Event Queuing in NodeJS

问题

NodeJS uses a event driven model in which only one thread executes the events. I understand the first event executed will be the user JS code. Simple example from nodeJS website of a webserver is below

var http = require('http');

http.createServer(function (req, res) {

  res.writeHead(200, {'Content-Type': 'text/plain'});

  res.end('Hello World\n');

}).listen(1337, '127.0.0.1');

console.log('Server running at http://127.0.0.1:1337/');

First event executed will perform the above steps. Later on the event loop will wait for events to be enqueued. My question is which thread enqueues event? Is there are a separate thread that does that? If yes can multiple threads enqueue events also?

Thank you.

回答1:

My question is which thread enqueues event? Is there are a separate thread that does that? If yes can multiple threads enqueue events also?

At it's core, the javascript interpreter is a loop around the select() sytem call. The select() system call informs the OS about the filehandles that your program is interested in and the OS will block execution of your program until there are events on any of those filehandles. Basically, select() will return if there's data on some I/O channel your program is interested in.

In pseudocode, it looks something like this:

while(1) {
    select(files_to_write,files_to_read,NULL,NULL,timeout)

    process_writable(files_to_write)
    process_readable(files_to_read)
    timeout = process_timers()
}

This single function allows the interpreter to implement both asynchronous I/O and setTimeout/setInterval. (Technically, this is only partly true. Node.js uses either select or poll or epoll etc. based on what functions are available and what functions are more efficient on your OS)

So, who enqueues events? The OS.

There is one exception. Disk I/O on node.js is handled by a separate thread. So for this I/O it is this disk I/O thread that enqueues events to the event loop.

It could be implemented without threads. For example, the Tcl programming language (which predates javascript but also has built-in event loop) implements disk I/O in the main thread using features such as kqueue (on BSD based OS like MacOS) or aio (on Linux and several other OSes) or overlapped-i/o (Windows). But the node.js developers simply chose threading to handle disk i/o.

For more how this works at the C level see this answer: I know that callback function runs asynchronously, but why?

来源：https://stackoverflow.com/questions/30291520/event-queuing-in-nodejs

标签

javascript

node.js

event-loop

event-queue