It is a replication method where you have a main Relational Database for reads and writes, and you replicate writes to one or more slaves, which serve only reads. Other slaves can replicate in a tree-like fasion.
When one of the masters goes offline, the system can continue to operate in read-only mode until a slave takes over and is promoted as a new master, and the old master, once it goes back live, becomes a slave, that is updated with the data.
Disadvantages
When using master-slave replication, you may end up in a situation where your system is not available for users. Also, it adds additional logic needed to promote a slave to a master