System Design Interview Tips

Below are the system design interview tips that might not be obvious or commonly mentioned. These tips helped me pass my job interview, and I hope they will help you to pass yours.

1. Take a walk before the interview to clear your mind and get your brain working.
2. Communicate with your interviewer, but don’t try to make friends with them; just focus on solving the problem.
3. When defining requirements, don’t look for the interviewer to give you requirements. Write down everything you believe makes sense, then review them with the interviewer.
4. Lead the conversation if you’re interviewing for a senior role. For junior roles, let the interviewer set direction.
5. Lead the conversation but don’t monologue. Pause at milestones to see if the interviewer has questions or feedback.
6. Figure out what makes your system challenging. Each system has a core puzzle. For live commenting, it’s real-time message delivery; for YouTube, it’s adaptive video serving based on device and network quality.
7. You usually don’t need back-of-the-envelope (BOE) calculations at the start of the interview. If you’re not comfortable with BOE, don’t ask the interviewer whether they’re needed; just say you can walk through estimates later if required.
8. For BOE, use quick numbers:
   1. 1MB or 1M -> 1e6
   2. 10M -> 1e7
   3. 1 day -> 86400 sec -> close to 100,000 sec -> 1e5
   4. Example: QPS for 10M posts/day -> 1e7/1e5 = 100 posts/sec
9. Connect your system design and technical decisions to the requirements and numbers.
10. Take notes. Your interviewer might not write feedback immediately and could forget key points.
11. Your biggest enemy might not be your technical skills, but time management. You only get around 35 minutes, so use them wisely.
12. Practice with someone who interviews at Big Tech. You’ll get relevant evaluation and targeted feedback. I personally used HelloInterview for mock system design, behavioral, and coding interviews and was happy with the experience. Use my referral link for 20% off your first mock interview.

Good luck!