当主数据库由于某些原因无法连接时，你可以配置多个灾备数据库。
例如可以像下面这样为一个连接配置灾备数据库:
config/database.php
'127.0.0.1',
'username' => 'demo8',
'password' => 'demo8',
'database' => 'demo8',
'DBPrefix' => 'dr_',
];
// 备用库
$db['failover'] = [
[
'hostname' => '127.0.0.122',
'username' => 'demo8',
'password' => 'demo8',
'database' => 'demo8',
],
[
'hostname' => '127.0.0.133',
'username' => 'demo8',
'password' => 'demo8',
'database' => 'demo8',
]
];