Record the default core bid ranking

[bretg]

As described in the response to the Audio/CTV committee requests, there's a proposal to define seatbid.bid.ext.prebid.rank as a way to flag the highest priority bids. A rank of "1" would mean highest priority. Note that highest priority doesn't always mean "highest price". Sometimes deals are prioritized over open market.

The 'response' document imagines a "bid ranking module" which would assign these ranks based on account configuration to cover advanced scenarios like "open market bids more than $5 above the deal price can beat out the deal, except for deal 123"

But at this point, neither Audio nor CTV needs support for that type of advanced scenario.

Since PBS-core is already sorting bids to apply ad server targeting, I'd like to explore adding the default seatbid.bid.ext.prebid.rank to every bid. Then someday, a more sophisticated bid ranking module could be built to override this default value.

The algorithm would be very basic:

• If ext.prebid.targeting.preferdeals is true, then sort by isDeal (true first) and then bid.price (highest first), else just bid.price (highest first).
• If bid.price is tied the sort order can be indeterminate
• loop through sorted bids
  • assign the current index to seatbid.bid.ext.prebid.rank. i.e. first bid gets 1, second 2, etc.

[bretg]

This was discussed in committee, and then more in a slack thread. Here's a summary of where we stand:

1. The Prebid proposal is that PBS should not be in the business of picking 'winners'. It currently has a primitive algorithm for doing this, but we are emphatically not an ad server, and do not provide enough enough controls around this choice to consider ourselves authoritative.
2. There's a community requirement that some host companies don't want to cache bids that don't "win".

So the proposed vision for the future is:

1. Caching behavior could have a flag that can disabled so PBS-core doesn't cache and a module can handle caching and updating the ORTB with the cache IDs.
2. PBS-core will supply a default "response ranking algorithm" and write the values into seatbid.bid.ext.prebid.rank
3. If turned off, PBS-core will not cache bids to PBC.
4. A 'processed auction response' module can refine the ranking however it wants.
5. A separate 'processed auction response' module can call PBC for whatever bids it care to call winners and add whatever ORTB it wants to the results. Specifically, this module would need to:
   1. add the ext.prebid.cache object where relevant
   2. update seatbid.bid.ext.prebid.targeting to add hb_uuid, hb_cache_id, hb_uuid_BIDDER, and/or hb_cache_id_BIDDER where relevant. (unless we already have a targeting module that can do this)

It's understood that there's some legacy cruft in core. At some point in a major release, we should pull the "winning bid" logic and the "targeting" out and make them modules. PBJS doesn't use our targeting anyhow. AMP and App do.