What is a good substitute for Master/Slave?

somewhat depends on context what the best replacement is.

Primary/Replica is often used in databases.

Distributed databases where by nature that often can change had already often adopted the terminology used by consensus algorithms, which typically speak of a "leader" (that's choosen out of "nodes", "members", "servers" by "leader election" of some kind. Sometimes "follower" for the non-leader nodes)

Primary/Secondary is an option.

In electronics there is the suggestion of Controller/Peripheral

Master slave has and always will be a lazy metaphor in databases. We can be more specific. Active / passive, primary / secondary, leader / follower are all very different and imply different actions and mechanisms.

Databases: primary/replica Repository: main branch CI/CD: server/agent

These are all fairly common terms I see or use for these scenarios.

There are a other terms you could use though in different systems that make sense.

Master: main/primary/broker Slave: agent/worker/replica/node

Bust out a thesaurus. Ace is apparently close to master. I kinda like that word. Ace of database.

There's a good list in this article...

https://www.zdnet.com/article/linux-team-approves-new-termin...

Ah detaro for the win!: "Controller/Peripheral" I'm a desktop/embedded guy (no DB's) so this makes the most sense for me. Thx detaro!

Primary/Secondary...

primary/replica