The HLD interviewThe lag
Your mistake happened at minute 4. Why does the bill arrive at minute 30?
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More questions.
Core conceptsThe partition
Two counters, one berth left, and the phone line dies. Who gets it?
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Networking and APIsRound trips
One order screen, three services: why does plain REST make your phone wait twice?
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Data and storageAtomicity
₹500 left Asha’s wallet and reached nobody. How?
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Caching fundamentalsSizing
Memory says 13 cache shards and ops say 12. Why build 20?
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Messaging and streamsAt-least-once
Why did one customer get two plates of the same biryani?
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Distributed coordinationO(log N)
Why does 10× the nodes add only about two rounds of gossip?
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Running in productionScaling lag
Your autoscaler picked exactly the right pod count. Why did users still fail?
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Why is it slow? Performance anti-patternsMetastable
The failover took 30 seconds. Why were you down for 40 minutes?
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SecurityPooled connections
Row-level security was on. How did tenant B still see tenant A’s rows?
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Multi-tenant SaaSHard TTLs
You cache tenant lookups for 60 seconds. Why are you down 29 minutes?
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Architecture stylesMerging back
Why did Segment merge 140+ services back into one?
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System design trade-offsEventual
How can one 2–5 game be read as 18 different scores?
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Key technologiesDurable execution
A worker dies halfway through your order. How does another pick up exactly there?
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Common patternsBatching
Why do 100-row batches give one database ten times the throughput?
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InfrastructureConsistency
You cut the price to ₹899. Why can the cache keep saying ₹999?
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Search & discoveryTail latency
Each server is slow once in 1,000 calls. Why are 63% of searches slow?
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Storage & mediaAdaptive bitrate
Your train enters a tunnel. Why does the video blur instead of freezing?
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Social & feedsTrends
“#GoodMorning” gets more tweets than “#IPLFinal”. Why does only “#IPLFinal” trend?
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Real-time & collaborationSimulcast
A friend joins on 4G from a train. Why isn’t everyone’s video blurry?
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Location & mapsRain night
Why do restaurants vanish from your app when it rains at 8 pm?
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E-commerce & bookingThe draw
Why doesn’t tapping Buy faster, or again, get you the phone?
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Payments & fintechBuying power
Asha has ₹50,000 and buys on two devices at once. Can she overspend?
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Data, analytics & monitoringDistinct counts
Unique visitors: 100 at nine, 120 at ten, 90 at eleven. Why not 310?
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Developer platformsLate events
Usage from 23:58 on 31 October arrives at 00:20. Which bill is it on?
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AI & LLM systemsFailover
Provider B fails mid-answer. Why not just finish the answer on provider A?
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