Design an ad frequency capping system

Q: Design an ad frequency capping system

This question evaluates a candidate's ability to design scalable, low-latency distributed systems that enforce ad frequency caps across users, devices, and profiles, testing competencies in system architecture, data modeling, consistency, and real-time enforcement; it falls under the System Design domain and requires both high-level architectural understanding and practical application-level trade-off reasoning. It is commonly asked to assess how applicants reason about scalability, availability, latency-consistency trade-offs, API and data flow design, and failure modes when enforcing multi-window caps at high QPS.

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Question

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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?