The HLD interviewDo the arithmetic
“Collisions are astronomically rare.” Why did that one line sink the round?
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More questions.
Core conceptsAlerting
An outage burned 139% of the budget before the page. What went wrong?
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Networking and APIsThe API step
You have five minutes to design the API. Where do you start?
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Data and storageSame names, different writes
“Withdraw ₹5,000 if my balance is still over ₹20,000.” Why three different answers?
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Caching fundamentalsAdmission
Same memory, same requests. How does one policy serve 18% more reads?
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Messaging and streamsRaces
Two consumers hold the same message. Which one gets to debit?
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Distributed coordinationFalse alarms
Why not fail over the moment a node’s heartbeats stop?
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Running in productionAmplification
How does one tap become 243 attempts at a struggling database?
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Why is it slow? Performance anti-patternsThe cliff
Your cache works beautifully. Why is that the scary part?
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SecurityBlast radius
One edited URL and a script at 10 a second. What walks out?
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Multi-tenant SaaSNames vs IDs
A company leaves and frees its subdomain. Who inherits its traffic?
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Architecture stylesWrong boundaries
One service per table sounds tidy. Why does checkout then make 11 calls?
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System design trade-offsThe ceiling
How far up can one machine go before you must scale out?
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Key technologiesData model
Why does a Pune distributor write every order into two ledgers?
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Common patternsPre-signed URLs
Why should your API never touch the bytes of an upload?
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InfrastructureCampaigns
Why not send all 100 million sale pushes at once?
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Search & discoverySpider traps
Why would a crawler still be fetching calendar pages for the year 9999?
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Storage & mediaConsistency
Before December 2020, why could S3 miss a file you’d just written?
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Social & feedsHubs
Your friend knows 30,000 people. Why does that break the nightly job?
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Real-time & collaborationRouting
Your phone hops from Wi-Fi to 4G. Where do your messages go?
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Location & mapsMap matching
How does Maps know you’re on the flyover, not the road below?
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E-commerce & bookingFamous outage
How did a cache failure take down Amazon’s site and its warehouses?
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Payments & fintechLive P&L
Your P&L ticks live with every price. Who does the maths?
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Data, analytics & monitoringInvalid traffic
Yesterday your ad had 18,342 clicks. Today it shows 18,101. Why?
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Developer platformsVolume tiers
Why would one more message cut a bill by about ₹5,000?
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AI & LLM systemsAutoscaling
Your GPU dashboard reads 100%. Is it serving 2 requests or 200?
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