The HLD interviewWhy rounds are lost
Most candidates who fail a design round knew enough to pass. What sank them?
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More questions.
Core conceptsHidden SPOFs
Two cooks, two tawas. Why does the dosa stall still shut?
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Networking and APIsHTTP versions
Why can HTTP/2 be slower than HTTP/1.1 on a shaky mobile network?
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Data and storageSnapshots
Asha’s bottle now costs ₹379. Why does her old invoice still say ₹349?
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Caching fundamentalsStampedes
Why does one empty samosa tray jam the whole wedding kitchen?
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Messaging and streamsGoodput
Your servers were 100% busy. Why did 72,000 users still time out?
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Distributed coordinationLeases
Two riders, one parcel, both holding a valid slip. Who should get it?
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Running in productionReplication lag
A region dies. Why are 2,400 writes you already confirmed missing?
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Why is it slow? Performance anti-patternsThe symptom
50,000 people view the same products. Why does the database run 50,000 queries?
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SecurityBOLA
She changed one digit in the URL and saw a rival’s invoices. How?
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Multi-tenant SaaSForgotten filter
One UPDATE forgot its tenant filter. Whose invoices got archived?
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Architecture stylesA 2 GB patch
One server needs 18.5 days. How does a swarm do it in 53 minutes?
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System design trade-offsPush vs pull
Why does WhatsApp push, while your email app polls every 15 minutes?
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Key technologiesThe bill
Storing the images costs $2,250. Why does serving them cost $13,800?
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Common patternsThe race
Two people book seat C7 in the same second. Who gets it?
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InfrastructureStorage
Why can’t a message queue just be a table you delete from?
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Search & discoveryReal-time
Last over, you search “ind vs aus”. How are one-second-old posts already there?
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Storage & mediaOpen Connect
Why is tonight’s episode already sitting inside your internet provider?
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Social & feedsViral threads
200,000 fans refresh one match thread. Why doesn’t Reddit fall over?
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Real-time & collaboration250 OK
A bank’s server hands you an email. At what moment do you own it?
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Location & mapsFriend graph
Why would a map search find 200,000 people to show you 30 friends?
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E-commerce & bookingRoom counts
Forty identical rooms on New Year’s Eve. How do you never sell 41?
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Payments & fintechThe opening bell
It’s 09:15. Two million phones open the app. What breaks first?
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Data, analytics & monitoringAlerting
IPL final, last over, payments failing. Who gets paged, and how fast?
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Developer platformsExactly once
How do you bill 10 billion events a day, counting each exactly once?
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AI & LLM systemsGangs
Why does a 1,024-GPU job need 128 whole machines free at once?
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