Assign priority to nodejs tasks in a event loop

Question

Is there any way by which one can apply priority to Node.js task's in a event loop. I want to assign priority to task which are present in a event loop of nodejs.
Suppose in a event loop there are 5 jobs A,B,C,D,E which having same priority and then next job is received whose priority is higher than last five jobs. Then event loop starts executing that higher priority job.

@neelsg: https://en.wikipedia.org/wiki/Dynamic_priority_scheduling is a well-known problem with lots of applications — Bergi, Sep 30 '14 at 12:13
Actually in my application multiple functions' are executing parallelly i.e in a event loop. So if there are some important high priority functions are there so I want these functions to be executed first and then other. — Sanket, Sep 30 '14 at 12:34
@Bergi I'm aware that this is a well known problem, but don't think it is possible to do in Node.js. Sometimes you can avoid such issues by re-factoring a problem. If the asker has some specific reason why he/she needs to do this, we may be able to suggest a different approach. — neelsg, Sep 30 '14 at 12:39
@Sanket Could it be possible for you to move the high priority functions to a dedicated **[Child Process](http://nodejs.org/api/child_process.html)**? — neelsg, Sep 30 '14 at 12:41
No if I did so then it will giving problem in a database CURD operations. — Sanket, Sep 30 '14 at 12:43
@neelsg: Well, you can always write your own scheduler :-) With generators/continuations this should be quite possible. — Bergi, Sep 30 '14 at 12:54

score 3 · Accepted Answer · answered Sep 30 '14 at 11:33

3

The event loop in node.js does not support priorities. See some documentation:

Short of rewriting it, I don't think there is much you can do about that.

answered Sep 30 '14 at 11:33

neelsg

4,802
5
34
58

score 2 · Answer 2 · answered Oct 07 '14 at 20:30

You should use a priority queue, something like priorityqueuejs

In this way you can dequeue an item with the max priority and execute it.

Some code:

'use strict';

var PriorityQueue = require('priorityqueuejs');

var queue = new PriorityQueue(function(a, b) {
  return a.value - b.value;
});

queue.enq({ value: 10, func: function() { console.log("PRIORITY: 10"); } });
queue.enq({ value: 500, func: function() { console.log("PRIORITY: 500"); } });
queue.enq({ value: 300, func: function() { console.log("PRIORITY: 300"); } });
queue.enq({ value: 100, func: function() { console.log("PRIORITY: 100"); } });

(function executeNext() {
  if(queue.size()) {
    var next = queue.deq();
    next.func();
    if(queue.size()) {
      setTimeout(executeNext, 0);
    }
  }
})();

And the output is:

PRIORITY: 500
PRIORITY: 300
PRIORITY: 100
PRIORITY: 10

Here is the executeNext function extracts the next top-priority item and executes it.

Assign priority to nodejs tasks in a event loop

2 Answers2