health checks

Anycast turns BGP convergence into your failover mechanism. Sessions stick to the site they landed on, monitoring must go per-site, and a route you cannot withdraw cleanly is worse than DNS.

DNS failover is a blunt instrument with three delays stacked on top of each other: detection, propagation and client caching. Knowing their sizes tells you when DNS is the right layer at all.

Self-healing unavailability is a timeout arithmetic story. Pull the balancer's removal timestamps, lay them against the GC log, and the pause that ejects the instance will be sitting there in plain sight.

A health endpoint that returns 200 because the process is running tells you the process is running. Interviewers want to hear where you draw the line between a check that pages and a check that removes traffic.