This document describes the mechancis of a Prebid Server cookie sync.
Many Bidders track users through Cookies. Since Bidders will generally serve ads from a different domain than where Prebid Server is hosted, those cookies must be consolidated under the Prebid Server domain so that they can be sent to each demand source in /openrtb2/auction calls.
Start by calling /cookie_sync. For each element of response.bidder_status,
call GET element.usersync.url. That endpoint should respond with a redirect which will complete the cookie sync.
Bidders who support cookie syncs must implement an endpoint under their domain which accepts an encoded URI for redirects. For example:
GET some-bidder-domain.com/usersync-url?redirectUri=www.prebid-domain.com%2Fsetuid%3Fbidder%3Dsomebidder%26uid%3D%24UID
This example endpoint would URL-decode the redirectUri param to get www.prebid-domain.com/setuid?bidder=somebidder&uid=$UID.
It would then replace the $UID macro with the user's ID from their cookie. Supposing this user's ID was "132",
it would then return a redirect to www.prebid-domain.com/setuid?bidder=somebidder&uid=132.
Prebid Server would then save this ID mapping of somebidder: 132 under the cookie at prebid-domain.com.
When the client then calls www.prebid-domain.com/openrtb2/auction, the ID for somebidder will be available in the Cookie.
Prebid Server will then stick this into request.user.buyeruid in the OpenRTB request it sends to somebidder's Bidder.