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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