In photography,
secondary, or slave, flash units are connected to a master unit to provide synchronized lighting. Parallel audio duplication often entails multiple
recording with devices (i.e., for
cassette tape or
compact disc) linked together so that operating the controls of a master device triggers the same commands on slave devices. Railway
locomotives 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. See
Multiple-unit train control. In a
hydraulic system, a
master cylinder is a control device that converts
force into hydraulic pressure that drives movement in a slave cylinder at the other end of the hydraulic line. A common application is a vehicle
brake system. A
master clock provides
time signals used to synchronize one or more
slave clocks as part of a
clock network. 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.
Computing Computer bus protocols often use a master-slave relationship. For instance, a
USB host manages access to the USB bus shared by any number of USB devices. A
serial peripheral interface (SPI) bus typically has a single master controlling multiple slaves.
I2C and
I3C may even have multiple masters on a bus.
Modbus also uses a master device to initiate connection requests to slave devices. An edge-triggered
flip-flop can be created by arranging two
gated 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. In
database 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 of
event 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) 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 is used. These databases are used in scenarios where performance is an acceptable tradeoff for
ACID properties, for example, non-mission-critical data, like suggesting similar purchases. == Non-examples ==