Design an ad frequency capping system

Design a frequency capping system for an ads platform.

An ad server must decide, for each ad impression opportunity, whether showing an ad would violate frequency caps such as:

• Per-user per-campaign cap (e.g., max 3 impressions per user per 24 hours)
• Per-user per-ad cap (creative-level)
• Optional: per-user per-advertiser cap
• Optional: cross-device identity (multiple device IDs map to one user)

Requirements

1. Online decisioning : At serve time, given (user_id, candidate_campaign_ids, now) , filter out campaigns that would exceed caps.
2. Event ingestion : After an ad is shown, an impression event must update the counters used for future decisions.
3. Time windows : Support rolling windows (e.g., last 1 hour, 24 hours, 7 days).
4. Scale (assume): 100K–1M QPS serving; similar order impression events.
5. Latency : p99 decisioning latency budget ~10–20 ms at the ad server.
6. Correctness : Prefer not to exceed cap; small under-delivery is acceptable. Handle duplicates and late events.

Deliverables

• APIs and data flow
• Storage/data model for counters
• How you enforce rolling time windows
• Consistency strategy (exact vs approximate)
• Failure modes, edge cases, and monitoring