Header bidding

Header bidding is a process that lets publishers capture bids for their ad units from demand sources that might otherwise have been missed. Publishers offer their inventory to multiple ad exchanges simultaneously, before calling their ad server, and those bids then compete directly with the ad server's own.

What it changed. Without it, the ad server runs through its inventory as a page is displayed, picks an ad and sends it back to the publisher. Prebid describes header bidding as a way to make the auctions for a publisher's ad space more competitive and transparent.

How it works. The page starts to load, and the header bidding code asks a set of demand partners for bids. The partners reply, and the ad server is called with some or all of those bids attached. The ad server considers every ad that could be displayed, including the header bids, and shows the winner for each ad unit — which may or may not be a header bid.

The latency trade-off. Header bidding delays the call to the ad server until outside bidders have been contacted, which can slow page loading and ad display. Sending requests to all bidders at once, rather than one at a time, is one way Prebid reduces the delay.

Prebid and server-side bidding. Prebid is a free, open-source header bidding solution. Prebid.js, launched in 2015, is a JavaScript library that runs in the browser. Prebid Server handles header bidding server-side, which is required in environments without an HTML header for JavaScript, such as mobile apps and AMP pages. The term now also covers bidding that never touches the page header, such as video auctions in the player.