How does the Node.js event loop work?

Question

After playing with Node.js and reading about async i/o & evented programming a lot I\'m left with some question marks.

Consider the following (pseudo) code:

Answer 1

Run parrallel async code pattern:

To 'Wait for result from both async functions' you can do: in both async calls callbacks check both result and if all ready, call your DoSomethingWithTwoDependantResults.

In your example you probably need to execute queries sequentially:

query(sql1, function(sqlres1) {
    query(sql2, function(sqlres2) {
        writeResultUsingDataFrom(sqlres1, sqlres2);
    }
});

your original code, modified to execute two queries in parallel:

function writeReply(res, template, username, event_name)
{
    var body = renderView(username, event_name, template);
    res.writeHead(200, {'Content-Type': 'text/plain'});
    res.write(body);
    res.end();
} 

function onRequest(request, response)
{
    // some non-blocking db query
    query('SELECT name FROM users WHERE key=req.params['key']', function (err, results, fields) {
        if (err) {
            throw err;
        }
        username = results[0];
        if (username && event_name)
            writeReply(res, template, username, event_name);
    });

    // some non-blocking db query
    query('SELECT name FROM events WHERE key=req.params['key']', function (err, results, fields) {
        if (err) {
            throw err;
        }
        event_name = results[0];
        if (username && event_name)
            writeReply(res, template, username, event_name);

    });
};

Answer 2

Have you seen this? I'm still getting the hang of it all and I can't answer your question in detail, but basically you're right about the thread-pool... Ryan explains quite a lot in the video.

EDIT: And this one from about a year later, when he goes into more detail.