The master is configured with the endpoints for all the chunkaervers. A separate control/config system supported this including things like automatic replacement of dead nodes.
Awesome! Just checked the whitepaper there was a cluster with 300 machines, so all machines had to be described in configuration file that was used by master, right?
Mostly. Binaries of that era took everything on the command line. The config system would generate that from files and keep the running binaries up to date via ssh.