Advanced Topics(Chapter 7 of MongoDB The Definitive Guild)

　　In addition to these basic operations, MongoDB supports a wide range of advanced operations that are implemented as commands. Commands implement all

of the functionality that doesn’t fit neatly into “create, read, update, delete.”

　　The command response will always contain the key "ok". If "ok" is true, the command was successful, and if it is false, the command failed for

some reason. 

　　The basic idea behind GridFS is that we can store large files by splitting them up into chunks and storing each chunk as a separate document. Because MongoDB supports storing binary data in documents, we can keep storage overhead for chunks to a minimum. 

　　db.eval is a function that allows you to execute arbitrary JavaScript on the MongoDB server. It takes a string of JavaScript, sends it to MongoDB (which executes it), and returns the result.