Mastering JavaScript

Server-Side JavaScript
An asynchronous evented-model in a
browser
Before we try to understand Node, let's try to understand JavaScript in a browser.
Node relies on event-driven and asynchronous platforms for server-side JavaScript.
This is very similar to how browsers handle JavaScript. Both the browser and Node
are event-driven and non-blocking when they use I/O.
To dive deeper into the event-driven and asynchronous nature of Node.js, let's first
do a comparison of the various kinds of operations and costs associated with them:
L1 cache read
0.5 nanoseconds
L2 cache read 7 nanoseconds
RAM
100 nanoseconds
Read 4 KB randomly from SSD
150,000 ns
Read 1 MB sequentially from SSD 1,000,000 ns
Read 1 MB sequentially from disk 20,000,000 ns
These numbers are from https://gist.github.com/jboner/2841832 and show
how costly Input/Output (I/O) can get. The longest operations taken by a computer
program are the I/O operations and these operations slow down the overall program
execution if the program keeps waiting on these I/O operations to finish. Let's see an
example of such an operation:
console.log("1");
var log = fileSystemReader.read("./verybigfile.txt");
console.log("2");
When you call fileSystemReader.read(), you are reading a file from the
filesystem. As we just saw, I/O is the bottleneck here and can take quite a while
before the read operation is completed. Depending on the kind of hardware,
filesystem, OS, and so on, this operation will block the overall program execution
quite a bit. The preceding code does some I/O that will be a blocking operation—
the process will be blocked till I/O finishes and the data comes back. This is the
traditional I/O model and most of us are familiar with this. However, this is costly
and can cause terribly latency. Every process has associated memory and state—both
these will be blocked till I/O is complete.
[ 202 ]
www.it-ebooks.info