clarification re: OpenRTB macro substitution capabilities

[rdgordon-index]

There have been a few existing issues regarding macro substitution -- however, they are focused on aggregate reporting APIs (#477) or on the renderURL (#286 (comment)).

As per https://github.com/InteractiveAdvertisingBureau/openrtb2.x/blob/main/2.6.mdsubstitutionmacros :

Note that OpenRTB compliance exchanges must support all macros for which data is available and support substitution in both markup and URLs for win and billing notification.

In other words, this requirement for macro substitution is not limited to the renderURL -- so I'm looking to better understand how macro support will function in PAAPI.

Let's take the ${AUCTION_PRICE} macro as an example -- unlike WIDTH and HEIGHT for responsive ads (mentioned in #417), the clearing price is not needed for rendering the creative; and if the creatives are hosted on a CDN, the ${AUCTION_PRICE} would not be much use in that context.

How would a seller be able to indicate this to the buyer? There's a mechanism for passing metadata from reportResult to reportWin, but this value for ${AUCTION_PRICE} can only be determined within scoreAd() -- it's not necessarily the value of the winning bid noted in bid -- and there's no way to exfiltrate this signal via event-level reporting. And even if such a mechanism existed, the API doesn't provide a method to facilitate such substitution.

In summary: how does the seller's obligation about replacing such macros in both creative markup and win notices change in PAAPI?

[rdgordon-index]

Based on the WICG meeting on 2023-11-08 based on @michaelkleber's feedback (cf. https://github.com/WICG/turtledove/blob/main/meetings/2023-11-08-FLEDGE-call-minutes.md#macro-substitutions---httpsgithubcomwicgturtledoveissues817)

• buyers can no longer assume that any macros will be replaced in the rendered creative
• signalsForWinner is the mechanism for communication information from reportResult to reportWin
  • buyers and sellers would need to agree on how to name and signal these macros
  • buyers would be responsible for using this in their sendReportTo calls, or using them in reportEvent (subject to all the requirements for this new API)

However, as per the original post, it's still not clear how to get any price adjustments from scoreAd into reportResult to make this possible.

Potential solution -- can we get trustedScoringSignals passed into reportResult? These adjustments can be based on the K/V that we're using in scoreAd -- so if those K/V were made available, then this would be a solved problem.