Server Side Header Bidding

Server side header bidding, also called server-to-server (S2S) header bidding, refers to a process where the header bidding auction takes place on a server instead of within the user’s browser. In contrast, client side header bidding (C2S) is a process where header bidding auctions are run on the user or client’s browser. To learn more about the basics of header bidding see our what is header bidding guide.

8 April 2021

How does server side header bidding work?

  1. The user enters a URL
  2. The browser starts loading the page
  3. The header bidding JavaScript tag located between the <head> tags of the page sends a request to the ad server hosted by an S2S header bidding company like Snigel
  4. The ad server sends out bid requests to SSPs and ad exchanges
  5. The highest bidder wins
  6. The winning creative is retrieved and rendered on the page

Why use server side header bidding?

While client side header bidding allows publishers to increase their advertising revenue by connecting to more SSPs and ad exchanges, it also has some drawbacks. The real-time auction that enables the SSPs and ad exchanges to compete runs in the user’s browser. This increases the amount of processing power required by the browser and as a result, can mean slower page load speeds. Server side header bidding addresses some of these challenges but also comes with its own issues.

The upside of server side header bidding

Improved page load speed

S2S header bidding moves the real-time auction from the user’s browser to the server side header bidding company’s ad server. This means less processing power from the user’s browser is required and faster page load times can be achieved. In particular, users with older devices or a poor internet connection will find sites using server side header bidding provide a smoother experience.

Unlimited number of connections to advertising demand sources

Whereas browsers limit the number of network connections allowed at one time, server side header bidding lets publishers connect to an unlimited number of advertising demand partners. This means publishers can send out more bid requests, resulting in more competition for their ad space and increased ad revenue.

A better user experience for native video

Video ads require substantial processing power and generally slow down the page load time. Server side header bidding significantly speeds up this process, providing a better user experience.

The downside of server side header

Cookie Matching

When the real-time auction takes place in the user’s browser, advertisers can scan for cookies that identify the user. Since advertisers aim to match their ads to specific user profiles, they are willing to pay more when they can identify users. With server side header bidding, much of the user’s data can’t be sent to the ad server. As a result advertisers get a lower match rate and bid less for the publisher’s ad space.

CafeMedia ran a one week test where it sold advertising inventory only through server-to-server header bidding on some of its sites. Page load speed increased 40 percent on average, but CafeMedia EVP Paul Bannister also said revenue declined nearly 30 percent.


Since the real-time auction is now happening on an external server, publishers may have less insight into the auction process. In contrast, with client side header bidding publishers can see which buyers are participating in the bidding process and who won. This process can become problematic when the bidding partner is also providing the server side header bidding “black box” along with their own demand. Here there is a potential conflict of interest as the bidding partner is bidding to win in the auction and getting to decide who wins.

For this reason, many top publishers have instead select an established and demand agnostic header bidding partner like Snigel.

Hybrid header bidding

Publishers can get the best of client side header bidding and server side header bidding by using a hybrid setup. In this setup the best performing SSPs and ad exchanges are run on the client side while the long list of advertising demand sources with lower performance compete on the server side. 

In general, S2S is best for demand partners with broad advertising goals like brand awareness where the bids are generally lower. C2S is best for demand partners that are using specific data like cookies and visitor demographics to target audiences and therefore bid more.

Hybrid header bidding can be used in combination with other tools like anti-adblock and native video header bidding.

Overall, hybrid header bidding lets publishers connect to more advertising demand sources while minimizing page load times and the amount of processing power required by the user’s browser.

It’s crucial that publishers customize their hybrid header bidding setup based on:

  • The SSPs and ad exchanges that perform the best for the site
  • The regional location (Geo) of users
  • The target page load speed for the site
  • Having performed a supply path optimization on their bidders. This determines which way (server or client) is the most effective for a specific bidder

Snigel can help publishers find which header bidding configuration works best on their site. See how this worked for here. We use multivariate tests and extensive data analysis to maximize ad revenue and minimize page load times. Contact us here to learn more.

About the Author

Ben Rycroft

Ben is Snigel's Head of Publisher Success. He works on business development and marketing - spreading the word about how Snigel can help publishers supercharge their ad revenue.


Let our team of experts assess how Snigel can increase your ad revenue.
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