Supply chain · also called schain, SupplyChain object
SupplyChain object (schain)
The SupplyChain object (schain) is a field in OpenRTB bid requests that lists, in order, every seller or intermediary that handled the impression on its way from the publisher to the buyer.
The schain is the travel history of a bid request. Each company that sells or resells the impression adds a "node" containing its ad system domain, its seller ID (which can be looked up in sellers.json), and whether it handled payment. A "complete" flag says whether the chain is known all the way back to the original publisher.
The IAB Tech Lab released the SupplyChain object in 2019 alongside sellers.json, and it is carried in OpenRTB (originally as an extension, now a standard field in OpenRTB 2.6). Prebid and major SSPs populate it.
Buyers use schain to see how many hops sit between them and the publisher, check each hop against ads.txt and sellers.json, and cut long or suspicious paths through supply path optimization. Incomplete chains, unknown seller IDs or chains longer than necessary are common red flags for traffic arbitrage and fraud. Like all bid request data, schain is asserted by sellers; its value comes from cross-checking.
Think of it like this
The schain is like the chain-of-custody form attached to a parcel: every handler signs it, so you can see if it passed through anyone unexpected.
An example
An schain with two nodes, the publisher's direct SSP account and one reseller, is complete. Another request for the same site shows four nodes and complete=0, so the buyer skips it.
Related terms
sellers.json
sellers.json is a public file in which SSPs and exchanges list their seller accounts, naming who owns each one and whether it is a publisher or an intermediary.
ads.txt
ads.txt (Authorized Digital Sellers) is a public text file on a website's domain listing which ad sellers and accounts are authorised to sell that website's ad inventory.
OpenRTB
OpenRTB is the open technical standard, maintained by IAB Tech Lab, that defines how bid requests and bid responses are formatted and exchanged in real-time bidding.
Supply path optimization (SPO)
Supply path optimization (SPO) is a buyer's practice of choosing the most direct, cheapest and most trustworthy routes to the same inventory, and cutting unnecessary resellers and duplicate paths.
Reseller
A reseller is a company authorised to sell a publisher's ad inventory through an ad system account that it controls, rather than the publisher's own account, and labelled RESELLER in ads.txt.