The HLD interviewThe order
How does a doctor cover a whole patient in fifteen minutes?
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More questions.
Core conceptsBandwidth
Why doesn’t a fifth lane get anyone to the stadium sooner?
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Networking and APIsCarrier NAT
Why does your rate limiter think 500 phones are one abuser?
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Data and storageWrite cost
Why does one status update touch four indexes in PostgreSQL, one in MySQL?
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Caching fundamentalsStale sets
Why does the canteen board still say ₹15 after the price went up?
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Messaging and streamsFan-out
How does checkout notify six teams without calling any of them?
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Distributed coordinationAddress churn
The restaurant service changes address close to 300 times a day. Who keeps track?
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Running in productionSlow vs down
How can a slow recommendations widget take down checkout in 0.4 seconds?
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Why is it slow? Performance anti-patternsThe symptom
Same code. A 3-item cart takes 9 ms, an 80-item cart 240. Why?
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SecurityDelegated access
How does a budgeting app read your bank transactions without your password?
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Multi-tenant SaaSBridge
Your bank customer wants its own database. Do you rebuild your SaaS?
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Architecture stylesModular monolith
Shopify’s monolith has over 2.8 million lines of Ruby. Why didn’t they split it?
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System design trade-offsTwo kinds of service
Why can any bank teller serve you, but only your doctor will do?
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Key technologiesGuarantees
Your payment app says “done”. What exactly has PostgreSQL promised?
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Common patternsThe playbook
Assume 500,000 phones ping every 4 seconds. Can one database keep up?
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InfrastructureScale
Sale opens at 00:00. What keeps 9 million reads a second off the database?
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Search & discoveryQuery understanding
You type “red running shoes under 2000”. How does the app read that?
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Storage & mediaSync
You save a wedding video in Pune. How is it in Toronto minutes later?
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Social & feedsBig accounts
An account with 100M followers tweets. Why doesn’t it take minutes to reach you?
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Real-time & collaborationFan-out
A million people are online in one server. Who actually gets your message?
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Location & mapsNear me
You type “biryani” in Bengaluru. How do twenty nearby places appear so fast?
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E-commerce & bookingExactly once
Double-tap “Place order” on flaky 4G. Why do you pay only once?
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Payments & fintechDouble entry
You pay ₹1,249.50. Why does the ledger say it adds up to zero?
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Data, analytics & monitoringIPL final
How does a hashtag nobody used at 7 pm top India by 7:40?
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Developer platformsCanary
A bad build passes every test. What stops it reaching every user?
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AI & LLM systemsRAG
Why isn’t “vector database plus LLM” enough for company search?
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