Quick Reference

Last-minute rapid recall.

Design checklist (any system-design prompt)

Clarify → estimate scale → API → components → data/storage → scale (LB, cache, async) → resilience (timeout/retry/circuit-breaker/bulkhead) → trade-offs.

Resilience

Timeout · Retry (idempotent + backoff+jitter) · Circuit breaker · Bulkhead · Fallback/graceful degradation.

Caching

cache-aside/read-through · TTL + LRU · local (per-instance) vs distributed (Redis) · invalidation + stampede.

REST

Verbs & idempotency (GET/PUT/DELETE idempotent, POST not) · status 400/401/403/404/409/429/5xx · URI versioning · pagination (offset vs cursor) · statelessness · mask sensitive data.

Java quick hits

== vs equals · equals+hashCode · HashMap (buckets, 0.75, treeify@8) · ConcurrentHashMap vs synchronizedMap · streams lazy+single-use · Optional (no .get) · records = immutable DTOs · constructor injection · singleton beans stateless · @Transactional rolls back on unchecked.

Concurrency quick hits

volatile=visibility · synchronized/Lock=mutual exclusion · Atomic=CAS · deadlock 4 conditions · ThreadLocal confinement · immutability = free thread safety · virtual threads (Java 21).

Theory

ACID vs BASE · CAP (C vs A under partition) · SOLID/DRY/KISS/YAGNI · idempotency · statelessness · at-least-once vs exactly-once.

Coding-round process

Clarify → example → brute force + Big-O → optimize → clean code → test edges → state complexity. Patterns: hashmap · two-pointer · sliding window · binary search · BFS/DFS · heap/top-k · DP basics · LRU cache.

In the room

Think out loud · state trade-offs · lead with impact then how · quantify · "I don't know, here's how I'd find out".