The HLD interviewThe clock
In a 45-minute design round, how many minutes are actually design?
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More questions.
Core conceptsSplit brain
Why are two primary databases worse than none?
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Networking and APIsWhose side?
An office runner and a kitchen counter. Which one is the reverse proxy?
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Data and storageUpdate anomaly
Why did one restaurant’s new name mean rewriting 110,000 rows?
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Caching fundamentalsThe idea
Why does a dairy put a milk booth on every street?
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Messaging and streamsDuplicates
B-204 paid once. Why does the society register say ₹9,000?
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Distributed coordinationHalf-done orders
₹450 charged, then the restaurant says no. Who gives it back?
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Running in productionStop, slow, wrong
Why is a slow toll lane worse than a shut one?
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Why is it slow? Performance anti-patternsThe symptom
Restaurants upload menu photos at 11 am. Why does customer login slow down?
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SecurityJWT
Anyone can read a JWT. So why can’t anyone forge one?
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Multi-tenant SaaSDiwali sale
One seller exports 2 million orders. Why does everyone else wait 8 minutes?
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Architecture stylesNo best style
A scooter, a truck fleet or a hired cab. Which vehicle is best?
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System design trade-offsTwo ways to process
Why does a prepaid meter catch the geyser your monthly bill misses?
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Key technologiesHNSW
Checking 10 million vectors takes 5.6 s. How does HNSW check only thousands?
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Common patternsThe contract
Your export takes 10 minutes. Why can’t the request just wait for it?
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InfrastructureTop of the hour
Assume six million jobs are due at 09:00:00. Which one goes first?
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Search & discoveryStories
300 outlets report the same landing. How does your app show it once?
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Storage & mediaPlayback
How does a song start playing before your thumb leaves the glass?
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Social & feedsMatching
Both swipe right in the same 50 ms. Why might nobody get a match?
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Real-time & collaborationClocks
Your opponent is on fibre, you’re on mobile data. Who loses on time?
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Location & mapsSegments
How does Strava know you beat a stranger on a hill?
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E-commerce & bookingThe final second
One signed bat takes 2,000 bids in its final second. What breaks?
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Payments & fintechUPI intent
You pay in your UPI app. How does the shop find out?
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Data, analytics & monitoringLogging at scale
Checkout errors hit 3%. Which of 86 billion log lines explains why?
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Developer platformsFair timing
Same code, same tests: Accepted on one submit, TLE on the next. Why?
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AI & LLM systemsPrompt injection
A support ticket tells your agent to email card numbers out. What stops it?
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