The HLD interviewWorked example
Assume 25 million phones poll a live cricket score. How can one server cope?
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More questions.
Core conceptsGray failure
Its heartbeats are perfect. So why can’t customers reach the VM?
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Networking and APIsThe unknown outcome
You tapped Pay once, it failed, you tapped again. Why two debit SMS?
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Data and storageThe rule
Why does writing every change twice make commits faster?
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Caching fundamentalsCold start
Your cache restarts mid-sale. Why does the database fall over?
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Messaging and streamsRetention
Your consumer broke on Friday evening. How close were you to losing everything?
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Distributed coordinationRedlock
Five Redis servers and a majority vote. Why isn’t that a safe lock?
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Running in productionLatency
Why does an Indian user of a US-only app wait 390 ms?
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Why is it slow? Performance anti-patternsFair queues
Your one job is queued behind 60,000. Why wait five minutes?
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SecurityKMS quota
12,000 encrypted reads a second, one KMS. Why does it throttle?
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Multi-tenant SaaSDatabase per tenant
Assume 1,000 tenant databases and 20 app instances. Why 200,000 connections?
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Architecture stylesConnections
1,000 Lambda environments each open a Postgres connection. Why do 900 fail?
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System design trade-offsBrokers
Why did Kafka make its consumers pull instead of pushing to them?
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Key technologiesConsumer groups
Why does adding a consumer sometimes make Kafka no faster at all?
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Common patternsResume
Redis dropped your chat message. Why did no user notice?
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InfrastructureConflicts
Your laptop adds rice after your phone adds dal. Why might rice vanish?
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Search & discoveryPrivacy
Could autocomplete ever suggest your PNR number to a stranger?
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Storage & mediaRedirects
Why can’t you kill a phishing link once you’ve sent a 301?
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Social & feedsCelebrities
A film star posts during the IPL final. Why doesn’t your feed stall?
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Real-time & collaborationStorage
Most people never delete email. How much new mail piles up each year?
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Location & mapsDouble booking
Two dispatchers grab the same driver at once. Who wins?
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E-commerce & bookingChannel sync
A villa is also listed on Booking.com. Why can it sell twice?
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Payments & fintechMarket data
1,000 subscribers want every order event within microseconds. Why not TCP?
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Data, analytics & monitoringCardinality
One label added “just for today”. Why did every team lose data?
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Developer platformsNoisy neighbours
In one shared job queue, why does a 200,000-job import delay everyone 40 minutes?
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AI & LLM systemsEdits
You edit a question from ten minutes ago. What happens to the old answer?
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