System design mock interviews with an AI interviewer
A system design interview hands you a deliberately vague prompt (design a URL shortener, a chat service, a news feed) and watches what you do with it. Strong candidates scope the problem, sketch a design that works, then go deep on the parts that break at scale, saying out loud which trade-offs they're making.
It's the round that's hardest to practice alone, because the value is in the pushback. PrepLinx's AI interviewer gives you the problem in stages, reads the diagram you draw on the whiteboard, and asks what a real interviewer would: where's the bottleneck, what happens when this region goes down, how does this behave at 10x the load.
What this round tests
Requirements and scope
Clarifying functional and non-functional requirements and estimating scale before you design.
High-level architecture
A clear, working design whose components and data flow you can explain.
Scalability
Sharding, caching, replication and queues, used where the numbers call for them.
Reliability
What fails, how you'd detect it, and how the system degrades.
Trade-offs
Naming the alternatives you considered and why you chose one.
Communication
Driving the discussion and explaining your design choices clearly.
Practice questions
Examples of the questions and follow-ups you can expect. Practice saying your answers out loud, not just reading them.
- Design a URL shortener that handles 100 million new links a month.
- Design the backend for a group chat app with read receipts.
- Design a rate limiter shared by a fleet of API servers.
- Design a news feed for a social network with 50 million daily users.
- Design a notification service that sends email, SMS and push notifications at scale.
- Your primary database region goes down. Walk me through what happens.
How a PrepLinx session works
- 1
Choose the round
Pick the role, the interview type and a difficulty (easy, medium or hard). Optionally add a job profile with your resume and the job description, so the questions relate to that job.
- 2
Interview out loud
The AI interviewer speaks, listens to your answers and asks follow-ups. It doesn't give hints or say whether you're right, just like the real thing. The session runs up to 60 minutes.
- 3
Get scored feedback
A report with a 0–10 score for each dimension the round is graded on, strengths quoted from your own words, weaknesses, concrete improvements and an overall hiring signal, plus the full transcript.
How to prepare
Spend the first minutes on requirements
Agree on features, scale and latency targets, and do a quick back-of-the-envelope estimate. Every later decision should trace back to these numbers.
Draw the simple version first
Get a complete, working design on the board before optimizing any part of it. Interviewers want to see the whole system before the deep dives.
Say the trade-off out loud
"I'm choosing X over Y because Z" is the most valuable sentence in this round.
Plan for failure
For each component, say what happens when it fails. Interviewers almost always ask, and it's better if you get there first.
Drive the conversation
The interviewer won't tell you what to cover next. Senior candidates especially are expected to lead.
Frequently asked questions
Can I draw diagrams?
Yes. Every session has a whiteboard. When you ask the interviewer to look at your diagram, it reads it and asks about the components, data flow and failure modes, the way a real interviewer would.
How long is a system design session?
Up to 60 minutes.
Is there system design practice for ML and data roles?
Yes. Choose the machine learning engineer role for ML system design (training, serving, monitoring), or the data engineer role for pipeline and warehouse design.
Is it suitable for beginners?
Choose easy difficulty for a smaller problem. The feedback report explains what a stronger answer would have covered, which is one of the fastest ways to learn the format.
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Practice your next system design interview
4 full interviews free, with scored feedback. No credit card, and they don't expire.
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