Ads Frequency Cap
An Ads Frequency Cap system limits how many times a user sees a specific ad or ad category within a given time window. This is crucial for advertisers—showing the same ad too many times leads to user fatigue and wasted impressions, while showing it too few times means missed opportunities.
Unlike a standard Rate Limiter, ads frequency capping has a unique characteristic: the read and write paths are separated. During ad bidding, you check if an ad is eligible (read), but a successful bid doesn't mean the ad gets shown. The actual impression is recorded later through event tracking (write). This separation fundamentally changes the design.
Why Ads Frequency Capping Matters
Without frequency capping:
- User fatigue — Seeing the same ad 50 times in a day annoys users and hurts brand perception
- Wasted spend — Advertisers pay for impressions that provide diminishing returns
- Poor allocation — Budget could be better spent reaching new users
Frequency caps are typically set by advertisers as part of their campaign configuration:
- "Show this ad at most 3 times per user per day"
- "Show ads from this campaign at most 10 times per user per week"
- "Show this ad category at most 5 times per user per hour"
In interviews, connect this to the broader ad-serving ecosystem. Frequency capping is one of many signals in the ad selection process—alongside targeting, bidding, and relevance scoring. Understanding where it fits shows depth.