The HLD interviewReal numbers
India makes 800 million UPI payments a day. How many a second at peak?
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More questions.
Core conceptsCascades
One payment provider slows down. Why does every endpoint go down with it?
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Networking and APIsEnd to end
TCP already checksums every packet. Why do storage systems add their own?
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Data and storageRaces
Asha tapped “Create account” twice. Why does she now have two accounts?
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Caching fundamentalsDelete, don’t update
On a write, why delete the cache key instead of updating it?
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Messaging and streamsPhantom events
Why did payments charge for an order that never existed?
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Distributed coordinationCost of action
Why does etcd act after one second, but HDFS wait 10.5 minutes?
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Running in productionBlast radius
Your app runs in three zones. Why does one bad deploy kill all three?
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Why is it slow? Performance anti-patternsThe symptom
Every day at 10 am, “Cannot assign requested address”. What ran out?
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SecurityThe handshake
Someone records an entire mTLS handshake. Why can’t they replay it?
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Multi-tenant SaaSCross-tenant leaks
Can editing one header let you read another company’s invoice?
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Architecture stylesThe plumbing
A retry bug in a shared library. Why must 60 teams redeploy?
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System design trade-offsSame price
One big box or 24 small ones cost the same. Which do you run?
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Key technologiesConsensus
How do your servers agree on one leader, and never two?
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Common patternsThe problem
The pod crashed right after charging the customer. Who remembers the order?
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InfrastructureTwo holders
Two workers both think they hold the lock. Who pays the merchant?
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Search & discoveryFreshness
Why not just update the search index every time a price changes?
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Storage & mediaPrivacy
Could skipping an upload reveal a stranger’s salary?
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Social & feedsHot streams
Assume 20,000 people comment the second a wicket falls. What do you see?
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Real-time & collaborationTicks
Two grey ticks on your message. Which machine put each one there?
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Location & mapsGeo index
Why can’t a B-tree on latitude and longitude find places near you?
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E-commerce & bookingTail latency
Each call is slow 1% of the time. How often is the page?
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Payments & fintechDouble spend
Two ₹800 payments hit a ₹1,000 wallet in the same millisecond. Then what?
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Data, analytics & monitoringClick IDs
You double-tap an ad in a cricket app. Is the advertiser charged twice?
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Developer platformsOutbox
The payment committed, then the pod died. Where did the webhook go?
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AI & LLM systemsHybrid search
Why does vector search miss circular HO/OPS/2025/118?
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