The following setup has one master and two slaves with cross failover. If master fails, system will continue in read-only mode. If both slaves fail, system will continue only using the master.
<?php
$databases['default']['master'] = array (
'database'=> 'mydatabase',
'username'=> 'readwrite',
'password'=> 'readwrite',
'host'=> 'master',
'port'=> '',
'driver'=> 'mysql',
'prefix'=> '',
);
$databases['default']['slave'][] = array (
'database'=> 'mydatabase',
'username'=> 'readonly',
'password'=> 'readonly',
'host'=> 'slave1',
'port'=> '',
'driver'=> 'mysql',
'prefix'=> '',
'readonly'=> TRUE,
);
$databases['default']['slave'][] = array (
'database'=> 'mydatabase',
'username'=> 'readonly',
'password'=> 'readonly',
'host'=> 'slave2',
'port'=> '',
'driver'=> 'mysql',
'prefix'=> '',
'readonly'=> TRUE,
);
$databases['default']['default'] = array (
'driver'=> 'autoslave',
'master'=> array('master', 'slave'),
'slave'=> array('slave', 'master'),
);
?>