The HLD interviewLatency too
One line of arithmetic turned 560 ms into 18 ms. What did it change?
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More questions.
Core conceptsThe kitchen
One cook makes 40 orders an hour. What happens when 400 arrive?
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Networking and APIsLive commentary
You tap “older” and read the same two balls again. Why?
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Data and storageRead-modify-write
The board says 37 Parle-G. Why does the shelf hold 32?
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Caching fundamentalsSkew
How can a cache holding 1% of the items serve 68% of requests?
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Messaging and streamsGlobal order
Why can’t more machines buy you one global order?
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Distributed coordinationBlocking
Three vendors said yes, then the planner fell ill. Why is everyone stuck?
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Running in productionBiggest line
Your team tunes CPU all quarter. Why does the bill barely move?
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Why is it slow? Performance anti-patternsBig users
Why does order history time out only for your best customers?
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SecurityShort-lived certs
Why would you want your service certificates to expire every 12 hours?
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Multi-tenant SaaSFleet averages
12 customers are suffering. Why does your latency dashboard stay green?
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Architecture stylesNAT traversal
Two phones on mobile data want to video call. Why can’t they just connect?
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System design trade-offsStaleness
Your hourly job takes 20 minutes. How stale can its answer get?
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Key technologiesGuarantees
Redis said OK. Why was the write gone after a failover?
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Common patternsREAD COMMITTED
Why does wrapping SELECT and UPDATE in a transaction still double-book?
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InfrastructureScale
3 GB/s comes in. Why must 18 GB/s go out?
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Search & discoveryTrending
Cricket toss at 7:00 pm. How does autocomplete know within minutes?
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Storage & mediaHot links
50 million watch an IPL final; 2% scan one QR. What breaks?
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Social & feedsHot score
Why does a 10-vote post tie a 1,000-vote one on Reddit?
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Real-time & collaborationReconnects
Your train enters a tunnel mid-move. Did your move count?
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Location & mapsH3 cells
Why does Uber carve the world into hexagons?
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E-commerce & bookingProxy bidding
Why can you be outbid the instant you place a bid?
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Payments & fintechLate top-ups
Money left your bank, but the wallet shows nothing. Where did it go?
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Data, analytics & monitoringNeedle search
One trace ID, 162 TB of logs. How is it found in seconds?
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Developer platformsSSRF
Why would an attacker register 169.254.169.254 as a webhook URL?
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AI & LLM systemsFairness
Launch night, demand runs 30% over capacity. Who gets the GPUs?
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