Design an ad frequency capping system

Q: Design an ad frequency capping system

This question evaluates competency in distributed systems architecture, real-time stateful services, high-cardinality time-series data modeling, API design for low-latency decision paths, and operational monitoring including consistency and latency trade-offs.

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Question

Design a real-time ad frequency capping system for an ads platform.

When the ad server is deciding whether to show an ad, it must enforce caps such as:

Per user per campaign : at most K impressions in the last T hours (rolling window).
Optional: per user per advertiser, per user per creative, etc.

Requirements:

Very low latency on the serving path (single-digit ms budget).
Very high QPS (millions/sec) and large cardinality (users × campaigns).
Events are generated on every impression and may arrive late/out of order.
Define what consistency you need (strict vs best-effort) and justify tradeoffs.
Support multiple time windows (e.g., 1 hour, 24 hours, 7 days).
Provide APIs for: (1) check-and-reserve (decision time) and (2) impression logging.

Explain data model, storage choices, write/read paths, deduplication, TTL/expiry, sharding strategy, and how you monitor/alert on correctness and system health.

Design an ad frequency capping system

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