Local vast cache: only store bids that aren't cached remotely yet

[robin-crazygames]

When using the IMA player + GAM, passing an URL to the IMA player and letting the IMA player make the request to GAM still results in better revenue.

That is why, in the above case, using a hybrid approach is the best solution that doesn't require an external VAST cache:

• When you have a winning prebid bid, you ask Prebid to contact GAM and download the VAST XML, and pass that to the IMA player using the getVastXml method
• When you don't have a winning prebid bid, you ask Prebid for the GAM URL and let the IMA player do the request using the buildVideoUrl method

I want to be able to use the second approach (pass a GAM url to IMA, and let IMA do the request) when I have a winning prebid bid that doesn't need the local cache.

Currently that isn't possible because e.g. for Rubicon, when I receive a bid which contains

vastUrl: https://prebid-server.rubiconproject.com/cache?uuid=foo

the GAM URL I get from Prebid will contain

hb_uuid=bar
hb_uuid_rubicon=bar

where bar is a uuid from the local cache.

However, if at this point I would let the IMA player make the request, the replacing of bar with foo won't happen and the IMA player cannot download the VAST XML.
That forces me to use the getVastXml method instead of being able to use the buildVideoUrl method with the improved GAM auction.

I think this issue would need changes in 2 places:

---

Skip local storage when not needed:

export const storeLocally = (bid) => {
  //When the bid is already stored in a remote cache, no need to store it locally
  if(bid.vastUrl){
    return
  }
  const vastXml = getVastXml(bid);
  const bidVastUrl = URL.createObjectURL(new Blob([vastXml], { type: 'text/xml' }));

  assignVastUrlAndCacheId(bid, bidVastUrl);

  vastLocalCache.set(bid.videoCacheKey, bidVastUrl);
};

---

Only try replacing UUIDs in the bid when it is coming from the remote cache

In getVastForLocallyCachedBids, replace the

if (!vastAdTagUriElement || !vastAdTagUriElement.textContent) {
      return gamVastWrapper;
}

check with

if (!vastAdTagUriElement || !vastAdTagUriElement.textContent || !vastAdTagUriElement.textContent.startsWith(config.getConfig('cache.url'))) {
      return gamVastWrapper;
}

This relies on the cache.url being specified, but according to the docs, this is a required property anyway.

[dgirardi]

Hey @robin-crazygames, I'm not sure I understand the scenario here (and in #14638 which follows your suggestions).

when I have a winning prebid bid that doesn't need the local cache

Are you setting up a GAM creative specifically for rubicon, and relying on the rubicon adapter to provide hb_uuid (through videoCacheKey)? Or otherwise how are you getting GAM to serve the same url that's in the bid?

Only try replacing UUIDs in the bid when it is coming from the remote cache

That piece of logic is part of the getVastXml flow, which would not run if you're trying to get IMA to do it. It should also already skip the replacement if it can't find a cached document corresponding to the uuid.

[dgirardi]

Is the issue that the local cache always overrides cache keys provided by adapters? In

Prebid.js/src/auction.ts

Lines 646 to 657 in 92d5f0a

	if (useLocal) {
	// stores video/audio bid vast as local blob in the browser
	storeLocally(bidResponse);
	} else if (cacheUrl && (useCacheKey || context !== OUTSTREAM)) {
	if (!bidResponse.videoCacheKey || ignoreBidderCacheKey) {
	addBid = false;
	callPrebidCache(auctionInstance, bidResponse, afterBidAdded, videoMediaType);
	} else if (!bidResponse.vastUrl) {
	logError('videoCacheKey specified but not required vastUrl for video bid');
	addBid = false;
	}
	}

:

the "normal" cache flow used to run when all of the following are met:

1. cache.url is set
2. the ad unit's .mediaTypes.video has either context !== 'outstream' OR useCacheKey == true
3. the adapter does not fill in the bid's .videoCacheKey OR cache.ignoreBidderCacheKey is set

But if the local cache is enabled (cache.useLocal) it always runs regardless of what else is in the ad unit / bid. Perhaps we should replace (1) above with

1. either cache.url OR cache.useLocal is set?

[robin-crazygames]

What we currently do

• Do a Prebid auction with local VAST cache enabled
• Depending on the outcome of the auction:
  • When no bid: ask Prebid for a GAM url, and pass that URL to the IMA player. IMA will go to GAM where another auction will happen and then the VAST XML is returned to the IMA player
  • When Prebid bid: ask Prebid to download VAST XML (meaning Prebid goes to GAM, and the GAM auction happens), and pass that VAST XML document to the IMA player.

The drawback of this approach is that our GAM auctions (including AdX and our direct campaigns) result in way lower CPMs when Prebid downloads the VAST XML compared to when the IMA player contacts GAM. There is still something missing/different in the request that Prebid makes compared to the request from the IMA player.

So, if we have a winning Prebid bid that doesn't really benefit from the local cache, we want to avoid using it so that we can pass an URL to IMA, and have a better GAM auction.

Are you setting up a GAM creative specifically for rubicon

Yes, we have creatives for each bidder. The one for Rubicon looks like

https://prebid-server.rubiconproject.com/cache?uuid=%%PATTERN:hb_uuid_rubicon%%

While for example the one for TTD looks like

https://prebid.adnxs.com/pbc/v1/cache?uuid=%%PATTERN:hb_uuid_ttd%%

I hope this answers all your questions

[dgirardi]

Thanks for clarifying. Did you use the same setup before local caching was available? I think that

a winning Prebid bid that doesn't really benefit from the local cache

would also not benefit from any other cache, so the fix should be to trigger local caching on the same conditions that a remote cache would, as outlined above. Then adapters like rubicon that provide a .videoCacheKey can skip it if you set up a GAM creative for them.

[robin-crazygames]

Did you use the same setup before local caching was available?

Yes, we did. We haven't changed anything to our GAM setup. It was always the same, independent of our usage of the local cache.

would also not benefit from any other cache

True. But as you said, at the moment Prebid always enter the local cache path. There aren't any conditions in there. Which is why I logged this issue.
But my proposed solution might be far from the ideal one (which is also why I only logged an issue and not just opened a PR).