Class modelling, SOLID and extensibility. Practise the round where the follow-up is always "now add this requirement".
Low level design rounds have a predictable shape: you model a domain in classes, and then the interviewer adds a requirement that your first design did not anticipate. Electric charging bays in the parking lot. Multiple elevators with a dispatch policy. Tournament mode in the chess game. How much you have to rewrite is the score.
That is why SOLID matters here as a practical tool rather than an exam topic. A design where pricing is a strategy, vehicle types are open for extension, and the allocation policy sits behind an interface absorbs the new requirement in one class. A design with a switch statement over vehicle type in three places does not.
The other half of the round is restraint. Candidates who apply five design patterns to a problem that needed one read as someone who has memorised patterns rather than someone who has maintained code. Interviewers notice the difference, and the simpler defensible design usually wins.
A free LLD mock interview on InstaMock runs the round end to end. You get a problem statement with its requirements, a timer, and an AI interviewer that asks you to justify your classes, then changes the requirements the way a real interviewer would. At the end you receive a scored feedback report with the areas to improve, and you can retake the same problem or move to the next one. There is no credit card and no trial period.
If you want a person on the other side, a 60-minute 1:1 LLD mock interview with a working engineer is ₹499, or ₹899 for two. Most candidates use the free AI rounds to get the structure right and book a live round in the week before the real interview.
5-10 min
Clarify behaviour and boundaries. Ask what is in scope before modelling.
15 min
Classes, relationships and responsibilities. Where the round is usually won.
15 min
Strategy, factory, observer and state - applied where they earn their cost.
10-15 min
A new requirement is added. How much of your design has to change?
10 min
Shared state, race conditions and invalid transitions.
0-2 years
Sensible entities, single responsibilities and working code. Patterns are a bonus, not the bar.
2-5 years
Interfaces at the points that change, a pattern you can justify, and a new requirement absorbed without rewrites.
5+ years
Thread safety, invalid state transitions, and an explicit account of what the design makes easy and what it makes hard.
A low level design interview asks you to model a bounded problem in classes - a parking lot, an elevator, a booking system - with clear responsibilities and extension points. It tests object-oriented judgement rather than distributed systems knowledge.
HLD is about services, storage and scale. LLD is about classes, responsibilities and extensibility inside one service. HLD asks how it survives a million users; LLD asks how it survives the next requirement.
As few as the problem needs. Applying five patterns where one would do is a recognised negative signal - it reads as memorisation. Use a pattern when you can name the change it makes cheap, and say so.
Closely related. A machine coding round is an LLD problem you must also implement in working code within 60 to 90 minutes. Indian product companies run it frequently, so practise designing and typing under the same clock.
Yes. AI LLD mock interviews on InstaMock are free, with scored feedback and no credit card. A live 1:1 LLD round with an engineer is the paid option: ₹499 for one 60-minute interview or ₹899 for two.
You get a problem statement and a timer. The AI interviewer asks you to scope the requirements, model the classes and explain your choices, then adds a new requirement to test how well the design extends. You finish with a scored feedback report that lists what to improve.
Parking lot, elevator system, vending machine and snake and ladder. Between them they cover strategy and state patterns, extensibility, and concurrency, which are the ideas most LLD rounds return to.
Plan for 45 to 60 minutes, which matches most real LLD rounds: about 10 minutes on requirements, 30 on the design and its patterns, and the rest on the extension challenge and edge cases.
Whichever object-oriented language you write most fluently - Java, Python, C++ or another. Interviewers grade the class design, not the syntax, but they expect the code to be idiomatic for the language you chose.
Model a domain, then defend it when the requirement changes.