Programmatic · also called S2S header bidding, server-to-server bidding
Server-side header bidding
Server-side header bidding runs the header bidding auction on a remote server rather than in the user's browser or app, reducing page weight and latency while connecting to more bidders.
In classic client-side header bidding, the user's browser calls every bidder directly, which slows pages and drains mobile data. Server-side header bidding sends a single request from the page to a server, which then calls all the bidders, runs the auction and returns the winner.
Prebid Server is the best-known open-source option; Google's Open Bidding (formerly Exchange Bidding) and Amazon's Transparent Ad Marketplace are other server-side systems. Server-side setups are standard for CTV and mobile apps, where there is no browser header, and help pages with many ad slots.
The trade-offs: server-side bidders often receive less user data, because the browser's cookies are not sent directly to each bidder, so cookie matching rates and bids can be lower. The server operator also gains control and visibility over the auction. Many publishers therefore run a hybrid: key partners client-side, the rest server-side.
Think of it like this
Server-side header bidding is like phoning one broker who calls ten lenders for you, instead of filling in ten loan applications yourself.
An example
A news site moves eight of its twelve bidders into Prebid Server; page load time drops by 400 ms while total bids per impression stay roughly the same.
Related terms
Header bidding
Header bidding is a technique where a publisher offers each ad impression to many SSPs and exchanges simultaneously before calling its ad server, so all buyers compete at once.
Prebid
Prebid is the most widely used open-source header bidding software, including Prebid.js for websites, Prebid Server for server-side auctions and Prebid Mobile for apps.
Latency
Latency is the time delay between a request and a response; in advertising, it is how long auctions, ad servers and creatives take to return and display an ad.
Auction timeout
An auction timeout is the maximum time an auction waits for bids; responses that arrive after it are ignored, even if they would have won.
Unified auction
A unified auction is a single auction in which all demand sources, such as direct campaigns, header bidders and exchanges, compete for an impression under the same rules at the same time.
Sources: Prebid Server overview