Common patternsDurable execution
A worker dies mid-order. How does another finish it without charging twice?
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More questions.
InfrastructureFencing
Jepsen found etcd’s data rock-solid, yet two clients held one lock. How?
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Search & discoveryScale
Results day: 72,000 shared-page requests a second. Why do only 333 reach the origin?
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Storage & mediaSearch
You type “tum hi ho”. How does search find “तुम ही हो”?
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Social & feedsRouting
Your comment on a tiny stream reaches 2 of 750 servers. How?
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Real-time & collaborationScale
Zoom went from 10M to 200M daily participants in three months. What breaks?
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Location & mapsScale
Assume 625 uploads a second on Sunday morning. What breaks first?
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E-commerce & bookingExpiry
Stock is gone in seconds. Why does a second sale start at 12:10?
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Data, analytics & monitoringElection night
Election night: one news site sends 30,000 events a second. What breaks first?
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AI & LLM systemsSandboxes
Why can’t code an AI agent wrote just run in a container?
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Core conceptsLoad parameters
Twitter took just 5,000 tweets a second. So why was it hard?
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Networking and APIsThe handshake tax
Gmail went HTTPS for everyone and added no machines. So what makes HTTPS slow?
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Data and storageORMs
Why does a page of 50 restaurants send 101 queries?
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Messaging and streamsSnapshots
How do you copy 400 million rows while they keep changing?
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Distributed coordinationOdd sizes
Why does a fourth node make your cluster no safer than three?
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Running in productionFlag debt
Seven of eight servers got the code. How did Knight lose over $460 million?
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Why is it slow? Performance anti-patternsBuffer cache
PostgreSQL already keeps hot rows in memory. Why add Redis at all?
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Multi-tenant SaaSWildcards
Why would 400 sign-ups in a week need 8 weeks of certificates?
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Architecture stylesTrust
Your app sends price: 1 for a seat. What should the server charge?
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System design trade-offsThe trap
Why does “make everything async with Kafka” make interviewers wince?
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Key technologiesSizing
8 million sessions fit in 4 GB. Why do you still need five primaries?
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Common patternsHot keys
Why doesn’t adding cache shards help one hot product?
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InfrastructureRule changes
How could one typo reject almost every UPI payment within 30 seconds?
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Social & feedsPaging
Three friends post while you scroll. Why don’t you see repeats?
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Real-time & collaborationScale
A million ops a second at peak. Why can one server own each document?
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E-commerce & bookingLate payments
Your UPI payment lands after your seat hold expires. Now what?
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