsuccessful.