ConnectAd Chronicles


Prebid Client vs Prebid Server – which integration wins?

The one thing we love most at ConnectAd is being there for our customers. We care for them, want them to do well and therefore take their needs and wishes very seriously. However, there is one thing that can easily drive us up the wall. Publisher with a full ad stack. Full ad stack? As we already explained in an earlier blog post there simply is no such thing as a 'full ad stack'.  

Why header bidding is a must in 2023

The harsh reality is that publishers still rely on a 'full ad stack' instead of using all the options available to them – and that's a pity.

We all know that header bidding is the only way to grow and increase your yield. To be more precise it has replaced the traditional waterfall for good and therefore revolutionized the digital advertising world for publishers by allowing them to sell ad inventory on multiple programmatic marketplace auctions in real time. You wanna do that too and are thinking about implementing header bidding soonish?

Great! Then you should read on now and learn everything about Prebid Client and Prebid Server – the two open-source header bidding solutions meant to help publishers maximize their ad revenue.

How Prebid Client works

Prebid Client is a client-side header bidding solution that runs entirely in a user's browser which means that the bidding happens on the browser. By doing so it adds a piece of JavaScript to a publisher's website in between the tags. Whenever a page loads, the auction code sitting on the publisher’s website is executed in the user’s browser. This means: The user’s browser sends ad requests to a number of demand partners. Subsequently, bids are received from DSPs via SSPs and ad exchanges, and the one that bids the most wins the auction.

Pros of client-side header bidding 

+ Higher Cost per Mille (CPM) and higher fill rates: Publisher can receive more qualified bids from buyers who are willing to pay a higher price. They also are able to reach more buyers which brings them more chances to sell each and every available ad space.

+ Higher cookie match rates: As the whole bidding process happens in the browser publishers and advertisers are able to sync their cookies which makes it possible to target specific users.

+ Greater control and transparency: Publishers are the owner of their websites and the remain the ones to manage the header bidding codes. They have full control and can be sure that they are getting the highest bid from the buyers.

+ Industry Standard: As the server-side header bidding solution often isn’t running as smoothly as the client-side header bidding, Prebid Client still is THE go-to implementation for most publishers and advertisers. 

Cons of client-side header bidding

- Latency issues: Demand partners take their time to get bid responses from the buyers. Therefore the code has to wait longer to receive their bids. This leads to a longer page loading time, which is not only frustrating for users but might also result in fewer impressions loading and lowering the likelihood of ads even being viewed.

- Compatibility: Client-side header bidding works in the browser, so compatibility with multiple browsers is very important. If compatibility is not guaranteed header bidding auctions could end up being inefficient.

- Duplicate bids: There is a risk in offering the same impression for sale multiple times, if a publisher connects to multiple header partners. However, this can also happen in the server-side implementation.

- Performance: Adding more logic slows down the performance of the browsers and the website itself. This is no problem for up-to-date hardware, but older devices or systems may struggle with it.

- Limited ad requests: The number of ad requests sent by the client-side header bidding wrapper is limited to about a dozen. 

How Prebid Server works

Prebid Server is a server-side header bidding solution that runs entirely on a publisher's server. So it's very similar to client-side header bidding with one key difference: the requests are no longer sent to the various demand partners from the browser, but from a server. Therefore the bidding happens on a server provided by a technology partner instead of the user’s browser. This means: When a web page is loaded, an ad request is sent to the server, which then sends the bid request to SSPs, DSPs, ad exchanges etc. and also receives corresponding bids again to determine the highest bidder and admit the winner. 

Pros of server-side header bidding

+ Reduced latency: By moving the header-bidding process off the browser and into a server the latency issue is significantly reduced. In fact, if the auction process is done through a server-side implementation rather than a browser, the content of the web page can be loaded independently of and without being affected by the ad auction. The loading speed of the ads is faster, the user experience better.

+ Infinite scalability: Unlike browsers that have a limited number of network connections they can make at one time Prebid server makes it easy for publishers to add more and more bidders to their requests and therefore generate more and ore revenue from demand partners. The server-side approach essentially creates unlimited scalability without any price on latency.

+ Works better for videos and rich media: Videos themselves tend to load very slow. When loading video ads it has proven to be worthwhile to rely on server-side header bidding. The same applies to advertising on OTT devices.

+ Unified auctions: Managing and setting up ad units per wrapper can be quite a challenge for publishers. However, with a Prebid server solution, you don't have to face this at all. Because: It combines server-side and client-side platforms and creates performance reports for all bidders or demand partners in one place. 

Cons of server-side header bidding

- Lack of control and transparency: The publisher sets the floor price, but he can’t choose the buyers as good as with client-side. The ad server is in control and the publisher must rely on the server to do its job correctly. To cut to the chase: The auction process is a “black box” and remains a secret as it is “hidden” on the server.

- Lower match rate and bidding prices: As the ad request is sent from an external ad server – and not the user’s browser – to the SSPs, DSPs and Ad Exchanges, not only the match rate but although the bidding prices are lower as the relevancy of ads will be diminished.

- Harder to identify users: Server-side header bidding lacks cookie matching because most user data is filtered when it is transferred to a server. This causes a significant drop in revenue.

- Lack of trust: Prebid server is the newer implementation method. It still raises many concerns among publishers and advertisers and it will take some time before it is fully adopted and accepted in the industry. 

In summary: It’s up to you, but we’re here to help

As you can see, each of these header bidding solutions has its advantages and disadvantages. As a publisher, you should weigh these up carefully and consider what makes sense.

However, the following cannot be denied:

o Header bidding beats waterfall – everyday of the week!

o Prebid Client is easier for the publisher, but can get "full" after about 10-12 bidders.

o If the Prebid Client is full then the Prebid Server must be used. But please keep in mind: Prebid Server is the technical "top class" and requires appropriate know-how and a suitable infrastructure. As a publisher, you have to work cleanly here. It is especially important to use identifiers and take good care of functioning user matching. If you pay attention to this, however, there is virtually no limit to the number of bidders you can integrate on the server route. Or as we like to say: The sky is the limit ;-)


Good news at the end: No matter where in this whole topic you are standing at the moment, we at ConnectAd are here to help – and we can do everything. From good old waterfall to Prebid Client and Server integrations. So feel free to contact us in case you need support!