The HLD interviewSpeed of light
No optimisation brings Frankfurt closer than 67 ms to Pune. Why not?
0%
Page 9
More questions.
Core conceptsMod N
A fifth depot opens. Why do four parcels in five switch depot?
0%
Networking and APIsContracts
You rewrite the payments service before a Flipkart-style sale. Why does no app notice?
0%
Data and storageChild index
Why did deleting 1,000 test orders take 11 hours?
0%
Caching fundamentalsScans
Why does one nightly export make your cache forget everything popular?
0%
Messaging and streamsAppend-only
Why does your bank never rub out a wrong entry?
0%
Distributed coordinationDNS caching
Why does your Java client keep calling a server that died?
0%
Running in productionObservability
Checkout’s p99 doubled. How do you find a cause nobody graphed?
0%
Why is it slow? Performance anti-patternsThe symptom
One merchant starts a sale. Why do 999 other merchants time out?
0%
SecurityNonces
Encrypt 20,000 records a second under one key. Why stop after 2.5 days?
0%
Multi-tenant SaaSCost
Assume 2,000 tenants. Why does a stack each cost 58× more than sharing?
0%
Architecture stylesSessions
One app server crashes. Why is everyone on it logged out?
0%
System design trade-offsThe kill test
Kill one server at noon. How do you know if anyone noticed?
0%
Key technologiesWatermarks
How does Flink know a minute of clicks is finally complete?
0%
Common patternsHit rate
Why does going from 95% to 99% cache hits cut database load fivefold?
0%
InfrastructureAlgorithms
Why can a limit of 100 a minute let 200 through in 20 seconds?
0%
Search & discoveryURL-seen
Why is “have I seen this URL?” harder than downloading the page?
0%
Storage & mediaErasure coding
Why not just keep three copies of every file?
0%
Social & feedsRanking
Out of a thousand posts, how does the app pick your top 20?
0%
Real-time & collaborationUnread badges
You @everyone ten million people. Why does that cost one write?
0%
Location & mapsRouting
Why can’t Maps just run Dijkstra from Bengaluru to Mumbai?
0%
E-commerce & bookingCart merges
You deleted the phone on your laptop. Why is it back in your cart?
0%
Payments & fintechOne thread
LMAX ran 6 million orders a second on one thread. Why not more threads?
0%
Data, analytics & monitoringThe merge trap
Every server sends its top 2. Why is the real #1 missing?
0%
Developer platformsTenancy
Why not give each of 50,000 companies its own database?
0%
AI & LLM systemsChunked prefill
One 6,000-token prompt arrives. Why do 128 other answers freeze?
0%