What is ad server?
An ad server is the system that decides which ad fills a slot and records that it was shown. A publisher ad server, such as Google Ad Manager, holds direct-sold campaigns, priorities, and pacing, and weighs them against programmatic demand. An advertiser ad server hosts creatives and counts impressions and clicks independently of the seller. Neither is an OpenRTB role, though both shape what bidders see.
Also called: publisher ad server, advertiser ad server, first-party ad server, third-party ad server.
How it works
On the sell side, the publisher ad server usually makes the final call on an impression. Header bidding wrappers collect bids from several exchanges and pass the best ones to the ad server as targeting values, and the ad server compares them with direct campaigns and its own exchange demand. OpenRTB signals this arrangement with Source.fd: a value of 1 means an upstream source, not the exchange, makes the final sale decision.
That matters to bidders. A bid that wins the exchange auction can still lose in the ad server, so a win notice from Bid.nurl is not proof of delivery, and the billing notice in Bid.burl becomes the event to count. The transaction ID in Source.tid, shared by every exchange bidding on the same opportunity, helps buyers see when several requests describe one impression.
On the buy side, the advertiser ad server delivers the creative behind the bid. A VAST wrapper or display tag in Bid.adm often points to it, and it serves the final asset and fires its own impression tracking. Gaps between the exchange's billing count and the advertiser ad server's count are normal and usually come from render failures, latency, and invalid traffic filtered on one side only.
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Related terms
Every term is in the glossary.