Design a vaccine booking platform like CoWIN
PremiumBackend systemHard
- Slot inventory
- Release time traffic spike
- Atomic conditional update
Quick answer
This backend system interview question asks you to design a vaccine booking platform like CoWIN. 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 vaccine booking platform like CoWIN, a national system where citizens register, find a nearby centre, book a dose slot and get a signed certificate. The hard parts are millions of people booking the moment new slots open, never overbooking a session, and dose-interval rules.
Centres publish daily sessions, each with one vaccine, an age group, a dose number and a capacity. Citizens log in with a mobile number and an SMS OTP; one mobile number can register up to 4 people, each with a government photo ID number. At the centre, a vaccinator checks the person, records the dose, and the certificate follows.
What must a vaccine booking platform like CoWIN do?
- Search and book: Search centres by district or PIN code and date, see remaining capacity per session, and book one slot for one registered person.
- Eligibility rules: Check age group and dose number; dose 2 only 28-42 days after dose 1 for vaccine A or 84-112 days for vaccine B, same vaccine.
How well must it do it?
- No overbooking: A session never confirms more appointments than its capacity, even with 20,000 requests for its last slots in one second.
- Spike tolerance: Search and booking stay up when new slots open at a fixed time and tens of millions of people arrive at once.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- Slot inventory
- Release time traffic spike
- Atomic conditional update
- Eligibility rules
- Idempotent sync
- Signed QR certificates
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 vaccine booking platform like CoWIN and explain your choices the way you would in an interview.
- Is this question asked at entry level?
- It is marked Hard. It suits engineers preparing for senior interviews, where you are expected to drive the discussion and go deep on trade-offs.
- 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