The HLD interviewLargest wins
Writes say 18 shards; storage says 75. Which number wins?
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More questions.
Core conceptsPower cut
Why does your housing society’s generator wait before starting?
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Networking and APIsThe problem
Three apps, six services. Why write auth, limits and logging six times?
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Data and storageTwo philosophies
One clubhouse register, many families. Hand out one key, or number every entry?
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Caching fundamentalsOffload
95% or 99% cache hits. Why is that a 5× difference?
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Messaging and streamsExpected version
Two phones pay from one wallet at the same moment. Which one gets refused?
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Distributed coordinationRandom timeouts
Without randomness, Raft elections took over 10 seconds. How does chance fix that?
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Running in productionRPO
Disaster at 14:00, last backup at 02:00. How many orders are gone?
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Why is it slow? Performance anti-patternsHandshakes
The bank answers in 30 ms. Why does each call take 92?
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SecuritySecret zero
How does a brand-new pod get its first secret without being given one?
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Multi-tenant SaaSHost header
One forged header. Can your own reset email hand an attacker the token?
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Architecture stylesAvailability
Chain ten services, each 99.9% up. Why does it fail 7.2 hours a month?
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System design trade-offsLoad levelling
5,000 orders a second, workers for 1,000. Why did no checkout fail?
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Key technologiesFilters
Asked for 10 matches, got 4, with no error. What did the filter do?
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Common patternsPartition keys
Why does keying writes by timestamp leave seven of eight shards idle?
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InfrastructureRouting
Add one cache node. Why could 99.6% of your keys suddenly miss?
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Search & discoveryRanking
How does a moon landing overtake the Budget in 23 minutes, with no editor?
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Storage & mediaFormats
Why doesn’t Spotify switch quality mid-song the way YouTube does?
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Social & feedsSpikes
Twitter took 143,199 tweets in one second. Why did users see no blip?
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Real-time & collaborationFailover
The server holding your document freezes mid-edit. Why don’t you lose a keystroke?
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Location & mapsPrivacy
How did a fitness heatmap reveal secret military bases?
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E-commerce & bookingFairness
Why doesn’t joining the queue early get you tickets first?
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Payments & fintechHot wallets
One merchant gets 1,900 payments a second. Its balance row takes 500. Now what?
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Data, analytics & monitoringANN indexes
A billion vectors take 512 GB. How does the index fit one server?
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Developer platformsScale
Why do the slowest 1% of endpoints hold a third of all connections?
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AI & LLM systemsDurability
A worker crashes mid-run, after the dispute is raised. Is it raised twice?
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