This article may be unbalanced towards certain viewpoints. (July 2020)
Master/slave is a model of asymmetric communication or control where one device or process (the "master") controls one or more other devices or processes (the "slaves") 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. Historically, the master/slave terminology has existed for many decades, although in the 21st century it has been a subject of controversy due to its association with slavery, and some organizations and products have since replaced it with alternative terms.
- In electronics, master/slave relationships are used to describe some of the following scenarios:
- 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.
- In AXI, master and slave have differing roles, with master initiating transactions and the slave responding to those transactions.
- In database replication, the master database is regarded as the authoritative source, and the slave (also called replica) databases are synchronized to it.
- In photography, secondary or slave flash units may be synchronized to the master unit to provide light from additional directions.
- 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.
- 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 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 incorrectness in 2004, and named it the most politically incorrect term of that year. In 2018, Python replaced the term, after a heated debate. Many developers in the discussion on whether to make the replacement agreed that the term "slave" should be replaced while "master" was not offensive, while others argued that changing "slave" was also unnecessary. There was similarly contentious debate among developers when Django and Drupal (in 2014) and Redis (in 2017) replaced the term. The Black Lives Matter movement sparked that discussion again in 2020. Some have argued that the change is superficial and that companies should make real change to support the black community.
Different phrases have been adopted to replace existing usage of the phrase. Some examples include:
- In May 2014, Django replaced "master/slave" with "leader/follower" in its documentation, and then within two days replaced that with "primary/replica".
- In June 2014, Drupal replaced "master/slave" with "primary/replica", drawing inspiration from Django.
- Amazon Web Services/Amazon Relational Database Service
- "What is master/slave? - Definition from WhatIs.com".
- Description of the Microsoft Computer Browser Service from Microsoft KnowledgeBase
- Information on Browser Operation from Microsoft KnowledgeBase
- "'Master' and 'slave' computer labels unacceptable, officials say". CNN. November 26, 2003.
- Master/Slave, Snopes
- '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.
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- "ISC explanation via Twitter".
- "#22667 replaced occurrences of master/slave terminology with leader/follower by fcurella · Pull Request #2692 · django/django". GitHub. Retrieved 2020-07-14.
- "Fixes #22667. Replaced leader/follower terminology with primary/replica by fcurella · Pull Request #2694 · django/django".
- "Multiple databases". Django Project. Retrieved 2018-05-08.
- "Replace "master/slave" terminology with "primary/replica"". Drupal.org. 2014-05-28. Retrieved 2018-04-17.
- "Configure Database Replicas for Management Points". technet.microsoft.com. Retrieved 2018-05-08.
- "Working with Read Replicas of MariaDB, MySQL, and PostgreSQL DB Instances – Amazon Relational Database Service". docs.aws.amazon.com. Retrieved 2018-05-08.
- "Set Up Database Replication". Engine Yard Developer Center. Retrieved 2018-05-08.
- "Replication – Redis". redis.io. Retrieved 2020-07-13.
- Gryp, Kenny (2020-07-01). "MySQL Terminology Updates". MySQL High Availability. Retrieved 2020-07-02.
- "PostgreSQL: Documentation: 8.2: High Availability and Load Balancing". www.postgresql.org. Retrieved 2020-07-14.
- "Configuring the Salt Minion". SaltStack Inc. Retrieved 2019-04-19.
- "Issue 34605: Avoid master/slave terminology - Python tracker". bugs.python.org. Retrieved 2020-07-13.