6 Painful Mistakes I Made in System Design Interviews
So you don’t have to
Hey friends 👋,
Today, I want to get a little personal and talk about something many of us dread: System Design Interviews. If you’ve ever found yourself spiraling mid-interview, feeling like you’re designing a spaceship when they asked for a bicycle... trust me, you’re not alone.
I used to think I could just "wing it" because I knew how to build things.
“System Design is just my daily job... but a bit bigger maybe, right?”
Oh, past me. So innocent. So wrong.
1. Jumping into solutions without understanding the problem
Interviewer: "Design a scalable chat app."
Me (in 0.5 seconds): "Okay, we'll use Kafka, Redis, WebSockets, Microservices, Kubernetes..."
😅 What I should have done:
Take 5–10 minutes just to ask questions.
How many users are we expecting?
Real-time chats or just message queues?
Mobile-only or desktop too?
Moral of the story:
Understand what they need before showing off how you’ll build it. (Trust me, it impresses them way more.)
2. Skipping the High-Level Design
Another classic me-move: jumping straight into nitty-gritty components.
But better High-Level Design first.
Draw that simple block diagram. API Gateway → Service Layer → Database.
Even a rough sketch helps everyone stay on the same page. Without it, you're basically explaining a city map... without a map.
Moral of the story:
Big-picture thinking first. Zoom in later.
3. Treating Every API Like REST (Because That’s All I Knew)
REST was my safe space.
But guess what? Sometimes RPC or GraphQL is a better fit!
Need high-performance for IoT devices? Go RPC.
Want to avoid overfetching data on mobile apps? Hello, GraphQL!
Moral of the story:
Learn the tools in your toolbox, not just the hammer.
4. Forgetting About Scalability and Caching
In my early interviews, I'd confidently say, "We'll just use a database."
Meanwhile, the interviewer is thinking:
"Cool... and when you hit 10 million users and your database explodes?"
Scaling, replication, partitioning, caching — all that scary grown-up talk — matters.
Even just mentioning ideas like sharding or distributed caching shows you’re thinking beyond Day 1.
Moral of the story:
Build for the traffic you hope to have, not just the traffic you have today.
5. Thinking “Deep Dive” Means “Talk Faster”
When they ask you to deep dive into a component, it’s about thoughtfulness, not speed.
Pick one area, like your caching layer or messaging system, and explain it clearly and calmly.
Moral of the story:
One deep, meaningful explanation > Shallow buzzword bingo for the whole system.
6. Forgetting the Final 5 Minutes: Wrap It Up
Old me: "Okay, that's the design. Any questions?"
Better me:
Summarize the main points.
Talk about potential trade-offs.
Suggest improvements for scale or resilience.
A strong wrap-up makes you sound like someone who could own the project, not just build it.
Moral of the story:
Close strong. Always.
Final Thoughts
System Design interviews aren't just about technical skills — they're a dance between problem-solving, communication, and showing that you can think for the long haul.
I'm still learning, and every conversation, every diagram, every stumble gets me a little better. If you’re prepping for interviews, give yourself permission to slow down, think deeply, and most importantly, enjoy the process.
Let me know if you have any other tips in the comments below. I'm always happy to learn.
Until next time,
Adlet
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