Inengineering,master–slave is a relationship between two systems in which one controls the other. In some cases one master controls just one slave system, but in others there are multiple slave systems controlled by the same master. Sometimes the master is a different kind of system than the slave, but sometimes there are multiple similar systems and one of them is designated the master in order to centralize external (i.e. user) control of the collection.[1]
Due to its connection toslavery, the terminology is a subject of controversy and has been replaced with alternative terms in some cases.[2][3][4]
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In photography,secondary, or slave, flash units are connected to a master unit to provide synchronized lighting.
Parallel audio duplication often entails multiplerecording devices (i.e. forcassette tape orcompact disc) linked together so that operating the controls of a master device triggers the same commands on slave devices.
Railwaylocomotives operating in the same train (for example, to pull a load too heavy for a single locomotive) may be configured for master–slave operation with all but one of the locomotives controlled from the first. SeeMultiple-unit train control.
In ahydraulic system, amaster cylinder is a control device that convertsforce into hydraulic pressure that drives movement in a slave cylinder at the other end of the hydraulic line. A common application is a vehiclebrake system.
Amaster clock providestime signals used to synchronize one or moreslave clocks as part of aclock network;[5] a slave clock receives and displays the time from a master; though it may be able to keep time itself if the master is not working.
Computer bus protocols often use a master-slave relationship. For instance, aUSB host manages access to the USB bus shared by any number of USB devices. Aserial peripheral interface (SPI) bus typically has a single master controlling multiple slaves.[6][7][8][9]I2C andI3C may even have multiple masters on a bus.Modbus also uses a master device to initiate connection requests to slave devices.
An edge-triggeredflip-flop can be created by arranging twogated latches in a master–slave configuration. It is so named because the master latch controls the slave latch's value and forces the slave latch to hold its value, as the slave latch always copies its new value from the master latch.
Indatabase replication, the master database is the authoritative source. The slave or replica database is controlled by the master database which repeats its update commands (for example by way ofevent log) to the slave. The slave therefore retains an exact copy of transaction processed by the source database (up to the most recently transmitted log)[10] This scheme is not as strict as electronic devices sharing a clock, however the slave database only does what the master tells it, unless the slave is promoted due to failure of the master. Some databases implement so called "multi-master replication", where a mix of writeable master nodes and readable nodes are used. These databases are used in scenarios where performance is an acceptable tradeoff forACID properties, for example non mission critical data, like suggesting similar purchases.
The termmaster is used in some technology contexts that do not refer to a relationship of control.Master may be used to mean a copy that has more significance than other copies in which case the term is an absolute concept; not a relationship. Sometimes the termmaster-slave is used in contexts that do not imply a controlling relationship.
Insource code management master may refer to the trunk branch.Indisk imaging the gold master is the version which will be released to manufacturing for duplication.
AParallel ATA (aka IDE)hard drive interface supports two hard drives on a cable, which are designatedmaster andslave. The distinction is required by the interface even though neither drive has control or priority over the other.
Themaster–slave terminology was first used in 1904.[11]
The terminology was used in 1988 for RFC 1059 and in 1997 for RFC 2136, related to thedomain name system. In 2020,Paul Vixie commented on his choice of words:
I introduced the master/slave terminology in RFC 2136, because I needed names for the roles in anAXFR/IXFR transaction, and the zone transfer hierarchy could be more than one layer deep, such that a server might initiate some AXFR/IXFR's to the "primary master" but then respond to AXFR/IXFR's from other servers. In retrospect I should have chosen the terms, "transfer initiator" and "transfer responder". However, thehydraulic brake and clutch systems in my car had "master cylinders" and "slave cylinders", and so I did not think I was either inventing a new use for the words "master" and "slave", or that my use of them for this purpose would be controversial.[12]
Saidhydraulic brakes for the automotive industry were patented in 1917 byMalcolm Loughead.[13] The termslave cylinder was used in other patent applications, including one byRobert Esnault-Pelterie, published in 1919.[14]
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Media analytics companyGlobal Language Monitor placedmaster/slave first in their annual list of politically charged language for 2004.[15] TheBlack Lives Matter movement in the United States sparked renewed discussion, and terminology changes occurred in 2020.[16] Some argued, however, that terminology changes were superficialperformative activism.[17][3] None-the-less, various alternate and generally context sensitive terms have been proposed including:[16][3][18][19][20][21][22][23][24][25]
In 2003, after receiving a discrimination complaint from a county employee, theCounty of Los Angeles inCalifornia asked that manufacturers, suppliers and contractors stop usingmaster andslave terminology on products.[26][27] Following complaints, the County of Los Angeles issued a statement saying that the decision was "nothing more than a request".[26]
In 2018, after a heated debate, developers ofPython replaced the term.[4] Python switched tomain,parent, andserver; andworker,child, andhelper, depending on context.[4][28] TheLinux kernel adopted a similar policy to use more specific terms in new code and documentation.[19][29]
In July 2018,Google's developerstyle guide was updated to include avoiding the termmaster in software documentation, especially in combination withslave. Instead, the guide recommends terms --when in combination -- such asprimary/secondary andoriginal/replica; Many individual variants of master and slave are given.[30]
In 2020, GitHub renamed the defaultmastergit branch tomain.[16]
The binary logs containing the changes are shipped from the master server to the slave servers so that they can exactly mimic what the main server is doing
"We found 'master/slave' to be the most egregious example of political correctness in 2004," said Paul JJ Payack, president of The Global Language Monitor.
