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Client-Side Header Bidding Vs Server-Side Header Bidding – What to Choose?

Client-side Vs Server-side Header Bidding
Now that you have two ways to execute header bidding - Client-side and Server-side, which one would you choose? We've analyzed and come up with the guide to help you make the right decision.

We’ve learned about header bidding and its use cases. But, what happens when we have Client-Side Header Bidding Vs Server-Side Header Bidding?

Header Bidding also known as pre-bidding, is an advanced method of integrating several demand partners (Ad Exchanges, SSPs, Ad Networks, etc.), collecting multiple bids from them at the same time, and selling the ad inventories at higher eCPMs unlocking higher yield. 

It allows publishers to collect bids from demand sources for all of a publisher’s inventory prior to the sale. The goal is to bring a greater level of transparency in the process to help publishers understand who is bidding what amount. The technique relies on the users’ browsers for execution. In this article, we will understand the difference between client-side header bidding and server-side header bidding, pros and cons of each type, and many more facts about them. So, shall we begin?

Table of Content:

Client-Side Header Bidding Vs Server-Side Header Bidding

Although header bidding increased the CPMs by up to 60%, there were some drawbacks in implementing header bidding on the client-side. Noticeably, page latency. So, to deal with the pitfalls and improve the user experience, adtech introduced a sister technique and named it ‘server-side header bidding‘.

But the server-side header bidding has its own advantages and disadvantages. Ultimately, it’s up to the publishers to settle on a decision. As a header bidding provider, we feel responsible to guide publishers like you to choose the best decision. There’s no one size fits all solution in programmatic advertising. Hence, we explained both client-side header bidding (browser-side header bidding) and server-side header bidding to help you pick the right solution.

What is Client-Side Header Bidding?

In client-side header bidding, bidding happens on the browser. Every time a page loads, ad requests are sent to the demand partners via a piece of JavaScript code added to the header of the publisher’s website. The demand partners conduct their own auctions and send the bid prices received from the buyers to the page header.

Now, the code that is residing in the header of the website, passes the highest bid value to the ad server (with the help of line item key-value targeting) to determine the final winning bid. Once the server receives the winning bid response from the header bidding line item, it compares with the eligible server line items (Sponsored, Standard, etc.) and selects the line item with the highest CPM rate. Then, the server serves the ad to the user.

Client-side Header Bidding


Higher Cost Per Mille (CPM): Publishers can now receive bids from buyers who are more interested and are willing to buy more price.

Higher Fill rates: More buyers means higher chances of filling all types of available ad inventories, including both premium and remnant (unsold) inventories.

Better Match rates: Since the bidding happens in the browser, publishers can synchronize the cookies and identify the relevant users for advertisers. This capability of delivering targeted advertising offers more revenue. It also improves the user experience because users don’t see irrelevant ads.

Greater Control and Transparency: Publishers are the owner of their websites, and they are the ones managing the header bidding codes on the side. So, they can remove or add any demand partner, change the header bidding codes, and so on. Besides, it creates a transparent auction as the publishers know they are getting the highest bid from the buyers.


Latency: Demand partners take time to get bid responses from the buyers. So, adding more demand partners means the JS code has to wait for a longer period to receive the bids from them. And this leads the page to take more time to load making users wait longer. It has a negative effect on the user experience, results in fewer impressions loading, and lowers the likelihood of ads even being viewed.

Compatibility: Compatibility with multiple browsers is important. Certain browsers may behave in certain conditions differently (e.g., some browsers may pool connections to external pixels or block them completely).

Duplicate Bids: Risk of putting the same impression or inventory up for sale since a publisher uses multiple bidding partners. However, this isn’t limited to client-side header bidding, it can also happen with server-side header bidding.

What is Server-Side Header Bidding?

It’s just like the client-side header bidding but the difference here is that instead of sending the requests from the browser, server-side header bidding sends the requests to the different demand partners from the ad server. This means instead of the user’s browser, the auction now happens on a server provided by a technology partner.

There’s just less implementation work to do because only one script (from the ad server) needs to be added for all partners. When a webpage loads, an ad request will be sent to the ad server. The ad server then sends the bid request to SSPs, DSPs, Ad Exchanges, etc., and receives the bids to determine the winner and serve the ad from the highest bidder.

Server-side Header Bidding


Reduced latency: As the browser is sending a single request (to the ad server) this time, the page load time is greatly reduced.

Scalable: The server-side approach essentially creates unlimited scalability without any price on latency. A publisher can add as many bidder partners as he/she wants. 


Match rates: Since the ad request is sent to the SSPs, DSPs, Ad Exchanges from an external ad server (not from the user’s browser) match rate will be lower. Hence, bidding prices will also be lower than the client-side header bidding as the relevancy of ads will be diminished.

Black Box: Ad Server will be in control and publishers have to rely completely on the server to pass the right values and decide the winner. Publishers consider this as a ‘Black Box Approach’. On the client-side, a publisher will be in control and can see what happens exactly.

Client-Side Header Bidding Vs Server-Side Header Bidding – Which One is for You?

If user experience* is more important than CPM for you, then the ideal way is to go with server-side header Bidding.  If CPM is more important than user experience*, then the right way is to go with client-side header bidding. However, latency can be reduced if you add the ideal number of demand partners (i.e. 5-6), then page load speed can be improved to a great extent via client-side header bidding.

*Note that we’re talking about very minimal differences here. For instance, there might be a few milliseconds difference when you execute and compare both the solution. 

Still confused about which way to go? Well, there is hybrid header bidding for those who don’t compromise on both. In the hybrid header bidding, a couple of the demand partners are shifted to the ad server. So, minimal latency and higher CPMs. A win-win condition. If you have any questions regarding how both types of header bidding can help, get in touch with us.

Automatad Team

At Automatad, we help publishers to monetize better without hampering the user experience. Our products are live across hundreds of publishers, earning them incremental ad revenue with every passing second. You can request a free audit to get an estimated revenue uplift today.

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