Question

as I use Node.js for not too long time yet, I’ve ran into a following problem. I understand that using callback-driven paradigma we need to convert loops we used in synchronous code into recursion. My problem is I cannot understand how deep node can go with recursion, results with different tests are inconsistent. For example, I tried code I’ve found on Web:

    var depth = 0;

    (function recurseBaby() {
        // log at every 500 calls
        (++depth % 500) || console.log(depth);

        // bail out ~100K depth in case you're special and don't error out
        if (depth > 100000) return;

        recurseBaby();
    })();

It gives me node exception (max recursion depth) after 18500 recursions.
So I tried to add some functionality, like working with queue:

var depth = 0,
    Memcached = require('memcached'),
    memcacheq = new Memcached('127.0.0.1:22201');


(function recurseBaby() {
    // log at every 500 calls
    (++depth % 500) || console.log(depth);

    // bail out ~10M depth in case you're special and don't error out
    if (depth > 10000000) return;

    memcacheq.set('test_queue', 'recurs' + depth, 0, function (error, response) {
        return recurseBaby();
    });
})();

It didn’t finish yet, but works so far for more than 4 million recursions (and queue is actually getting filled). So I would like to clarify, how recursion depth limit works in node. My guess would be that if I do something in that recursively called function, node has more time to free call stack, but I may be terribly wrong. Any clarifications from more experienced node users are welcome.

Answer 1

There is no recursion in your second example at all.

In the first example you clearly have a recursive call.

In the second example a call is made to memcacheq.set which returns immediately, and then your function returns. It will have made a node somewhere of the function to call when the triggered event happens at some later time.

— No recursive call has been made —

At some point in time later the memcache function will finish and queue an event to the event queue to trigger your callback.

But note that this is done from an event in the event queue, NOT by a direct recursive call from your function which has probably long since returned.