The HLD interviewEstimates
Why can one line of arithmetic, 2,500 disks, kill a design?
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More questions.
Core conceptsThe chain
A cook misreads the recipe. How do 800 wedding guests never taste it?
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Networking and APIsThe gRPC trap
Your balancer split connections perfectly. Why are half your servers idle?
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Data and storageWhen a copy pays
Why can’t a menu page just count each dish’s orders live?
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Caching fundamentalsHit ratio
Going from 90% to 99% cache hits sounds small. Why is it 10×?
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Messaging and streamsPolling
Why does search still sell a product you deleted an hour ago?
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Distributed coordinationClock skew
A rider is on the way, yet the order still says PAID. Why?
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Running in productionDeep checks
Why can a one-minute database blip restart every pod in your service?
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Why is it slow? Performance anti-patternsThe symptom
Payments slowed from 40 ms to 1 s. Why did the cart page fail?
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SecurityPKCE
A thief grabs your login code mid-redirect. Why does it get nothing?
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Multi-tenant SaaSExactly once
A retry resent 1,840 tokens of usage. Does your customer pay twice?
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Architecture stylesBounded contexts
Ask five teams what an “order” is. Why do you get five answers?
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System design trade-offsTwo ideas
One cook keeps four pans going. When do you need four cooks?
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Key technologiesGuarantees
Kafka acknowledged your write with acks=all. How can it still vanish?
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Common patternsThe problem
A 4 GB upload takes 27 minutes. The Wi-Fi drops every 10. Now what?
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InfrastructurePriority
Your OTP is queued behind 20 million sale SMS. How does it skip ahead?
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Search & discoveryData structure
Why not just query a database on every keystroke?
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Storage & mediaHashing
Why not just hash the long URL and keep seven characters?
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Social & feedsRanking
Why do four mutual connections count for less than three?
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Real-time & collaborationRanking
Why can’t a database just tell you your rank?
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Location & mapsScale
750,000 location updates a second. Why can’t a database keep up?
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E-commerce & bookingSale night
A TV drops ₹7,000 at midnight. How do 200,000 watchers hear in minutes?
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Payments & fintechImmutability
A bug took 10% commission instead of 6%. Why not just edit the row?
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Data, analytics & monitoringChange data capture
A seller cuts a phone’s price. How does search know within seconds?
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Developer platformsRunners
Why does CI for 5,000 engineers need 28 new machines a second at peak?
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AI & LLM systemsContext
The model remembers nothing. So what does your 60th message actually send?
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