Async task manager with maximum number of concurrent "running" tasks

Question

I am trying to implement two classes that can deal with asynchronous tasks in JavaScript:

Class Task: mimics the execution of a task with setTimeout. Once the timer expires, the task is considered completed.
Class TaskManager: has a capacity parameter to limit the numbers of tasks that can be executing in parallel.

I thought if I could just call the loop function recursively, just to keep checking if one job is done, I could proceed to the next job. But this leads immediately to a "Maximum call stack size exceeded" error.

Can someone explain how I can fix this?

class Task { constructor(time) { this.time = time; this.running = 0; } run(limit, jobs, index) { setTimeout(() => { console.log('hello', index); this.done(limit, jobs, index); }, this.time); } done(limit, jobs, index) { jobs.splice(index, 1); console.log(jobs); } } class TaskManager { constructor(capacity) { this.capacity = capacity; this.jobs = []; this.index = 0; this.running = 0; this.pending = []; } push(tk) { this.jobs.push(tk); this.index += 1; const loop = () => { if (this.jobs.length === 0) { return; } if (this.jobs.length <= this.capacity) { this.running += 1; tk.run(this.capacity, this.jobs, this.index-1); return; } loop(); } loop(); } } const task = new Task(100); const task1 = new Task(200); const task2 = new Task(400); const task3 = new Task(5000); const task4 = new Task(6000); const manager = new TaskManager(3); manager.push(task); manager.push(task1); manager.push(task2); manager.push(task3); manager.push(task4);

Javascript is strictly single threaded. No tasks will run in parallel, rather they run concurrently (giving the illusion of being parallel). Therefore, any infinite loop such as your recursive loop() function will cause setTimeout to never run thus no tasks will ever execute. The solution is that loop() itself must be setTimeouted — slebetman
– slebetman, Commented Feb 27, 2019 at 8:39
You can read my answer to this other question to understand how the asynchronous "engine" in javascript work: stackoverflow.com/questions/29883525/… — slebetman
– slebetman, Commented Feb 27, 2019 at 8:42

trincot · Accepted Answer · 2019-02-27 09:37:33Z

You should not implement the busy loop, as that will block the event loop and so no user UI events or setTimeout events will be processed.

Instead respond to asynchronous events.

If you let the setTimeout callback resolve a Promise, it is not so hard to do.

I modified your script quite drastically. Here is the result:

class Task { constructor(id, time) { this.id = id; this.time = time; } run() { console.log(this + ' launched.'); return new Promise(resolve => { setTimeout(() => { console.log(this + ' completed.'); resolve(); }, this.time); }); } toString() { return `Task ${this.id}[${this.time}ms]`; } } class TaskManager { constructor(capacity) { this.capacity = capacity; this.waiting = []; this.running = []; } push(tk) { this.waiting.push(tk); if (this.running.length < this.capacity) { this.next(); } else { console.log(tk + ' put on hold.'); } } next() { const task = this.waiting.shift(); if (!task) { if (!this.running.length) { console.log("All done."); } return; // No new tasks } this.running.push(task); const runningTask = task.run(); console.log("Currently running: " + this.running); runningTask.then(() => { this.running = this.running.filter(t => t !== task); console.log("Currently running: " + this.running); this.next(); }); } } const a = new Task('A', 100); const b = new Task('B', 200); const c = new Task('C', 400); const d = new Task('D', 5000); const e = new Task('E', 6000); const manager = new TaskManager(3); manager.push(a); manager.push(b); manager.push(c); manager.push(d); manager.push(e);

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