MasterSlave

Master-slave, often written master/slave, is a model of relationship in technical systems in which a controlling component, the master, directs one or more subordinate components, the slaves. The master issues commands, data, or timing, and the slaves carry out actions or respond to requests. The arrangement is defined by the direction of control rather than the inherent superiority of the devices involved.

Common domains include computing and electronics. In serial communications such as SPI and I2C, the master device initiates transactions and the slave devices respond or hold data. In data management, master/slave replication uses a primary database to propagate updates to one or more replicas. In distributed systems, a master node may coordinate worker nodes or tasks. In automation and robotics, a master controller governs slave actuators, sensors, or subsystems. The term is descriptive of function but can be misleading if taken as a value judgment rather than a mechanism.

Terminology and reception: The phrase has historical associations with human slavery, leading many organizations to favor alternatives such as primary/replica, leader/follower, controller/agent, or host/guest. Some standards and projects still use master/slave, but there is a trend toward neutral naming. When discussing systems, authors often clarify that the terms describe roles and do not imply qualitative assessments. The ongoing discussion reflects broader efforts to use inclusive language while preserving technical meaning, and the choice of terms can vary by domain and audience.