MySQL's replication system has been a core feature often touted for scaling (sort of) and redundancy (sort of). I'll describe a client's extensive use of MySQL replication (they have more than 200 MySQL instances replicating to one another) as a reference for the many uses and misuses of replication.
Not locked into proprietary space… many options. Based from the same code tree Lib mysql vs lib drizzle – licensing gotcha libmysql gpl libdrizz bsd drizzle Maria Our delta Google patches Percona xtradb
Google patches include a better malloc And completely changed locking
Choose 5.1 or 5.4 only if you need the featuresets.
Much performance pain come into play with lock contention
Understand
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API SHOULD KNOW THE DIFF proxy as an option if API not possible. scale writes. write own interfaces? check on it. btree index larger than memory -- writes get _slow_ Memcached – good for short lived data and caching wafflegrid -- patches to mysql -- innodb bufferpool over memcached
LVM and slaves give easy opportunities for creating clones for testing