Single server obsługuje ~1000 concurrent users. Powyżej tego — upgrade CPU/RAM i hit ceiling. Solution: multiple servers + load balancer.
Load balancer basics
Load balancer sits in front. User connects do load balancera. Load balancer routes request do jednego z 10 servers. Distributes load. Jeśli jeden server dies — load balancer routes do remaining 9.
Session persistence
WordPress sessions store login info. Jeśli user logs on server 1, session na server 1. Next request na server 2 — session lost. Solution: shared session store (Redis).
Database replication
Single database bottleneck. Replication: master writes, slaves read. Load balancer routes queries. Słowel writes (single master) ale fast reads (multi slaves).
Auto-scaling w cloud
AWS, Google Cloud: auto-scaling groups. Peak traffic? Launch more servers. Traffic drops? Terminate extras. Pay per usage.