Prebid.js Frequently Asked Questions

This page has answers to some frequently asked questions about Prebid.js. If you don’t find what you’re looking for here, there are other ways to get help.

When starting out, what should my timeouts

Below is a set of recommended best practice starting points for your timeout settings

  • 1,000 milliseconds or less for the internal auction timeout
  • 3,000 milliseconds or less for the prebid tag’s overall failsafe timeout

The former setting is used to track the auction once it started; if it expires, we will use whichever bidders have responded and select the winner(s) accordingly. 

The latter setting is used when for some reason Prebid did not load (or there’s some other serious issue); if it expires, we will default to the adserver.

For examples of setting up these timeouts, please refer to the Basic Example page.

How many header bidders should I have?

Every publisher is different. In order to answer this question you’ll need to run some tests, gather data, and decide what works for you based on your performance and monetization needs.

Generally speaking, in a client-side header bidding implementation, you should aim to bring in approximately 1-5 demand partners. In a server-to-server implementation, you have some flexibility to add more partners.

In both scenarios, your goal should be to see your inventory fill at the highest CPMs without adding too much latency in the process. When selecting your demand partners, it’s important to choose marketplaces that have premium demand at scale, high ad quality and low latency.

There is an analysis from the Prebid team here which may be useful:

How many bidders should I work with?

Does Prebid.js cache bids?

Yes. As of version 1.0, Prebid.js will re-consider previous bids under limited circumstances. It will cache and reconsider bids in refresh scenarios when the bid is:

  • for the same AdUnit,
  • on the same page view,
  • for the same user, and
  • up to a certain Time-to-Live (TTL) or until the bid wins and is displayed.

Since the storage is in the browser, cached bids only apply to a single page context. If the user refreshes the page, the bid is lost.

Each bid adapter defines the amount of time their bids can be cached and reconsidered. This setting is called “Time to Live” (TTL), documented here.

Examples of scenarios where a bid may be reconsidered in Prebid.js:

  • Auto-refresh: Some pages will reload an AdUnit on a set interval (often 60-240 seconds). Previous bids for that particular AdUnit can be reconsidered for subsequent refreshes of that unit up to the TTL or until they win the unit.
  • Infinite scroll: As the user scrolls, the same AdUnit may be dynamically created over and over. The bid can be reconsidered for dynamically-created AdUnits with the same name. Again, the bid is only re-considered on that AdUnit up to the bid TTL or until it’s displayed.
  • Galleries: Some pages feature carousel-style galleries that contain an AdUnit that refreshes as the user cycles through the content in the gallery.

Here’s how it works:

  • Bid responses are stored in an AdUnit-specific bid pool.
  • When the same AdUnit is called, Prebid.js calls the bidder again regardless of whether there’s a bid in that AdUnit’s bid pool.
  • When all the new bids are back or the timeout is reached, Prebid.js considers both the new bids on that AdUnit and previous bids on the AdUnit that haven’t reached their TTL.
  • The cached bid is only used if its CPM beats the new bid.
  • Bids that win are removed from the pool. This is automatic for display and native ads, and can be done manually by the publisher for video ads by using the markWinningBidAsUsed function.

Some of my demand partners send gross bids while others send net bids; how can I account for this difference?

You will want to adjust the gross bids so that they compete fairly with the rest of your demand, so that you are seeing the most revenue possible.

In Prebid.js, you can use a bidCpmAdjustment function in the bidderSettings object to adjust any bidder that sends gross bids.

Does Prebid.js support synchronous ad server tags?

Short answer: not out of the box, because of header bidding partners’ limitations. But there are workarounds.

Take GPT synchronous mode as an example - if you’re loading GPT synchronously, there is no simple way of delaying GPT library loading to wait for bidders’ bids (setTimeout() cannot be used).

Therefore, it requires Prebid.js to run in a blocking/synchronous fashion. This will require all header bidding partners’ code to be blocking/synchronous. We’re not even sure if this is possible. We do not have a great out-of-the box solution for turning Prebid.js blocking at the moment.

Here are a couple of alternative workarounds:

  1. Option 1Load a blocking script that has a load time of 300-500ms. This script does nothing but keep the page waiting. In the meantime Prebid.js can run asynchronously and return the bids. After the blocking script finishes loading, GPT can start synchronously; at this point there will be header bidding bids available.
    For the best user experience, you probably want to insert this blocking script after the above the fold page content has loaded. Or if you’re okay with additional 500ms latency added to your page load time, this can be easily done.
  2. Option 2Use post-bid. The downsides are that post-bid no longer allows your header bidding partners to compete with DFP/AdX, but they can still compete with each other. For more information, see What is post-bid?

How do I use Prebid.js on secure (HTTPS) pages?

All prebid adapters that get merged should automatically detect if they’re serving into a secure page environment and respond appropriately.

In other words, you shouldn’t have to do anything other than make sure your own page loads Prebid.js securely, e.g.,

(Except that you should never never never use the copy of Prebid.js at that URL in production, it isn’t meant for production use and may break everything at any time).

How often is Prebid.js updated?

See the github release schedule for more details.

When do I have to upgrade my version of Prebid.js? does not support any version of Prebid.js prior to version 1.0. If you want continued support through updates and documentation you should upgrade to a newer version.

How can I change the price granularity for different ad units?

If you need different price granularities for different AdUnits (e.g. video and display), the only way for now is to make sure the auctions don’t run at the same time. e.g. Run one of them first, then kick off the other in the bidsBackHandler. e.g. here’s one approach:

  • Call setConfig to define the priceGranularity for the first set of AdUnits
  • Initiate the first auction with requestBids
  • In the bidsBackHandler
    1. Set the adserver targeting for the first auction
    2. Call setConfig to define the priceGranularity for the second set of AdUnits
    3. Initiate the second auction with requestBids
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