It isn’t clear to me that master/slave has a specific meaning. For instance in the DNS world, it's better to talk about read-replicas, public authoritative servers, update servers, zone transfer servers, DNSSEC signers, etc. - the nuances aren't captured by a primary/secondary distinction. The old master/slave terminology is badly misleading since the DNS protocol doesn't allow the master to command the slaves to do anything: in the DNS it's the slaves that make the demands. (BIND now supports primary/secondary in its configuration though there are parts that still require the old words.)