Coordinating histogramIndex with delegation

[benjaminsavage]

Assuming an advertiser has purchased ads through multiple channels, with saveImpression being called by multiple different ad-tech vendors / publishers, it may be difficult to coordinate the values of histogramIndex used across these different entities.

This could lead to unreadable results in the event that the same value of histogramIndex is used by different entities to represent different things.

To use an example:

• Assume the advertiser would like a breakdown by channel x campaign.
• They are running 3 campaigns through intermediary number 1.
• They are running 5 campaigns through intermediary number 2.
• They are running 7 campaigns through intermediary number 3.

They need to coordinate so that intermediary number 1 uses the values 0, 1 and 2 to designate their three campaigns, while intermediary number 2 uses 3 through 7, and finally intermediary number 3 uses 8 through 14. If the advertiser decides to start running a fourth campaign through intermediary number 1, things are complicated.

It may be desirable to devise an approach to delegation which does not require various ad-tech vendors to coordinate their usage of histogramIndex.

[martinthomson]

As we discussed, the status quo -- where an ad tech only receives conversion information for the impressions they were responsible for placing -- isn't that bad here.

If we set intermediarySites to default to the site of the iframe, when not called from a top-level context, we get that status quo. It's not perfect, but it's what people are used to getting.