How (not to) fail a system design interview
Common mistakes software engineers make during system design interviews and how to avoid them.
Hi Friends,
Welcome to the 116th issue of the Polymathic Engineer newsletter.
This week, we talk about system design interviews. Every software engineer knows that these interviews can be the hardest part of the whole process.
While coding interviews are more focused on algorithms, system design interviews require you to think deeply about architectural problems, trade-offs, and scalability. This can be challenging even for the more experienced folks out there.
However, I noticed one thing after doing many such interviews. A lot of candidates fail because they make common mistakes they could have easily avoided. If you know about these mistakes, you can do better and more likely succeed.
We'll explore these blunders below.
Stream - Scalable APIs for Chat, Feeds, Moderation, Video & Audio
This issue is offered by Stream. Stream can help you create seamless user experiences without the hassle of backend infrastructure. You focus on building and Stream will handle the scaling.
Made for Creators & Builders – Launch chat, video, and feeds with minimal code
Free Maker Plan – Perfect for devs and startups (≤5 members, ≤$100K funding, ≤$10K revenue)
Lightning-Fast APIs – 9ms response times, 99.999% uptime on a global edge network
No Surprise Costs – Transparent pricing with hard usage limits
Prebuilt UI Kits – Customize and launch beautiful interfaces in minutes
Not Understanding the Requirements
One of the most common mistakes candidates make is jumping into the design part without fully grasping the requirements. If you want to craft a solid system, it’s crucial to clarify the problem’s boundaries before you start suggesting solutions.
Take the time to ask questions—lots of them. This will not only help you understand what the interviewer is looking for but also show your commitment to addressing the real problem.
Never start making diagrams until you have clear what you need to build. This clarity will save you precious time and effort later.
Not Discussing Trade-offs
Another pitfall that trips up many candidates is not talking about trade-offs. In system design, there's rarely a single "right" answer. Strong candidates can compare different choices and explain why they chose the one they did.
When you're proposing a solution, don't just say what you'd do. You should instead explain how you came up with your ideas. Talk about the different options you thought about and why you ultimately chose one over the others, pointing out both the pros and cons of your choice.
For example, if you have to choose between a relational database and a NoSQL option, don't just pick one. You should explain why each option might work, and then you should say what made you make your choice in this case.
By discussing trade-offs, you demonstrate that you understand the complexities involved in system design. You show that you can make smart choices while being aware of what they mean. This is a very important skill that interviewers look for because it's similar to how engineers have to think when they face problems in the real world.
The goal is not to make a perfect system(often impossible in the real world anyway) but to show that you can reason about complex design problems and explain your choices.
Not Talking to the Interviewer
A lot of candidates treat the interview like a solo performance. They don't talk back and forth but rush through their designs without involving the interviewer in the conversation. This way of doing things completely misses the point of the interview.
Interviews are meant to reproduce a scenario where you work with a team. Good candidates know how important it is to work together and aren't afraid to share their ideas and get feedback as they go.
The interviewer isn't just there to judge you - they're a potential collaborator. Ask them what they think about your approach, and be ready to make changes based on what they say without stubbornly sticking to your original plan.
Also, be honest about what you don't know. If you're unsure about something, admit it. This led to a more enjoyable and productive interview for both parties, and you might even learn something new in the process.
Treating the Interview Like an Interrogation
An interview is not an interrogation but a collaborative discussion. Don't try to show off all the things you know, even if they have nothing to do with the problem.
The goal isn't to prove you know everything about system design. It's to show that you can solve problems in the real world well.
Save time and focus on what the interviewer wants. Not every interview requires you to do back-of-the-envelope calculations or create API or database schema. Ask which aspects of the design the interviewer is most interested in.
Using Buzzwords Without Understanding
A candidate using buzzwords and technologies without knowing what they mean is one of the most embarrassing things to see.
Interviewers want to see that you understand what you're talking about, not just know the industry buzzwords on the surface.
Here are some rules you should follow:
• Don't use concepts or tools that you don't fully grasp. Leave something out if you can't explain how it works or why you'd use it.
• Only suggest solutions you can justify. Be ready to explain why you made every choice you made in the design process.
• Don't make fancy block diagrams just because you saw them elsewhere. Your design should be made to solve the problem at hand.
As you talk about your solution, make sure you relate it to the system's needs and explain why you think a specific technology or method will work best in this situation. This clarity helps build your credibility.
Not Preparing Enough
System design interviews are not like the work you normally do, and your day-to-day experience is not enough to pass.
Preparation is critical. The goal isn't to have ready-made answers for every possible question. It's to boost your confidence and make it easier to think through problems in a structured way.
These are all things I found very helpful:
- Do mock interviews to improve your communication skills. Get better at putting your thoughts into clear, concise sentences.
Get familiar with the interview setup. If you'll use a whiteboard or a specific drawing tool, get used to it first.
• Review common system design patterns and technologies. You don't need to memorize everything, but having a good grasp of the basics will help you think more quickly during the interview.
• Study real-world systems. Look at how big tech companies have solved similar problems. This will give you ideas and see the pros and cons of different methods.
Interesting Reading
Here are some interesting articles I read in the past weeks:
Good tips here, Fernado!
A lot of folks mess up by rushing in or throwing around buzzwords.
Just slowing down, asking good questions, and thinking out loud can make a huge difference.