What is a CACHE-Buster and why do we need it?

Mathias Freudendahl Updated by Mathias Freudendahl

What is a cache-buster?

A cache-buster is a small URL-parameter that should always be added to the tracking pixel URL. If the cache-buster is not added, the browser will re-use previously recognized elements of a website again and again on almost every page. This is a problem for ads, banners and tracking pixels. Therefore, we need the cache-buster to trick the browser into thinking that it is using different elements each and every time.

Purpose of a cache-buster

The purpose of the cache-buster is to enable us to register the correct amount of impressions of a campaign. Most advertisement systems allow the usage of a cache-buster token. Cache-busters can be different from ad server to ad server. You can usually find the cache-buster for your ad server in the ad server documentation. 

Why is it important?

Without a cache-buster we might be unable to register the correct impression count of your campaign. We would also encounter problems calculating frequency correctly, which will affect reach estimates in the system. So in order to ensure good data quality, always use 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 (RND-tag) to the tracking pixel each and every time it is requested. For example:




Both URL's point to the same tracking pixel, a 1x1 image on visitanalytics.userreport.com. However, 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 every time, thereby forcing it to request it from the web server instead of fetching it from the cache.

How did we do?

How to create a pixel?

What is a tracking pixel?