The HLD interviewThe notes
If you didn’t say it out loud, did it happen in the interview?
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More questions.
Core conceptsShared fate
Two 99.9% replicas make six nines on paper. Why only 99.95% live?
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Networking and APIsComposition
One product page, seven services. Why shouldn’t the app call them all?
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Data and storageWrite skew
Every check passed. How did user 42 send ₹1,20,000 against a ₹1 lakh limit?
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Caching fundamentalsResharding
Add one cache node. Why could the database suddenly see 19× the reads?
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Messaging and streamsReplication slots
Why can one stopped connector take your primary database down?
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Distributed coordinationConnections
You added pods. Why do all your gRPC calls still hit one?
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Running in productionFallbacks
Why did falling back to the database take down Amazon’s whole website?
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Why is it slow? Performance anti-patternsEvent loop
Why can one 50 MB JSON upload freeze a whole Node.js server?
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SecurityPasswords
Why would 1,000 logins at 10 a.m. need 19 GiB of RAM?
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Multi-tenant SaaSThe whale
One tenant’s flash sale. Why do 1,999 others slow down?
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Architecture stylesRetry storm
Your apps retry. The mesh retries too. How does one tap become 729?
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System design trade-offsTokens
JWTs skip the session lookup. So why is logout suddenly hard?
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Key technologiesShards
50,000 tenants, one index each. Why 100 data nodes before any data?
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Common patternsCompensation
Why should a refund never be called a rollback?
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InfrastructureRelease
Why can’t a worker just delete the lock key when it’s done?
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Search & discoveryPageRank
Two pages, one inbound link each. Why does one rank 8× higher?
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Storage & mediaCaching
Assume 200 Tbps at peak. Why do only 3% of bytes reach the origin?
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Social & feedsFan-out
Why not just send every comment to all 10 million viewers?
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Real-time & collaborationReconnects
Your train enters a tunnel. How does chat resume without reloading everything?
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Location & mapsScale
Only 100 orders a second. So why is food delivery hard?
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E-commerce & bookingPrice quote
Your coupon expired while you typed the OTP. What do you pay?
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Payments & fintechReconciliation
₹3,600 crore a day of other people’s money. Does every rupee match?
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Data, analytics & monitoringTop vs trending
#love got 900 uses in five minutes, #raininmumbai 120. Which is trending?
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Developer platformsLeaderboard
A test-data bug forces a rejudge mid-contest. How does the leaderboard stay right?
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AI & LLM systemsCaching
How can a cache hand you another customer’s refund answer?
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