understanding the node.js event queue and process.nextTick

Question

I'm having trouble understanding exactly how process.nextTick does its thing. I thought I understood, but I can't seem to replicate how I feel this should work:

var handler = function(req, res) {
    res.writeHead(200, {'Content-type' : 'text/html'});
    foo(function() {
        console.log("bar");
    });
    console.log("received");
    res.end("Hello, world!");
}

function foo(callback) {
    var i = 0;
    while(i<1000000000) i++;
    process.nextTick(callback);
}

require('http').createServer(handler).listen(3000);

While foo is looping, I'll send over several requests, assuming that handler will be queued several times behind foo with callback being enqueued only when foo is finished.

If I'm correct about how this works, I assume the outcome will look like this:

received
received
received
received
bar
bar
bar
bar

But it doesn't, it's just sequential:

received
bar
received
bar
received
bar
received
bar

I see that foo is returning before executing callback which is expected, but it seems that callback is NEXT in line, rather than at the end of the queue, behind all of the requests coming in. Is that the way it works? Maybe I'm just not understanding how exactly the event queue in node works. And please don't point me here. Thanks.

Bullate

Answer 1

process.nextTick put the callback on the next tick that is going to be executed, not at the end of the tick queue.

Node.js doc (http://nodejs.org/api/process.html#process_process_nexttick_callback) say: "It typically runs before any other I/O events fire, but there are some exceptions."

setTimeout(callback, 0) will probably work more like you describe.

Warmup

No, setTimeout behaved the same way. I guess I'm really asking how I'm supposed to start processing http requests before a previous request is finished. – Bullate

Actually, I realized that setTimeout does exactly what I was looking for, so I apologize. But what exactly is process.nextTick for if it doesn't allow for the interleaving of cpu intensive tasks and http processing? – Bullate

"not at the end of the tick" — so, is there a way to push an arbitrary function at the end of the event queue? Is setTimeout(fn, 0) a recommended way or just a workaround? – Savagism

Answer 2

You should certainly read the link fgascon provided, and perhaps

https://github.com/joyent/node/issues/3335 for more background.

Use process.nextTick for when you want to call some code before any IO, but after the calling context has returned (usually because you want to register listeners on an event emitter and need to return the created emitter before you can register anything).

Colour