Execute promises in batches

batch_promise.js

/**
 * Created by steve on 1/09/15.
 */
var q = require('q');

function Task(fn, arg) {
    this.fn = fn;
    this.arg = arg;
}

var state = {};
state.in_progress = 0;
state.started = 0;
state.finished = 0;
var tasks = [];

for (var i = 0; i < 17; i++) {
    var fn = function hello_async(obj)  { return hello(q.defer(), state, obj)};
    var task = new Task(fn, i);
    tasks.push(task);
}

console.log(tasks.length + " functions to execute");
var promise = q_batch(q.defer(), tasks, 5);
promise.then(function(data) {
    console.log(state.started + " functions started");
    console.log(state.finished + " functions finished");
}, function(err) {
    console.error(err.stack ? err.stack : err);
});

function q_batch($q, tasks, batch_size) {
    //Finished?
    if (!tasks || tasks.length == 0) {
        $q.resolve(tasks);
        console.log("Finished");
        return;
    }

    //Call the function batch_size times to get the promises
    var promise_arr = [];
    for (var i = 0; (i < batch_size) && (i < tasks.length); i++) {
        try {
            var task = tasks[i];
            console.log("Calling " + task.fn.name + " with " + task.arg);
            promise_arr.push(task.fn.call(this, task.arg));
        } catch (e) {
            $q.reject("Batch failed. " + e);
        }
    }

    //Execute the batch_size, then go again
    var promise_batch = q.all(promise_arr);
    promise_batch.then(function(results) {
        try {
            //Remove the first n funcs and go again.
            tasks.splice(0, batch_size);
            q_batch($q, tasks, batch_size);
        } catch (err) {
            console.error(err.stack ? err.stack : err);
        }
    }, function(err) {
        $q.reject("Batch failed. " + err);
    });
    return $q.promise;
}

function hello($q, state, my_num) {
    state.in_progress++;
    state.started++;
    setTimeout(function(){
        console.log("Hello world. " + state.in_progress + " in progress. I am: " + my_num);

        //if (state.finished == 13) {
        //    throw "awight!";
        //}
        //if (state.finished == 13) {
        //    $q.reject("unlucky number 13");
        //}

        state.in_progress--;
        state.finished++;
        $q.resolve(state);
    }, Math.random() * 1000);
    return $q.promise;
}

This helped me alot to figure out a good way to recursively call my promise chain batch, thanks for putting it out there.

I recommend this: https://github.com/Nicktho/batch-promises/blob/master/index.js