Service Discovery
How a service finds the current network address of another service, when instances are constantly starting, stopping, and moving.
In a system where instances scale up and down and get rescheduled to different hosts, a hardcoded IP address is useless within minutes. Service discovery keeps a live, queryable registry of "which instances of service X are healthy and reachable right now," so a caller can look up a current address instead of assuming a fixed one — typically backed by a coordination store like etcd or Consul, itself built on Raft consensus.