Waterfall Auction
A sequential ad auction model where demand sources are called one at a time based on priority.
What is a Waterfall Auction?
A waterfall auction (also called a daisy chain or sequential auction) is a method of selling ad inventory where demand sources are arranged in a priority order and called sequentially. The ad server starts with the highest-priority demand source. If that source cannot fill the impression (or bids below a set floor price), the request "waterfalls" down to the next source in line, and so on until the impression is filled or all sources are exhausted.
Priority in a waterfall is typically set based on historical average CPMs. The demand source with the highest historical eCPM gets first look at every impression. This seems logical but creates a fundamental inefficiency: historical averages don't reflect the real-time value of individual impressions.
Why It Matters for Publishers
Understanding the waterfall model is important because many publishers still use it, either exclusively or in combination with header bidding. While header bidding has largely replaced pure waterfall setups for web publishers, waterfall remains common in mobile app monetization and among publishers who haven't migrated to more advanced setups.
The primary problem with waterfalls is that they leave money on the table. A demand source ranked third in the waterfall might be willing to pay $8 for a specific impression, but it never gets the chance to bid because the first-ranked source fills it for $3. The publisher loses $5 on that impression because the waterfall doesn't allow real-time competition.
Tips for Optimization
- Migrate to header bidding: If you're still running a pure waterfall, implementing header bidding should be your top priority. The revenue uplift is typically 20-50%.
- If using waterfall, reorder frequently: Review demand source performance weekly and reorder the waterfall based on recent eCPM data, not just historical averages.
- Set floor prices at each level: Require minimum bids at each waterfall tier to prevent low-quality fills from blocking higher bids further down the chain.
- Configure passbacks properly: Ensure each demand source is configured to passback quickly when it can't fill, rather than holding the request and adding latency.
- Use hybrid setups: Combine header bidding with a waterfall fallback. Header bidding handles the primary auction, and the waterfall catches any impressions that don't get bids from the header bidding partners.