The HLD interviewPer partition
A table provisioned for 40,000 writes a second throttles at 1,000. Why?
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More questions.
Core conceptsRound trips
Your dashboard says 50 ms. Why did the user wait 195?
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Networking and APIsSilent data loss
No error, no crash. So where did the customer’s ₹20 tip go?
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Data and storageDouble booking
Both saw room R1 free. Why did both bookings go through?
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Caching fundamentalsLeases
How did Facebook cut herd queries from 17K a second to 1.3K?
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Messaging and streamsThe knee
Why does going from 90% to 99% busy make requests ten times slower?
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Distributed coordinationClock sync
Clocks agree within 1 ms. Why is write order still a guess?
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Running in productionStrangler fig
How do you replace a booking system that can’t close for a day?
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Why is it slow? Performance anti-patternsRestores
Why does restoring checkout take three hours when orders are only 400 GB?
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SecurityCrypto-shredding
How do you delete a customer from 35 backups you can’t edit?
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Multi-tenant SaaSVolume tiers
Why can one more API call cut a bill by ₹99,999.80?
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Architecture stylesScale
Stack Overflow served about 2,400 requests a second from one app. Why not split?
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System design trade-offsWaiting vs computing
How does one CPU core serve 200 requests a second at 100 ms each?
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Key technologiesPooling
3,000 app connections, one Postgres. Why does a pool of 40 suffice?
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Common patternsLeases
A worker dies at 90%. Who finishes the job, and from where?
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InfrastructureTiming
Your 09:00 payment reminder fired at 09:04. Why was that deliberate?
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Search & discoveryDocuments
How can “red, size 9” find a shoe nobody can buy?
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Storage & mediaConflicts
Two laptops edit one file offline. Whose work gets thrown away?
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Social & feedsSeen-set
How does Tinder remember 10,000 people you’ve swiped on in 12 KB?
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Real-time & collaborationLive
Two million fans watch one top 10. How many Redis reads a second?
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Location & mapsRanking
A 5.0 with two reviews or a 4.6 with 3,000: which ranks first?
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E-commerce & bookingFreshness
Why can your price alert arrive a whole day late?
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Payments & fintechFailover
The engine dies after matching your order, before replying. Did you trade?
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Data, analytics & monitoringPolling vs the log
An order is created, shipped and deleted between two polls. What do you see?
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Developer platformsTenant context
Anyone can type acme.helpdesk.example. What proves a request belongs to acme?
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AI & LLM systemsPermissions
A branch officer can open 5% of the bank. What does the AI read?
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