How can I limit Q promise concurrency?

Question

How do I write a method that limits Q promise concurrency?

For instance, I have a method spawnProcess. It returns a Q promise.
I want no more than 5

Answer 1

I wrote a little library to do this: https://github.com/suprememoocow/qlimit

It's extremely easy to use and is specifically designed to work with Q promises:

var qlimit = require('qlimit');
var limit = qlimit(2); // 2 being the maximum concurrency

// Using the same example as above
return Q.all(items.map(limit(function(item, index, collection) { 
  return performOperationOnItem(item);
}));

It can also be used to limit concurrency to a specific resource, like this:

var qlimit = require('qlimit');
var limit = qlimit(2); // 2 being the maximum concurrency

var fetchSomethingFromEasilyOverwhelmedBackendServer = limit(function(id) {
  // Emulating the backend service
  return Q.delay(1000)
    .thenResolve({ hello: 'world' }); 
});