What is the order of operations for the mongodb-native driver?

Let's say you have a class that's purpose is to save a document of some sort and let's say it has a long life. How many times should open be called? Once per db write? When is close supposed to be called? Essentially I want a class method that looks like this:

  var myMongoClass = new MongoDB(server,port)
  myMongoClass.write_file(filename,callback)
  myMongoClass.write_doc(doc,callback)

I posted this a while ago and got it working: Problem with MongoDB GridFS Saving Files with Node.JS

It's now not working at all and failing with TypeError: Cannot read property 'md5' of null

Every time I work with this library I want to bang my head through a wall.

It seems like the correct answer is create a client and keep that client open for the duration of the application (never explicitly calling close). I have a wrapper that keeps a reference to the connected client and my app only boots up if the connection is received.