node-mongodb-native的主要提交者说 ：

当您的应用程序启动并重用
db对象时，您打开一次MongoClient.connect。它不是一个单独的连接池每个.connect
创建一个新的连接池。

You open do MongoClient.connect once when your app boots up and reuse the db object. It's not a singleton connection pool each .connect creates a new connection pool.

重用由MongoClient.connect（）产生的db对象。这给你池，并将提供明显的速度增加与打开/关闭连接每个数据库操作相比。

So, to answer your question directly, reuse the db object that results from MongoClient.connect(). This gives you pooling, and will provide a noticeable speed increase as compared with opening/closing connections on each db action.