Prebid Server: Multiple Root SChain nodes

[bretg]

(This is the Prebid Server side of prebid/Prebid.js#3870)

Copying the use case from that issue for context:

I'm a publisher. I work with SuperAds, a fictitious monetization platform who provides me prebid.js implementation and access to multiple exchanges. SuperAds does this by configuring prebid.js for me, with multiple bidders (Exchanges A and B), making use of the seller account (publisher ID) for SuperAds on those exchanges. However, I also have a direct relationship with ShinyClicks, an ad exchange with a prebid.js adapter, and SuperAds configures that in prebid.js for me too.

SuperAds has assigned me publisher ID 123 internally.

In this case, the supply chains needing to be sent out in bid requests by prebid.js would be:

Bidder for Exchange A: superads.com, 123
Bidder for Exchange B: superads.com, 123
Bidder for ShinyClicks: (I own the account on ShinyClicks)

In the first two cases, the exchange pays SuperAds, who pays me. I don't control the accounts on the exchanges.
In the third case, ShinyClicks directly pays me, as I control that account.

In this scenario, Prebid Server is not receiving a single simple schain on source.ext.schain. Rather, it will receive a list of schains that are appropriate for different bidders. Here's the proposed location and format:

ext.prebid.schains: [
   { bidders: ["shinyClicks"], schain: { SCHAIN OBJECT }},
   { bidders: ["*"], schain: { SCHAIN OBJECT }}
]

Prebid Server's job would be to parse this list and pass the correct source.ext.schain to each bidder. It should also remove ext.prebid.schains before sending to the adapter.

If there's already an source.ext.schain and a bidder is named in ext.prebid.schains, the latter would override the former.

[bretg]

This is done in PBS-Java.

[bjorn-lw]

I'd like to bump this one. We have cases where this support is needed.

[SyntaxNode]

Noted. We've been focused on privacy policies a lot recently. I understand it's a common ask from the community and have bumped up its priority within Xandr. I added the 'help wanted' tag if someone else is able to get to it first.

[bsardo]

I'm picking this up for PBS-Go.

[bsardo]

@bretg I spent some time going through the spec and the applicable github issues/PRs and I didn't see anything about how to handle the case where multiple non-wildcard schains are defined for a bidder. PBS-Java appears to ignore all schains for a bidder if multiple are defined for a bidder, leaving source.ext.schain unmodified. Can you confirm that this is the expected behavior?

[bretg]

What's the use case here @bsardo ? Seems like a data problem to me, but I'm open to input: should we use the first or use the last. Right now it's considered invalid input and ignored.

@rpanchyk - heads up.

[bretg]

I'm told that PBS-Go is going to treat this scenario as invalid and reject the request. Am ok with this. @rpanchyk - let's update PBS-Java to do this too.

[SyntaxNode]

This has been implemented in PBS-Go. It's available in release 0.120.0 and newer.