When requesting an asynchronous file read, the command to get the file from the operating system is issues. Being asynchronous, everything continues along on its way, and no one is being held up while waiting for the server response. But, since the program has moved along without waiting, what happens when the response finally happens? When making asynchronous requests, we specify a callback function, which is simply a function that is called when an asynchronous request is responded to.
Asynchronous servers are considered scalable, as they can handle multiple synchronous users without much if any degradation in performance. For this reason, in node, use fs.readFile() which is asynchronous instead of the synchronous fs.readFileSync().