System design is in my opinion the highest signal hour you can spend with a candidate. If you had to do just one interview to evaluate engineers, this would be it.
If you think interviews are pointless, go here 👉 Insights from interviews from Kahneman's Noise. Good interview design gives you about 75% correlation with on-the-job performance. This matches my experience.
What a system design interview looks like
A system design interview is roughly the opposite of a leetcode interview. This is the one where you get to say "It depends", ask lots of questions, express strong opinions, and there is purposefully no specific right answer.
I like to keep introductions short (3min) so the candidate has more time to shine. I explicitly say this. At the end there's time for questions and I'm always happy to stay over.
The typical system design question will start with a problem statement, a thing for you to design, and scale numbers to guide your design. I've seen interviewers skip this last part but it's crucial – designing a system for 100 users is different than 1 billion.
I like to guide candidates into a 4 stage approach:
- Design the data schema
- Design the API
- Design the data querying
- Design the hosted architecture
Candidates can go in any order, but in over 100 of these interviews, I have never seen it go well in a different order. You do need to know the shape of your data before you can query.
To help candidates focus on the part they're solving, you have to hand-wave away large parts of the system. "Let's assume that part is implemented" is an important phrase to use.
Why this is a great interview
Unlike leetcode, people get better with experience. The interview is designed to highlight what you've done, not what you've read about.
Can they make it work
At a base level, you see "Can candidate design a working system". This is the hard skills part of the question and one that folks can study for. But that's not the fun part.
The fun part is seeing how they approach the problem. Does the candidate look at your scale numbers and design an appropriate system? Or do they rush to building the biggest baddest system they've read about online?
If they do over-design a thing, can they explain how they would deal with all the pitfalls? If they shard the database, do they truly grasp how bad of a last-resort idea that is in a world where AWS offers machines with 32TB of RAM? If they add a bunch of caching, do they have a good plan for managing invalidation?
Do they ask or assume
As the interviewer I play product owner / co-engineer.
Does the candidate ask business questions or assume details to make their life easier? When I offer engineering suggestions, can they incorporate those into their design? Do they recognize when they're out-of-depth and say "I don't know", or do they press on being confidently wrong?
If a candidate never says "I don't know" in a system design interview that's a yellow flag. Altho I did once interview a candidate so amazing that they got me to say I don't know and then taught me a thing. That was awesome.
Engineering over time
The best candidates say a lot of "Well, you could do A, B, or C", explain the pros/cons of different options, ask a question or two about the broader system, then pick an appropriate option. They then explain how this decision changes as the system evolves and build a design that preserves optionality/flexibility for later.
That takes experience. You can't study for that.
This makes system design a great leveling interview. Without reading their resume, you'll see if a candidate is fresh or has been doing this a while. And you'll know their blind spots and their strengths – important for building a team.
Cheers,
~Swizec
Continue reading about Why system design is my favorite interview
Semantically similar articles hand-picked by GPT-4
- Quick tip for system design interviews
- What to expect in senior level interviews
- Insights for interviews from Kahneman's Noise
- You're not asking for a job, you're selling a service
- Interviewing tips for experienced engineers
Learned something new?
Read more Software Engineering Lessons from Production
I write articles with real insight into the career and skills of a modern software engineer. "Raw and honest from the heart!" as one reader described them. Fueled by lessons learned over 20 years of building production code for side-projects, small businesses, and hyper growth startups. Both successful and not.
Subscribe below 👇
Software Engineering Lessons from Production
Join Swizec's Newsletter and get insightful emails 💌 on mindsets, tactics, and technical skills for your career. Real lessons from building production software. No bullshit.
"Man, love your simple writing! Yours is the only newsletter I open and only blog that I give a fuck to read & scroll till the end. And wow always take away lessons with me. Inspiring! And very relatable. 👌"
Have a burning question that you think I can answer? Hit me up on twitter and I'll do my best.
Who am I and who do I help? I'm Swizec Teller and I turn coders into engineers with "Raw and honest from the heart!" writing. No bullshit. Real insights into the career and skills of a modern software engineer.
Want to become a true senior engineer? Take ownership, have autonomy, and be a force multiplier on your team. The Senior Engineer Mindset ebook can help 👉 swizec.com/senior-mindset. These are the shifts in mindset that unlocked my career.
Curious about Serverless and the modern backend? Check out Serverless Handbook, for frontend engineers 👉 ServerlessHandbook.dev
Want to Stop copy pasting D3 examples and create data visualizations of your own? Learn how to build scalable dataviz React components your whole team can understand with React for Data Visualization
Want to get my best emails on JavaScript, React, Serverless, Fullstack Web, or Indie Hacking? Check out swizec.com/collections
Did someone amazing share this letter with you? Wonderful! You can sign up for my weekly letters for software engineers on their path to greatness, here: swizec.com/blog
Want to brush up on your modern JavaScript syntax? Check out my interactive cheatsheet: es6cheatsheet.com
By the way, just in case no one has told you it yet today: I love and appreciate you for who you are ❤️