Man, I’m gonna tell you the truth: until mid-2024 I was absolute trash at system design interviews.
I could run the actual backend for half a billion users at my day job, no sweat. But the second some Google or Meta interviewer said “Design Twitter” or “Design a URL shortener,” my brain turned into oatmeal. I’d either spend 25 minutes drawing seventeen microservices nobody asked for, or I’d forget to ask a single question and end up designing a system for 10 DAU. It was painful. I bombed loops I had no business bombing.
Then one weekend I sat in on a couple of staff-level mocks (friends prepping for principal rounds) and noticed every single one of them—Google, Meta, Snowflake, whatever—used the exact same skeleton. Not similar. Literally the same four beats, every time. I copied it shamelessly, drilled it until I hated myself, and suddenly system design went from my weakest round to the one where interviewers started running out of hard questions.
Here’s the dumb, boring, stupidly effective version I actually use in real interviews now. No fluff, no “95 % of interviewers say this” nonsense.
1. Shut up and ask questions first (5–8 min)
I do not touch the whiteboard until I’ve asked at least five questions. Period.
My default list (I literally have it memorized):
- How many daily active users? Monthly? Peak concurrency?
- Read-heavy or write-heavy? What’s the expected QPS ballpark?
- Latency budget? 100 ms p99? 1 second is fine?
- Global or single region? Eventual consistency OK?
- Any weird constraints (HIPAA, GDPR, offline support, etc.)?
Then I do one napkin math out loud:
“Alright, 400 M DAU, say 20 feed pulls per day on average with 3× spikes → call it 100–150 k QPS on the read path, maybe 5–10 k writes. Cool?”
They always clarify something and I jot it down. Instant “this guy’s done this before” points.
2. Big-picture diagram (15–18 min)
Only now do I start drawing. Same order every single time so I don’t forget half the system:
Clients → CDN → LB / API Gateway → App servers (stateless) → Cache → Queues → DB tier → Blob storage
I draw it left-to-right, label the main APIs as I go (POST /tweet, GET /home_timeline, etc.), mention WebSockets if needed, then walk one full request end-to-end while talking.
When the diagram feels complete I stop and say:
“Does this feel like the right shape before I start diving into the messy bits?”
They either nod or correct one thing. Either way, I’m aligned and they’re invested.
Side note: I keep a little scratch list in the corner for “rate limiting,” “sharding,” “cache invalidation,” “exactly-once delivery,” etc. so I don’t get distracted mid-flow.
3. Let them pick the pain points, then solve 2–3 properly (15–20 min)
I hand the pen over (figuratively):
“Which parts would you like to zoom in on?”
They always pick the hard ones—feed ranking at scale, celebrity write explosion, consistency model, failure modes, whatever.
For each I do the same four-step thing:
- Paraphrase the problem with numbers (“So one tweet from Taylor Swift = potentially 300 M writes if we fan out naively—yeah, no.”)
- List two or three real approaches I’ve either used or read post-mortems about
- Quick pros/cons with actual math or real-world examples
- Pick one and explain why (“I’d go hybrid fan-out: push to the 1–2 % power users, pull for everyone else. That’s what Twitter, Instagram, and LinkedIn all converged on.”)
4. Wrap and flip the script (last 3 min)
30-second recap of the two cleverest things in the design, then:
“By the way, what’s the gnarliest scaling problem your team is fighting right now?”
Boom—conversation turns human, they relax, sometimes you learn something useful for the onsite.
That’s the entire script.
I’ve used it in twelve final-round loops this year (Google L6, Meta E6, two startups that shall not be named, and a couple places that rejected me anyway). Works every damn time.
Practice it fifteen or twenty times on interviewing.io or with a friend who won’t go easy on you. By rep eight you’ll sound like the calmest person in the room.
Resources that actually helped me (no affiliate links, I swear):
- Grokking the System Design Interview courses on Educative.io
- ByteByteGo YouTube channel for quick refreshers
- Exponent’s system design course if you want someone to roast you live
That’s it. Go steal it, stop overthinking, and go get the offer.
You don’t need another framework. You need reps with this one until it’s boring.
Good luck—you’ve got this.
