Single-request architecture is a setup where, in a header bidding framework, the bidder sends one call to the ad server for multiple ad slots, and the server returns bids for all of those ad slots at the same time.
To explain why single-request might be advantageous to the publisher, and to the buyer/bidder, we should put it in context and explain multi-request architecture briefly. When header bidding adoption was on the rise, multi-request architecture was pretty common. With the multi-request arrangement, when the user loads the page, there’s essentially one auction that fires off for each of the ad slots on the page. As Andrew Casale from Index Exchange explained to AdMonsters in an interview at the end of 2016, this means that if you have four slots, the page initiates four requests (simple enough). But if there are five header bidders integrated on the site, each of those bidders will make an individual call for each of those slots, which means you’ll be sending out 20 requests total. The problem is, those calls require browser resources. There’s a cap on the number of tasks the typical browser queue can handle. The browser will handle however many of those calls it can, and the rest hit a roadblock.
With single-request architecture, the bidder would send one request for all the ad slots on the page. So in the above example, with five bidders, that’s five requests, even though you’re shooting to fill all four slots on the page. That can be appealing to the bidder, because they can ascertain what’s available on the whole page and make a bid to buy all of it. That makes it more likely for that bidder to be prioritized in the browser queue, and it opens the door for buyers to create ad experiences that are more customized for their target user’s experience moving through the page. And single-request can be appealing to the publisher, because keeping the number of tasks in the browser queue can cut back on header’s infamous latency problem and improve page performance.
So there’s a chance you’re wondering: If single-request is a win/win for bidders and publishers, why isn’t everyone doing it right now? Well, there’s a barrier to entry in the amount of engineering work and tech infrastructure involved for the bidder to adopt single-request. But adoption has been spreading as publishers become more curious about single-request and bidders ramp up their ability to support it.
Rethinking the Ad Server (2016)