Design an ad frequency capping system
PremiumBackend systemMedium
- Frequency capping
- Sliding window counters
- Ad decisioning
Quick answer
This backend system interview question asks you to design an ad frequency capping system. Aim for a design that meets its requirements at the stated scale, then explain its main components, data flow and trade-offs. Interviewers want to see how you scope the problem, size it and defend each choice.
The problem
Design an ad frequency capping system for a streaming service's ad-supported plan. It makes sure a viewer sees a campaign at most K times in T hours across TV, phone and web. The hard part is checking caps within 10 ms at ad decision time while impressions arrive from many devices and regions.
What must an ad frequency capping system do?
- Cap check: Given a profile and up to 50 campaigns, return in one call which ones are under every cap.
- Count impressions: Every played ad counts once toward its campaign's caps for that profile, whatever the device.
How well must it do it?
- Latency: Cap check p99 under 10 ms, measured at the ad decision service.
- Cap accuracy: A cap may be exceeded by at most 1 impression, and only in rare cross-region races; under-delivery is preferred to over-delivery.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- Frequency capping
- Sliding window counters
- Ad decisioning
- Redis
- Multi region replication
- Idempotency
How you’ll practise
- Requirements
- Estimate
- API and data
- High-level design
- Deep dives
- Wrap-up
Lala, personalized to you, gives a hint when you ask and reviews your design when you are ready.
What do interviewers look for?
In any backend system design round.
- Clarifies the requirements before drawing anything
- Uses a few numbers that actually change the design
- Draws a clear path from the client to storage
- Picks storage and caching with reasons, not by habit
- Explains what breaks first under load and how to fix it
- States trade-offs openly instead of claiming a perfect design
Questions people ask about this interview question
- Which design type is this question?
- It is a backend system question. You design an ad frequency capping system and explain your choices the way you would in an interview.
- Is this question asked at entry level?
- It is marked Medium. Entry-level engineers can practise it, but it expects you to reason about requirements and trade-offs, not only list components.
- What should I cover first?
- Start with the requirements: what the system must do, how well it must do it and which numbers change the design. Agree on scope with the interviewer before you design anything.
- Can I practise it for free?
- It is a premium question. Sign up and unlock it once for 10 credits; it then stays unlocked. When you finish, Lala, your personalized AI mentor, can review your design for 30 credits.
Related questions
Updated · Written by the SevyDevy team