Using promises instead of error first callbacks

Now, let's start considering several techniques that will come in handy when writing services.

Node runs as a single thread, so if every time it had to call a service, or read a file, or access a database, or do any other I/O-related operation, it would have to wait for it to finish, then attending requests would take a long time, blocking other requests from being attended, and the server would show a very bad performance. Instead, all operations such as those are always done asynchronically, and you must provide a callback that will be called whenever the operation is finished; meanwhile, Node will be available to process other clients' requests.

Node established a standard that all callbacks should...