Client-Side vs Server-Side Header Bidding: The Definitive Guide (2021)

what is server side header bidding

Serve-side header bidding and client-side header bidding are the two primary header bidding technologies. While both enable website owners to monetize their traffic, they provide distinct pros and cons. Knowing which one to use on your website, can improve your ad revenue and user experience.

Server-side header bidding, also called server-to-server (S2S) header bidding, refers to a process where the header bidding auction takes place on an external server. Moving the ad auction to an ad server speeds up the page load process and provides an opportunity to connect with an unlimited number of ad exchanges, ad networks, and SSPs. 

While client-side header bidding enabled publishers to increase their advertising revenue by connecting to more advertising demand partners, it came with some drawbacks. The real-time auction that enabled the SSPs, ad networks, and ad exchanges to compete, ran in the user’s browser. This increased the amount of processing power required by the browser. As a result, page load speed dropped.

Server-side header bidding was created to tackle these two challenges that occur with client-side header bidding. However, server-side header bidding also introduces some new challenges. There is less chance of a cookie being matched which the technology which results in lower ad revenue. In addition, server-side header bidding can provide less transparency since the auction takes place on an external server. To learn more about the basics of header bidding see our what is header bidding guide.

The upside of server-side header bidding

Improved page load speed

When the header bidding auction is moved from the client’s browser to the ad server, it means the browser has less work to do. As a result, the page load speed improves.

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.

Transparency

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 selected an established and demand agnostic header bidding company like Snigel.

What's the difference between client-side and server-side header bidding?

Client-side header bidding creates an ad auction within the user’s browser. In contrast, server-side header bidding creates an ad auction on an external server. As a result, server-side header bidding requires less processing power from the user’s browser and delivers faster page load times. In particular, users with older devices or a poor internet connection will find sites using server-side header bidding provide a smoother user experience.

Server side vs client side header bidding
Server-Side Header Bidding (S2S) Client-Side Header Bidding (C2S)
Lower latency Higher latency
Unlimited SSPs, ad exchanges, ad networks More advertising demand partners, leads to more latency
Lower revenue More revenue
Less transparency More transparency

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 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

Which is better, client-side or server-side 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 video ads.

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
  • A supply path optimization analysis for the bidders. This determines which method (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 WindFinder.com in our server-side header bidding case study. We use multivariate tests and extensive data analysis to maximize ad revenue and minimize page load times. Contact our ad ops experts 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.

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