Databases

User updates their profile. Refreshes the page — old data shows. They update again. Still old data. They're furious. Your system is eventually consistent — but nobody told the user (or the developer who designed the UI). Here's how to manage consistency expectations in distributed systems.

Two requests check inventory simultaneously — both see 1 item in stock. Both proceed to purchase. You ship 2 items from 1. Race conditions in distributed systems are subtler than single-process races because you can't use mutexes across services. Here's how to prevent them.

Network partition splits your 3-node cluster into two halves. Both halves think they're the primary. Both accept writes. Network heals. You have two diverged databases with conflicting data. This is split brain — one of the most dangerous failure modes in distributed systems.

Your server is in UTC. Your database is in UTC. Your cron job runs at "9 AM" — but 9 AM where? Customer in Tokyo and customer in New York both get charged at your server's 9 AM. Your "end of day" reports include data from tomorrow. Timezone bugs are invisible until they're expensive.