// Round 05 · 45–60 min · 30 credits
System design.
Design a system at scale, sketch it on the scratchpad, and defend every tradeoff.
System design interviews reward structure and judgement, not memorised architectures. The interviewer wants to see you scope the problem, make reasonable estimates, choose components for a reason, and adapt when the requirements change. Hardmode gives you a scratchpad, a problem matched to the level in the job description, and an interviewer that pushes on exactly the part of your design you were hoping it wouldn’t.
Scope
What this round covers
- Requirements gathering and scoping: functional, non-functional, and what’s out of scope
- Back-of-the-envelope estimates for traffic, storage, and bandwidth
- A high-level design, then a deep dive into the hardest component
- Tradeoffs around consistency, availability, latency, and cost
Session
How the session runs
01 —
The prompt
From the question bank, matched to the company’s domain and level, or a prompt you bring.
02 —
Design on the scratchpad
Sketch components and data flow while you talk. The interviewer follows both.
03 —
Pressure test
Scale it up, take a component down, change a requirement. See how the design holds.
Personalised
Fitted to you
Your CV
Starts near systems like the ones on your CV, then stretches you past them. Optional, and you can switch it off for any session.
Your difficulty
- Easy: A friendly interviewer who wants you to do well.
- Medium: A fair, professional interviewer. What most interviews feel like.
- Hard: A proper grilling, like a big-tech bar-raiser loop.
Review
What your feedback covers
- How your scoping set up the rest of the design
- Components that earned their place, and any that needed a clearer reason
- Tradeoffs you named, plus a few more worth raising next time
Detailed and supportive at every difficulty · No score, no verdict
Prep
Tips before you start
- Spend the first five minutes on requirements. Write them down.
- Do the estimates; they justify your design choices.
- Go deep on one component instead of shallow on ten.
// Next round
The rest of the loop
// Player 1
Get ready for hard mode.
Paste the job you want. Run the loop until the real one feels like a rematch.
20 free credits · No card required