Why do we need a cache-buster?

Before reading this, you may like to read our article "What is a cache-buster

When a browser loads a web page, the publisher’s web server will send an HTML file with instructions on where to retrieve all the images, text, pictures and other pieces that the web page consists of.

Downloading all this information takes time and bandwidth, so the browser tries to save as much of the information as possible for future use in temporary folders (the cache) - your hard drive.

This technique lets a browser surf through a website much faster because many elements of a website are re-used again and again on almost every page. Why fetch the same logo-file again and again, when the browser can save it to the temp folder once and then simply re-cycle it on every page? In other words, our browser is smart enough to read the HTML and recognize what files it already has loaded previously.

The problem is, that this behavior also works on ads, banners and tracking pixels. If you browse three different pages on the same website all containing the same banner, the browser would just re-use the banner it called the first time. The same happens with our tracking pixel.

So we need to trick the browser into thinking that the banner and tracking-pixel on page two and three are different from the one found on page one. Even when they are the same.

We do that by adding a random URL-parameter to the tracking pixel each and every time it is requested.

Take our example below:


Both URL's point to the same tracking pixel, a 1x1 image on visitanalytics.userreport.com.

But by using a different RND-tag each and every time the image is requested - we trick the browser into thinking it is a different image each and every time, thereby forcing it to request it from the web server instead of fetching it from the cache.

How did we do?

Which cache buster shall I use for my ad server?