Writing

Notes on backend systems

Incidents I debugged, systems I designed, and what they taught me.

May 18, 2026

The Redisson Lock Bug That Took Down Three Services

Three services threw 5xx at once from one line of lock code. A walk through the diagnosis, the thread-ownership root cause, and the fix.

distributed-systemsjavaredisincidentconcurrency

System design · interactive

Explorable architecture case studies — trace a request, break it, see the tradeoffs.

◆ Interactive case study

Async Notification Pipeline

Decoupling push delivery from the request path so a slow third party can never stall the app — how I built it, the tradeoffs, and how it scales to 100M users.

JavaAmazon SQSRedisAPNs / FCM

◆ Interactive case study

Account Migration & Merge

Merging a guest account into a registered one without losing progress — safe under partial failure, idempotent under retries, auditable end to end.

JavaMongoDBRedis3 services

◆ Interactive case study

Real-time Leaderboard

Live ranks for a multiplayer game — O(log N) updates, one push per interval instead of per score change, and recovery when the cache dies.

JavaRedis ZSETWebSocketsMongoDB