Design a personalized deals service
PremiumBackend systemMedium
- Personalized ranking
- One claim per user
- Atomic counters
Quick answer
This backend system interview question asks you to design a personalized deals service. 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 a personalized deals service for a shopping app: each week merchant deals are loaded, and each shopper sees the active deals ranked by their browsing history, filters and pages through them, and claims a deal once. The hard parts are ranking for millions of users, stable pages, and exact claims when thousands rush one deal.
Each deal has a start time, an end time, a category, a price, a discount and a quantity (how many claims are available). A claim gives the user a code to use later; using the code at checkout is handled elsewhere.
What must a personalized deals service do?
- Ranked feed: Show active deals (start time ≤ now < end time) ranked for the user, 20 per page.
- One claim per user: A user claims a deal at most once and gets a code; claims stop when the quantity runs out.
How well must it do it?
- Exact claims: No over-claiming and no double claim, even at 3,000+ claim attempts per second on one deal.
- Latency: Feed pages under 200 ms at p99; claims under 300 ms at p99.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- Personalized ranking
- One claim per user
- Atomic counters
- Cursor pagination
- Time windowed deals
- Candidate retrieval
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 a personalized deals service 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.
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Updated · Written by the SevyDevy team