The HLD interviewWeak vs strong
Kafka, Redis and Cassandra are all on the board. Why did the round fail?
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More questions.
Core conceptsNumbers first
Assume 1.5 million people chase about 60,000 berths at 10:00. Where do you start?
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Networking and APIsSSE vs WebSockets
An LLM types its answer token by token. Why isn’t that a WebSocket?
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Data and storageRead skew
Why can a bank report show ₹1,200 when only ₹1,000 exists?
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Caching fundamentalsHot keys
Adding 20 Redis shards didn’t help the live score. Why not?
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Messaging and streamsEphemeral delivery
Why did a subscriber deploy silently lose every order in that minute?
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Distributed coordinationFive copies
Five notebooks, one register, a silent secretary. Which entries are final?
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Running in productionRollouts
200 kitchens, one new biryani recipe. Which kitchens should try it first?
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Why is it slow? Performance anti-patternsCPU-seconds
A 40 ms query passed code review. Why did the sale melt the database?
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SecurityRotation
A thief copies your refresh token. How does the server ever notice?
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Multi-tenant SaaSMoving a tenant
Acme moved stamps a second ago. Where does its next request land?
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Architecture stylesCold starts
Your handler takes 100 ms. Why did the first request take 800?
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System design trade-offsChains
Ten services, each 99.9% up. Why is checkout down 7 hours a month?
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Key technologiesMeaning
“Light office wear for summer” finds linen kurtas. No word matches. How?
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Common patternsPolling
Polling every second for a million users. How many requests is that?
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InfrastructurePartitioning
Why could adding one machine to 120 reshuffle 342 TB?
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Search & discoveryRanking
Why does a post with 20,000 likes rank below one with 300?
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Storage & mediaFailover
A server dies mid-episode. Why doesn’t your TV even buffer?
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Social & feedsStorage
Your friend scrolls back to your 2019 Leh trip. What does that cost?
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Real-time & collaborationTransforms
Ravi deletes “IPL ” as Asha types “2026”. Where does her word land?
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Location & mapsPrivacy
In 2015, researchers pinned Nearby Friends users to within 5 metres. How?
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E-commerce & bookingWaiting room
Two million fans, 60,000 seats, noon. Why doesn’t the site melt?
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Payments & fintechWebhooks
A webhook arrives twice, the second forty minutes late. Which one counts?
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Data, analytics & monitoringFresh features
Skip three cricket clips. How does the next page know?
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Developer platformsContest spike
80,000 contestants start in the same minute. Where do the cores come from?
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AI & LLM systemsFragmentation
2,688 GPUs are free. Why can’t a job needing one whole block start?
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