Main is the replacement for master for git branches, not the general master-slave pattern. Wikipedia suggests:

Other replacement names include controller, default, director, host, initiator, leader, manager, primary, principal, root; and for slave: agent, client, device, performer, peripheral, replica, responder, satellite, secondary, subordinate, and worker.

I usually use controller / worker if it's a local process or controller / remote if the subordinates are on different hosts.