Prebid Server Adapter local storage issue affecting /cookie_sync

[bretg]

Type of issue

Bug

Description

Under certain conditions, the Prebid Server Adapter code is improperly skipping the send of /cookie_sync requests. The /cookie_sync request does appear to have a solid impact on cookie match rate based on a limited data set.

The main impact here is that the current design will affect users who hit web sites using different clusters of Prebid Servers.

The code in question is in prebidServerBidAdapter.js:

    if (_cookiesQueued || syncedList.length === bidderCodes.length) {
      return;
    }

So /cookie_sync isn't sent in two scenarios:

1. if there's already been a /cookie_sync request sent
2. if the number of adapters in localStorage (pbjsSyncs) matches the number of adapters in the AdUnit

I ran into this when I had 2 bidders in localStorage (rubicon and appnexus), and 2 bidders in an AdUnit -- rubicon and a different bidder (not appnexus). Didn't see cookie_sync go out, despite there not being a uids cookie. The code is taking a shortcut here by comparing list length rather than a deep comparison. Also, it doesn't seem right that it's comparing the the pbjsSyncs list with the bids in the AdUnits -- should be comparing the bidders configured for S2S.

But more importantly, the design doesn't currently consider that a single domain might hit two clusters of Prebid Servers (e.g. A/B mode), or change from one cluster to another. In that scenario, the page will suppress /cookie_sync, which has a noticeably positive effect on user ID match rates.

Steps to reproduce

1. Set localStorage (pbjsSyncs) to contain two entries. e.g. ["rubicon", "appnexus"]
2. Set up an AdUnit with 2 bidders. e.g. "appnexus" and "pubmatic"
3. Set up the S2S config with 1 bidder. e.g. "appnexus"
4. remove the uids cookie

Expected results

The /cookie_sync request should go out because the uids cookie is empty.

Actual results

/cookie_sync won't go out because there are two entries in pbjsSyncs localStorage and and two bidders in the AdUnit

[mkendall07]

thanks for the bug report. We can store sync flag in the localstorage namespace of the endpoint for the problem of multiple instances of prebid server if that makes sense. Although I'd think the cookies should probably be shared (technically not feasible with cookie but might be with localstorage). I'd propose fixing in 2 steps, the first fix being to make sure everything is synced and then the 2nd being handling multiple instance of PBS.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.