The HLD interviewRecovery
Your design is wrong and you just noticed. What do you say next?
0%
Page 20
More questions.
Core conceptsRetries
Every layer retries four times. How does one tap become 64 queries?
0%
Networking and APIsWhy limit
A bot triggers 100 OTP texts a second. What does an hour cost?
0%
Data and storageBloat
Your table is 400 GB but holds 50 GB. Why didn’t VACUUM shrink it?
0%
Caching fundamentalsWrite-behind
Write-behind turns 100,000 database writes into one. What does it cost?
0%
Messaging and streamsFan-out cost
Why do six subscribers cost six times the bill?
0%
Distributed coordinationPivot steps
Why can you refund ₹450 but never uncook a biryani?
0%
Running in productionGray failure
Every health check is green. So why are users getting errors?
0%
Why is it slow? Performance anti-patternsMultiplication
Three layers each added “just one retry”. How many queries per tap?
0%
SecurityShared compute
A customer’s notebook deleted its own firewall. What did it reach next?
0%
Multi-tenant SaaSLittle’s law
Your rate limit is holding. So why are 500 reports running at once?
0%
Architecture stylesDual write
The order saved, then the server crashed. Why did the kitchen never hear?
0%
System design trade-offsEvent time
A network blip moved 12:00’s clicks into 12:02. Who gets billed wrong?
0%
Key technologiesScaling
Why do ZooKeeper’s writes get slower every time you add a server?
0%
Common patternsMoving points
A million drivers ping every 4 seconds. Where should those pings go?
0%
InfrastructureJSON
Why would a browser open the wrong order from a perfectly valid ID?
0%
Search & discoverySale day
The sale opens at midnight and searches jump 10×. Why not autoscale?
0%
Storage & mediaNotifications
Why not just push every edited file to every device?
0%
Social & feedsPlayback
Swipe on a 4G phone. Why is the next video already playing?
0%
Real-time & collaborationSpectators
A world-championship game draws 100,000 viewers. Why don’t the players feel it?
0%
Location & mapsDensity
Why is a fixed 2 km radius wrong in Connaught Place and the desert?
0%
E-commerce & bookingEnding on time
Your bid left Pune at 19:59:59.990. The auction ends at 20:00. Did it count?
0%
Payments & fintechThundering herd
In March 2020, Robinhood went down two trading days running. Why?
0%
Data, analytics & monitoringRedaction
A developer logs card and Aadhaar numbers. Where do you stop them?
0%
Developer platformsPrepaid
A ₹100-a-second sender hits zero balance. How far past zero can it go?
0%
AI & LLM systemsInjection
A wiki page says “ignore your rules.” Will the company AI obey?
0%