Master/slave (technology)

• In electronics, master/slave relationships are common terms to describe various aspects of flip-flops, computer buses, and communication protocols.
  • In parallel ATA hard 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.
  • A master clock that provides time signals used to synchronize one or more slave clocks as part of a clock network.
  • On the Macintosh platform, rebooting into Target Disk Mode allows one computer to operate as a dumb disk enclosure presenting its storage devices to another via SCSI, FireWire, or Thunderbolt, essentially a slave mode bridge.
• In database replication, the master database is regarded as the authoritative source, and the slave databases are synchronized to it.
• Hydraulic and pneumatic systems may use a master cylinder to control one or several slave cylinders.
• 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.
• Duplication is often done with several cassette tape or compact disc recorders linked together. Operating the controls on the master triggers the same commands on the slaves, so that recording is done in parallel.
• In photography, secondary or slave flash units may be synchronized to the master unit to provide light from additional directions.

The terminology has sometimes been replaced with something different because the terms "master" and "slave" connote the practice of slavery.

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".[4][5] Following outcries about the request, the County of Los Angeles issued a statement saying that the decision was "nothing more than a request".[4] Following the controversy, Global Language Monitor found the term "master/slave" to be the most egregious example of political incorrectness in 2004, and named it the most politically incorrect term of that year; "This is but one more example of the insertion of politics into every facet of modern life, down to the level of the control processes of computer technology."[6]

"Main" and "secondary" has been proposed, enabling reusable interpretation of the remaining acronym labels for connecting. Another alternative for databases is "primary" and "replica", which is used in the documentation from IBM,[7] Microsoft,[8] Engine Yard,[9] Amazon Web Services/Amazon Relational Database Service,[10] and ACM[11] as well as in Python,[12] Django,[13][14] Drupal,[15] CouchDB,[16] Redis[17] and MediaWiki (several of which still uses "master").[18][19] Such issues are often met with aprehension because of the technical difficulty of changing a known identifier, and purely political motivation for doing so. For many use cases pointed out that such changes in terminology are cumbersome and unnecessary, because the master-slave dichotomy has been incorrectly conflated with mastering.[20]

The configuration management tool Salt uses the terms "master" and "minion".[21]

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.[22]