Ok. Thanks. I always knew something smart was happening, and this is the
first time I decided to not take it for granted.
On Mar 15, 2012 12:30 AM, "Axel Kittenberger" <[email protected]> wrote:
> There are not only callbacks, but also event emitters. In you client
> "conn.on('data', .." listens to an event emitter, it could be called
> multiple times, as long the connection exists. Node exists when you
> are no longer waiting for any event, neither event emitter or
> callback.
>
> On Thu, Mar 15, 2012 at 6:53 AM, C. Mundi <[email protected]> wrote:
> > I know *what* the following code does. I want to understand *why*.
> >
> > Consider the following client/server pair:
> >
> > client.js
> > var net = require('net');
> > var conn = net.createConnection(8000, function() { // 'connect' listener
> > console.error('connected to server');
> > conn.on('data', function(data) {
> > var msg = data.toString();
> > console.error('Got data: ' + msg);
> > conn.write('Back at ya!');
> > conn.end(); // Pay close attention!
> > });
> > });
> >
> > server.js
> > var net = require('net');
> > var server = net.createServer(function(client) {
> > console.error('Accepted connection');
> > client.on('end', function() {
> > console.error('Client has closed connection.');
> > });
> > client.on('data', function(data) {
> > var msg = data.toString();
> > console.error('Got data: ' + msg);
> > });
> > client.write('Welcome');
> > });
> >
> > server.listen(8000, function() {
> > console.error('Listening on port 8000');
> > });
> >
> > So we start the server ('node server.js') and then we run the client
> ('node
> > client.js') and see (as expected) that the server accepts the connection
> > from the client, they exchange a pair of messages and the client exits.
> Now
> > let's comment out the 'conn.end()' from the client. Now the client hangs
> > around until we kill it. What I want to understand is this: how does
> the V8
> > thread know that it should or should not hang around? The conn.end() is
> in
> > the 'connect' event listener, which gets registered only after the
> > connection is established. (And it is important to notice that if the
> > connection had been refused, then the client would have exited with an
> > error.) My mental model goes like this, and I would like some feedback:
> >
> > During the execution of each event loop, node memoizes every async call
> long
> > enough to see if the callback could be called. As long as there is at
> least
> > one pending callback (and no error or explicit exit() call) the event
> loop
> > keeps running. Ok. Now let's suppose the server accepts the connection.
> > The 'connect' event is caught by the listener callback in the client.
> And
> > after this 'connect' callback executes... node hangs around, because the
> > 'connect' callback registered a callback for the 'data' event. And event
> > handlers are persistent, so execution keeps looping if the conn.end() is
> not
> > present. Now, somehow, conn.end() breaks us out of the loop, at which
> point
> > there is nothing left to do *and* nothing left to wait for, so client.js
> > exits. My guess is that the conn.end() call unregisters all the event
> > handlers associated with the connection object conn, at which point node
> has
> > nothing left to do.
> >
> > Am I even close to the right way of thinking about this?
> >
> > Thanks!
> >
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