Simple async batch with concurrency control and progress reporting.
$ npm install batch
var Batch = require('batch')
  , batch = new Batch;
batch.concurrency(4);
ids.forEach(function(id){
  batch.push(function(done){
    User.get(id, done);
  });
});
batch.on('progress', function(e){
});
batch.end(function(err, users){
});
Contain the "job" index, response value, duration information, and completion data.
{ index: 1,
  value: 'bar',
  pending: 2,
  total: 3,
  complete: 2,
  percent: 66,
  start: Thu Oct 04 2012 12:25:53 GMT-0700 (PDT),
  end: Thu Oct 04 2012 12:25:53 GMT-0700 (PDT),
  duration: 0 }