Tech Interview Preparation Module 7
LLD and system design rounds interview questions
Interview questions on LLD and system design rounds: short warm-ups, then what each harder question tests, the first step of a good answer and its follow-ups.
Warm-up questions
Short questions with short answers, each answered in full.
-
Fresher to mid
What is a machine-coding round, and how is it different from a design discussion?
Answered in the lesson LLD and machine-coding rounds: delivering on time
-
Fresher to mid
How do you start a system design interview?
I say the prompt back in one sentence, so that we are designing the same thing, and then I ask for requirements before I draw anything. First the functional ones, what users must be able to do, then the non-functional ones: how many users, how fast, how available, and which promise must never break, such as a seat never being sold twice. I write both lists where the interviewer can see them. Then I spend about three minutes on the estimates that decide something, such as peak requests and writes a second, and say what each decides, before the API and the data model. The first five minutes produce no boxes, but they decide where the rest of the 45 go.
From the lesson Running a 45-minute system design round
-
Mid to senior
What does an interviewer expect from a senior candidate in a design round that they do not expect from a mid-level one?
The same design, with more questions answered about it. A mid-level answer shows a design that works for the stated requirements. A senior answer starts by scoping the ambiguous prompt into promises, a boundary and a number, and then says how each part fails, how anyone would notice, and what the trade-off of each choice costs, including for other teams. It also removes parts the numbers do not need, because every component has to be run by someone. At staff level the answer adds what the design costs the organisation to own, how to migrate to it from what exists, whether a second region is worth it, and how to agree the parts other teams own.
From the lesson Design rounds at senior and staff level
Questions with model answers
What each question tests, the first step of a good answer and the follow-ups to expect.
-
Fresher to mid
Forty minutes into a 90-minute machine-coding round you have every class typed, but no command runs yet. What do you do with the next fifty minutes, and what do you tell the interviewer?
- What it tests
- Re-planning a build around working slices, protecting the tests and the demo, and saying the new plan aloud.
- First step
- Stop adding classes. Say where you are and what you will do: wire the one command the prompt cannot do without from the parser to the reply, run it against two script steps within about 15 minutes, then add the next command the same way.
- Follow-ups
-
- The first run fails in two layers at once. Where do you look first?
- The interviewer asks what you will leave out. What do you say?
- What would you change in the first ten minutes next time?
From the lesson LLD and machine-coding rounds: delivering on time
Model answer, Pro. Model answers open with any pass; prices are in the notice on this page.Opening the model answers with your pass…
Model answers open with any MySmartCoPilot pass.
- Pro pass · 1 week₹79$2.99
- 30 days₹179$5.99
- 1 year₹1,699$45
One payment; a pass never renews. The price shown is the total you pay: no fees or charges are added at checkout.
Passes can’t be bought right now, so the model answers can’t be opened at the moment. If you have a pass, enter your licence key.
It also opens every other in-depth part in Learn and every Pro tool. Sales are final, except where the law gives you a right to withdraw, for example in the EU and the UK (refund policy).
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 3 minutes
-
Mid to senior
At minute 80 of a machine-coding round the interviewer asks you to support shelves in two sizes, and your desk has one size. With ten minutes left, what do you build, what do you say and what do you leave out?
- What it tests
- Judging what fits in ten minutes, keeping working code working, and showing where a change belongs in the design.
- First step
- Before typing, name the parts the change touches and the parts it leaves alone, and estimate the minutes it needs. If it fits in ten, offer to build it with new script steps while every old step keeps passing; if not, offer to describe it instead.
- Follow-ups
-
- Halfway through, an old step starts failing. What now?
- The interviewer asks you to describe the change instead of coding it. What do you say?
- How would sizes work if new ones had to be added without changing code?
From the lesson LLD and machine-coding rounds: delivering on time
Model answer, Pro.In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 3 minutes
-
Fresher to senior
Your machine-coding solution keeps everything in memory. The interviewer asks why you did not use a database, and what it would take to add one. How do you answer?
- What it tests
- Keeping storage apart from the rules, defending a choice of scope, and describing a change that touches one part.
- First step
- Say the choice was deliberate: the prompt asks for rules, and an in-memory store behind the desk's methods keeps them testable in milliseconds. Then name the change: a store that saves to a database behind the same methods, with the same acceptance script proving that nothing else changed.
- Follow-ups
-
- Two desks now share the shelves. What breaks?
- How would you test the database store?
- When would you have used a database from the start?
From the lesson LLD and machine-coding rounds: delivering on time
Model answer, Pro.In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 3 minutes
-
Mid to senior
Twelve minutes are left for deep dives, and you can see three risky parts in your design. How do you choose what to go deep on, and what do you say about the parts you leave?
- What it tests
- Ranking risks by their consequences and by the numbers, involving the interviewer in the choice, and closing the rest with a sentence each.
- First step
- Name the three risks in a sentence each and rank them by what breaks if they fail and by what the estimates said is hard. Propose the top one or two for the twelve minutes and ask whether the interviewer would rather see another.
- Follow-ups
-
- The interviewer picks the risk you ranked lowest. What now?
- Your first deep dive runs over. How do you protect the second?
- What do you say about the part you never reach?
From the lesson Running a 45-minute system design round
Model answer, Pro.In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 3 minutes
-
Fresher to senior
Your estimate says the system needs about 200 requests a second, but the interviewer seems to expect a large distributed design. What do you do?
- What it tests
- Trusting and explaining an estimate, sizing a design to the stated load, and showing how it would grow without building for a load nobody asked for.
- First step
- Say the estimate and what it decides: at 200 requests a second a few servers and one database are enough. Then ask whether the requirements should change, for example a larger audience or tighter latency, and offer to show what would change at ten and a hundred times the load.
- Follow-ups
-
- The interviewer says the load is in fact 200,000 requests a second. What changes first?
- How do you avoid sounding as if you cannot design at scale?
- What would still need care at 200 requests a second?
From the lesson Running a 45-minute system design round
Model answer, Pro.In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 2 minutes
-
Mid to senior
Your recruiter tells you that the design round is product architecture rather than systems design. How do your preparation and your 45-minute plan change?
- What it tests
- Telling the two kinds of design round apart, moving the minutes towards the API and the data, and preparing the topics a product round weighs.
- First step
- Keep the seven blocks, but plan to move minutes from the boxes and the deep dives to the API and the data model. Then prepare the API side of each practice prompt: its calls with their inputs and outputs, and who owns each piece of data.
- Follow-ups
-
- The interviewer starts asking about sharding anyway. What do you do?
- Which mistakes are most common in an API design?
- How do you practise for this kind of round?
From the lesson Running a 45-minute system design round
Model answer, Pro.In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 2 minutes
-
Senior to staff
In a deep-dive round you are asked about the most complex system you have supported, and its exact numbers are confidential. How do you present it so that it is both convincing and safe?
- What it tests
- Preparing a past system as evidence, keeping confidences while still giving scale, and showing your own decisions and their trade-offs.
- First step
- Before the day, choose the system and decide what you may say about it. Open with the context in general terms and your own part in it, then ask whether the interviewer would rather start from the architecture or from a decision you made.
- Follow-ups
-
- The interviewer asks for the exact figure. What do you say?
- How much of the system was your own work, and how do you show it?
- What would you change in it today, and why?
From the lesson Design rounds at senior and staff level
Model answer, Pro.In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 2 minutes
-
Senior to staff
Your design replaces an old notification system that 40 other services call. How do you migrate them without downtime, and how do you present that plan in the round?
- What it tests
- Planning a migration as part of a design, limiting the risk of each step, and keeping a way back.
- First step
- List the 40 callers and how critical each one is, and put a front in place that keeps the old interface, so that no caller has to change on the day the new system starts.
- Follow-ups
-
- Two of the forty callers cannot change for six months. What now?
- How do you know that the new system is delivering correctly?
- When do you switch the old system off?
From the lesson Design rounds at senior and staff level
Model answer, Pro.In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 2 minutes
-
Senior to staff
The interviewer asks what you would change in your design to cut its running cost by a third without breaking any promise. How do you answer?
- What it tests
- Finding where the cost is, separating what each promise needs from what it does not, and removing components with a reason.
- First step
- Find where the money goes before changing anything: break the bill down by component and by channel, and write next to each large item the promise it serves.
- Follow-ups
-
- The biggest cost is a component another team depends on. What now?
- How do you prove that no promise was broken?
- When is cutting cost the wrong goal?
From the lesson Design rounds at senior and staff level
Model answer, Pro.In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 2 minutes
All Tech Interview Preparation interview questions · The Tech Interview Preparation track