MySQL Slave Scaling (and more)

"At Booking.com, we have very wide replication topologies. It is not uncommon to have more than fifty (and sometimes more than a hundred) slaves replicating from the same master. When reaching this number of slaves, one must be careful not to saturate the network interface of the master. A solution exists but it has its weaknesses. We came up with an alternative approach that better fits our needs: the Binlog Server. We think that the Binlog Server can also be used to simplify disaster recovery and to ease promoting a slave as a new master after failure. Read on for more details. "

 http://blog.booking.com/mysql_slave_scaling_and_more.html