yii2 的配置都是在启动时加载的，所以mongo的配置也同样在component里面配置。
具体实现(无用户和密码)：
[ ‘mongo1‘ => [ ‘class‘ => ‘\yii\mongodb\Connection‘, ‘dsn‘ => ‘mongodb://server:27017/mongo_db, ], ], ‘mongo2‘ => [ ‘class‘ => ‘\yii\mongodb\Connection‘, //‘dsn‘ => ‘mongodb://server1:27017,server2:27017/mongo_db‘, ‘options‘=> [ //https://docs.mongodb.com/manual/reference/connection-string/#connections-connection-options ], ]
原文：https://www.cnblogs.com/jack-jt-z/p/10515599.html