The HLD interviewThe method
How does a wedding caterer know she needs twenty cooks, not four?
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Page 3
More questions.
Core conceptsError budgets
Why would a tiffin service plan to get 300 lunches wrong?
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Networking and APIsHop by hop
A router forwards your HTTPS request. Why can’t it read a single word?
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Data and storageShared nothing
Why did a 1986 paper bet on databases that share nothing?
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Caching fundamentalsEviction
The reserve shelf is full. Which book goes back upstairs?
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Messaging and streamsLost answers
Your QR payment says “Something went wrong.” Should you tap Pay again?
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Distributed coordinationEpidemic spread
How does news reach 1,000 flats with no notice board?
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Running in productionSlow, not down
Payments is healthy. So why is checkout returning 503s?
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Why is it slow? Performance anti-patternsThe symptom
The screen shows 20 orders. Why does the API send all 500?
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SecurityZero trust
One pod gets hacked. Why can it reach your payments database?
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Multi-tenant SaaSStatic stability
A restaurant chain’s head office crashes. Why do 300 outlets keep billing?
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Architecture stylesConcurrency
Serverless scales automatically. Why would 440,000 results-day requests still fail?
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System design trade-offsRead your writes
You save your new bio, refresh, and the old one is back. Why?
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Key technologiesLineage
DynamoDB shares Dynamo’s name. So why does it have a leader?
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Common patternsFan-out
Asha’s on gateway A, Ravi’s on gateway B. How does her message reach him?
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InfrastructureAlways writable
A machine dies mid-sale. How does your cart still save?
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Search & discoveryPoliteness
Crawl a billion pages a day without hammering a small restaurant’s website. How?
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Storage & mediaDurability
How do you keep a trillion files when a disk dies every day?
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Social & feedsUploads
Why doesn’t Instagram let your photo pass through its own servers?
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Real-time & collaborationTopology
Why doesn’t your video go straight to everyone else on the call?
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Location & mapsLive traffic
How does your phone know Hebbal is jammed before you get there?
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E-commerce & bookingOverselling
10,000 phones, 5 million shoppers, one counter. How do you never sell 10,001?
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Payments & fintechEscrow
Your wallet app shows ₹1,380. Where is that money, really?
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Data, analytics & monitoringNo cookies
How do you count unique visitors without storing a single cookie?
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Developer platformsIsolation
One merchant’s server hangs. How can it stall every other merchant’s webhooks?
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AI & LLM systemsBatching
Why is answering eight chats at once about 8× cheaper than one?
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