Design an ad frequency capping system

System Design: Ads Frequency Capping

You are designing an ad-serving platform (e.g., for a streaming service with ads). The product requires frequency capping so that a user does not see the same ad/campaign too many times.

Requirements

Functional

• Given an ad request (user_id, device_id, profile_id?, placement, timestamp, candidate_ads) , return an ad that respects frequency caps .
• Support caps at multiple levels:
  • Per creative (exact ad)
  • Per campaign / line item
  • Optionally per advertiser
• Support common cap windows, e.g.:
  • N impressions per 24 hours
  • M impressions per 7 days
  • K impressions per session
• Caps must work across devices (same user on TV/phone/web).
• Handle multiple profiles under one account (if applicable): clarify whether caps are per-account, per-profile, or both.

Non-functional

• Very low latency (ad decisioning is on the critical path).
• High availability.
• Cap enforcement should be close to real-time ; define acceptable staleness (e.g., up to a few seconds).
• Must scale to large traffic (think millions of QPS globally).

What to design

• High-level architecture and data flow.
• APIs/interfaces.
• Data storage model for tracking impressions.
• How you enforce caps during ad selection.
• Trade-offs: strictness vs latency, consistency choices, and failure behavior.

Edge cases

• Duplicate impression events (retries).
• Offline/late-arriving events.
• Multiple ad servers/regions making decisions concurrently.
• What happens if the frequency-cap store is unavailable?