7 Simple Ways to Fail System Design Interview

#13: ⚠️ Warning - Don’t Try This in a Real Interview (4 minutes)

Neo Kim

Oct 10, 2023

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Once upon a time, there were 2 engineering students - Bob and Bill.

Bob was the class topper and the favorite student of every teacher.

While Bill scored average grades.

Yet Bob failed the system design job interview round and Bill passed it.

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This post outlines the best tips to fail the system design interview. And this is Bob’s story.

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Here are 7 reasons why Bob failed the system design interview:

1. Don’t Bother About Requirements

Bob was the type of guy who answered every question right away. So he started the design without understanding the exact requirements.

But Bill asked many counter questions. And didn’t start the design until he understood the requirements.

2. The Perfect Design

Bob was good at drawing fancy block diagrams. But he didn’t discuss the trade-offs of his architectural decisions. And didn’t care to identify the bottlenecks and single points of failure.

But Bill talked about the alternative technologies. And mentioned the trade-offs of each architectural decision. This way he justified his decisions.

3. Showcase Knowledge Even if Interviewer Didn’t Ask For It

Bob loved to do capacity planning and did it without checking with the interviewer. Besides he didn’t ask whether API and database schema design was necessary.

But Bill checked with the interviewer whether there was a need for capacity planning. This helped to save time and focus on important things.

4. Do Monologue

Bob didn’t have a 2-way conversation with the interviewer and tried to finish the design fast.

While Bill often asked for feedback and adjusted his design.

With every architectural decision, he asked the interviewer:

What his thoughts were on this approach?
And if the trade-off was acceptable?

Besides he shared each assumption with the interviewer. And admitted the knowledge gaps when he got stuck. This helped to get hints from the interviewer.

5. Know Buzzwords

Bob loved to use buzzwords in the interview. So he missed many important design details.

But Bill loved proven technologies and had a broad knowledge of alternative technologies. So he knew when to use what.

His knowledge was T-shaped and it helped him justify design decisions.

6. Time Management Is Not Important

Bob didn’t do mock interviews and considered himself a lonely wolf.

Also he wasted time creating fancy block diagrams in the interview.

While Bill did many mock interviews to improve his communication skills. And used extra time to get familiar with the drawing tool before the interview.

Also he kept the design simple.

This is how he spent time in the interview:

Understand requirements: 5 minutes
High-level design: 10 minutes
Design deep dive: 15 minutes
Validation: 5 minutes

7. Don’t Validate Design

Bob didn’t mention cross-cutting concerns to the interviewer. Or check whether his design met the requirements towards the end of the interview.

But Bill mentioned what improvement he would make if he had extra time. And validated requirements with his design.

Takeaways

Don’t bother to understand requirements. This guarantees failure even with a perfect design.
Don’t discuss tradeoffs or identify bottlenecks.
Do capacity planning and other design steps without checking with the interviewer.
Don’t keep an open communication channel. Because the interviewer’s interest might be different.
Don’t read many books and white papers if you get extra time.
Don’t keep track of time in the interview and don’t take extra time if something is unclear.
And keep the design complex.

Be like Bob if you want to fail.

The system design interview is difficult because you want to design a system in 45 minutes. And it usually takes many engineers a few years to build it in the real world.

The job offer might get down-leveled if you don’t perform well in the system design interview. So prepare well and learn from Bob’s mistakes.

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