System design · Spaced repetition
System design, one napkin at a time.
Short daily drills on latency, caching, databases and the rest, scheduled by spaced repetition so they're still there when the interview starts.
Every cache entry gets an identical 60-second TTL. Under load, the database sees synchronized latency spikes every 60 seconds. What's the standard fix?
Keys written together with one TTL expire together, so their misses hit the origin in one burst. Jitter spreads expirations out and flattens the spike.
How it works
Ten questions over coffee. Scheduled so they stick.
- 01
Pick a session.
3–20 questions, or 1–25 minutes.
- 02
Answer.
Multiple choice, written to test reasoning rather than trivia.
- 03
Learn why.
See the explanation and why each wrong option is wrong.
- 04
Rate it.
Hard, Good or Easy. The FSRS scheduler brings it back right before you'd forget it.
Speaking practice
Recognizing a term isn't the same as using it.
Picking idempotent out of four options is easy. Saying “retrying the webhook is safe because the handler is idempotent” in a design review is the real skill.
Speaking cards name a concept, and you say a sentence out loud. You check the transcript, then an AI judge decides whether you used the term correctly. There's no typing option on purpose, because typing is the easier skill.
- 1 · The promptSPEAKING CARD · API DESIGN
Use idempotency in a sentence.
Tap to record - 2 · Check your transcript
“Retrying the webhook is safe because the handler is idempotent.”
Speech recognition can mangle jargon, so you confirm it first.Re-recordSubmit - 3 · The verdictSounds right
You tied idempotency to safe retries, which is exactly what it buys you.
HardGoodEasy
The judge returns pass or fail and one sentence of reasoning. No score out of 100.
The tutor
Stuck on a word? Ask. It won't tell you the answer.
Don't know what “Read Committed” means? Open the tutor next to any card and ask. It explains the term but won't tell you which option is right.
What are the four standard SQL isolation levels, ordered from loosest to strictest?
What's in it
850+ questions.200+ concepts.21 topics.
- API design
- Caching
- CAP theorem
- Coordination
- Data modeling
- Data structures
- Databases
- Design process
- Estimation
- Geospatial
- Hardware
- ID generation
- Load balancing
- Messaging
- Networking
- Scalability
- Search
- Security
- Storage
- Stream processing
- Transactions
Classic design problems
- URL shortener
- Ticket booking
- Chat
- Ride-sharing
- Social feed
- Business reviews
- Local delivery
- File sync
- Video streaming
- Photo sharing
- News aggregator
- Post search
- Dating app
- Fitness tracker
- Online judge
- Price tracker
- Distributed cache
- Rate limiter
- Job scheduler
- Web crawler
- Online auction
- Flash sale
- Stock trading
- Payments
- Live comments
- Notifications
- Collaborative editor
- Online chess
- Trending top-K
- Ad click aggregator
- Metrics monitoring
- AI chat app
Stats
Know where you're weak and where you're strong.
Accuracy by topic, lesson and concept tells you what to drill next. The forecast shows how many cards come due each day, so tomorrow's reviews aren't a surprise.
More features
Think before you see the choices.
Optionally hide the options until you tap, or until a timer runs out, so you practice recalling the answer instead of recognizing it.
Go back through a session.
Revisit earlier cards and their explanations with Back and Forward, without losing your place.
Flag bad questions.
Flag a card and it leaves your rotation while it's reviewed.