Drupal Master/Slave Replication
Regarding Master/Master (High availability) is possible to do load-balancing when no master is down.

The following will make all writes go to master1 and all reads go to master2. If master1 fails, all queries will go to master2. If master2 fails, all queries will go to master1.

'master' =&gt; array('master1', 'master2')
'slave' =&gt; array('master2', 'master1')


The following will make all queries go to master1. If master1 fails all queries will go to master2:

'master' =&gt; array('master1', 'master2')
'slave' =&gt; array('master1', 'master2')