Primary/secondary means they're all doing their thing, but one is preferred. There's no instruction going on between them

If you have a primary and secondary web servers, you'll use the primary first, but the secondary or secondaries are a fallback

If you have a primary and secondary drive, you have two drives, one of which is more important (probably because you booted from it). The secondary could be a copy or just another drive, either way the OS or a raid controller is managing it, one drive doesn't manage another

Similarly, we have dispatch/worker- the difference between that and master/slave is that they're different things. A master should be able to work without a slave, and a slave should be capable of being promoted to master - a dispatcher can't do the work and the worker can't take over if the dispatch goes down

The funny thing is we don't use master/slave much anymore, the whole premise is that the slave doesn't start to do what it does when it starts up. I can't think of any examples of it in the past decade - other paradigms, with a different relationship and a different name, have replaced it