Node.js httpserver.listen method ambiguity

Question

I have been working in Node.js and I am wondering what exactly does listen method do, in terms of eventloop. If I had a long running request, does it mean that server will never listen since it can only do one work at a time.

var http = require('http');
function handleRequest(request, response) {
    response.end('Some Response at ' + request.url);
}
var server = http.createServer(handleRequest);
server.listen(8083, function() {
    console.log('Listening...')
})

Is server.listen listening to some event?

Answer 1

You can think of server.listen() as starting your web server so that it is actually listening for incoming requests at the TCP level. From the node.js http documentation for .listen():

Begin accepting connections on the specified port and hostname.

The callback passed to server.listen() is optional. It is only called once to indicate that the server has been successfully started and is now listening for incoming requests. It is not what is called on every new incoming request. The callback passed to .createServer() is what is called for every new incoming request.

Multiple incoming requests can be in process at the same time though due to the single-threaded nature of node.js only one request is actually executing JS code at once.

But, a long running request is generally idle most of the time (e.g. waiting for database I/O or disk I/O or network I/O) so other requests can be processed and run during that idle time. This is the async nature of node.js and why it is important to use asynchronous I/O programming with node.js rather than synchronous I/O processing because asynchronous I/O allows other requests to run during the time when node.js is just waiting for I/O.