Design a gym class booking system
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- Waitlist
- SOLID principles
- Strategy pattern
Quick answer
This machine coding interview question asks you to design a gym class booking 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
Build a gym class booking system in memory, like the class schedule in a fitness chain's app. Admins add centres and workout classes; members book, cancel and join waitlists. The hard part is keeping capacity, waitlists and member rules correct, in code clean enough to change live.
The round has two parts. First you write the code. Then the interviewer reads it with you and asks for changes on the spot, so object design, design patterns and SOLID matter as much as a working demo.
What must a gym class booking system do?
- Book a class: book(userId, classId) confirms a seat if one is free, else adds the member to a waitlist of at most 5, else fails with CLASS_FULL.
- Cancel and promote: cancel(bookingId) frees the seat and promotes the first eligible waitlisted member; members may cancel until 30 minutes before the start.
How well must it do it?
- Correct capacity: Confirmed bookings never exceed capacity, and waitlist order is first come, first served.
- Easy to change: New sort orders, member rules or promotion rules plug in without editing the booking service.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- Waitlist
- SOLID principles
- Strategy pattern
- Observer pattern
- Overlap detection
- Capacity limits
How you’ll practise
- Requirements
- Entities and interfaces
- Core implementation
- Edge cases and tests
- Extensions
- 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 machine coding 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 machine coding question. You design a gym class booking 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.
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Updated · Written by the SevyDevy team