How Does Ad Serving Work?

Interactive ads are everywhere these days, but when it comes to the technical process of getting an ad on the page and how publishers and marketers verify it delivered, not many people can explain what actually happens in detail.  Read this article though and you’ll be one of them!  Below I’ve detailed step-by-step how a browser gets from the initial call to a publisher’s website to the final ad creative, and when and how each party counts an impression.  You can view a diagram of the ad serving process at the bottom of this post – the numbers in the text refer to the steps labeled in the diagram.

So, without further ado -

When a browser navigates to a publisher website (1), the publisher’s web server sends back a bunch of HTML code (2) that tells the browser where to get the content (3) and how to format it.  Part of the HTML code returned to the browser (4) will include a coded link known as an ad tag.

Here’s an example of what an ad tag from Doubleclick, one of the major ad serving companies, looks like:;topic=abc;sbtpc=def;cat=ghi;kw=xyz;tile=1;slot=728x90.1;sz=728x90;ord=7268140825331981?

Click here to read more about ad tags and how they are constructed.

The ad tag points the browser to the Publisher’s Ad Server (5), a system designed exclusively for delivering and tracking advertising.  In most cases, the Publisher’s Ad Server is actually a network of cloud servers owned and maintained by a separate company.  In this case, the content server tells the browser to fetch the ad from Doubleclick, a company owned by Google, that then makes the very complex decision on which ad to serve using a program called an Ad Selector.

In many cases the ad server is deciding among thousands upon thousands of potential options in mere milliseconds.  The computational power behind the Ad Selector is mind blowing – Atlas, the major rival to Doubleclick calls the supercomputer running its Ad Selector “WARP” and it is among the most powerful in the world, making billions of decisions a day and trillions in its lifetime.  The Ad Server makes a decision, and in most cases sends back another ad tag (6), or redirects the browser by pointing it to the Marketer’s Ad Server.  These redirects are technically speaking 302 redirects, which tells the browser the page has been “temporarily moved”.  This allows Ad Servers to count the 302 call as an impression and host the actual ad content on a different server.  Once the publisher’s adserver sends the browser a redirect to the marketer, it counts a delivered impression in its own database (star).  The only exception here is if the publisher decides to deliver a house ad or the marketer has asked the publisher to “site-serve” the ads, both of which requires the publisher load the actual creative files into their ad server, meaning the publisher is the final destination, and the browser can skip the loop through the marketer side (steps 7,8,11,12).

Click here to read more about why Publishers and Marketers have their own Ad Servers.

The browser now calls the Marketer’s Ad Server (7) and is redirected yet again to a Content Delivery Network, or CDN, (8) a global network of cloud servers that actually house the raw creative graphics to fetch the actual ad.  Why, you ask?  Well, as powerful as ad servers are, they just aren’t equipped to handle the volume and bandwidth required to deliver content as heavy as image files.  Redirects are often nothing more than a 1×1 pixel requiring just a few bytes of memory.  Image files on the other hand are kilobytes or even megabytes in size, could be called millions of times a day, and require a much faster and robust infrastructure.  Ad Servers might maintain three to six data centers across the world, but a CDN can process the heavy bandwidth and deliver the content faster because they operate hundreds of data centers and can route requests to the one nearest to the user, no matter where they are on earth.  You can think of the ad server as the brain and the CDN as the brawn.  Ad Servers aren’t the only companies that use CDNs, in fact many websites host their bandwidth intensive files in these cloud networks.  A CDN is almost always another independent company, such as Akamai, that hosts the heavy creative assets so the Ad Server doesn’t have to. There used to be a handful of these companies out there, but Akamai has acquired almost all of them and is the largest player by far in the space.

Here’s what a CDN redirect to an Akamai server hosting a flash file looks like:

In addition to sending back the redirect to the CDN, the Marketer’s Ad Server also appends a second redirect (10) back to itself with a query string to fetch a 1×1 pixel (11) after the ad content has been called.  When the browser fires this last redirect calling a 1×1 pixel from the Marketer’s Ad Server (11), the Ad Server knows the ad was successfully downloaded and it finally counts an impression in its own database (star).

In many cases, your browser has to make at least four calls for site served ads and six in the case of third-party served ads for this whole process to work, if not even more, but shouldn’t take more than a second regardless of the number of parties involved. To visualize the process explained above, please see the diagram below – 302 redirects are highlighted in blue, and the ad creative is highlighted in red.

In the next post I’ll write about how this process changes when an impression is sold in real-time (RTB) on a Supply Side Platform or Ad Exchange.