DNS
also called Domain Name System
The distributed directory that resolves names to addresses, and a surprisingly load-bearing part of most architectures.
Beyond name resolution, DNS is used architecturally for traffic management: weighted records for canary releases, latency-based or geo-based routing to the nearest region, and health-checked records for failover.
The property that repeatedly causes trouble is caching. TTLs are honoured inconsistently — resolvers, operating systems, JVMs and application libraries all cache, sometimes indefinitely — so DNS-based failover is never as fast as the TTL suggests. Plan for a long tail of clients still resolving to the old address, and prefer an anycast address or a load balancer for failover that must be quick.
It is also a single point of failure worth taking seriously. A registrar or DNS provider outage takes down everything you own at once, which is the argument for a secondary provider.