Master/slave is a model of asymmetric communication or control where one device or process controls one or more other devices or processes and serves as their communication hub. In some systems a master is selected from a group of eligible devices, with the other devices acting in the role of slaves.
In database replication, the master database is regarded as the authoritative source, and the slave databases are synchronized to it.
Railway locomotives operating in multiple (for example: to pull loads too heavy for a single locomotive) can be referred to as a master/slave configuration with the operation of all locomotives in the train slaved to the controls of the first locomotive. See Multiple-unit train control.
In parallel ATAhard drive arrangements, the terms master and slave are used but neither drive has control over the other. The terms however indicate which device has priority in using the shared communication interface.
Rmpi package in R is a standard master/slaves programming model.
The configuration management tool Salt uses the terms "master" and "minion".
In December 2017, the Internet Systems Consortium decided to allow the words "primary" and "secondary" as a substitute for master/slave terminology in their DNS server software BIND.
In 2003, the County of Los Angeles in California asked that manufacturers, suppliers and contractors stop using "master" and "slave" terminology on products; the county made this request "based on the cultural diversity and sensitivity of Los Angeles County". Following outcries about the request, the County of Los Angeles issued a statement saying that the decision was "nothing more than a request". Following the controversy, Global Language Monitor found the term "master/slave" to be the most egregious example of political correctness in 2004, and named it the most politically incorrect term of that year.
^'Master/slave' named most politically incorrect term, Seattle PI, December 2, 2004, The computer term "master/slave," which was banned as racially offensive by a Los Angeles County purchasing department, was named the most politically incorrect term of the year.[...] Among other terms on the top 10 list of politically charged words and phrases, issued by the word usage group Global Language Monitor, were "non-same sex marriage" to describe heterosexual unions, "waitron" for waiter or waitress and "higher being" for God, a term some people found too religious.