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1.1.4 • Public • Published


Easy to use asynchronous job queue.

Build Status NPM version

It stores jobs in the memory and runs it asynchronously in parallel with a given concurrency. Successor of caolan/async.queue.

Basically tress is a clone of queue from famous caolan/async but without all other implements of that Swiss Army Knife of asynchronous code. Although tress was intended to be an improved, better maintained and more safe alternative to caolan/async.queue, but first and foremost tress is backward compatible. It means, that everywhere you use async.queue (except undocumented features) you can write tress instead and it must work.

You can do like this:

// old code:
var async = require('async');
var q = async.queue(function(job, done){/*...*/});
// new code:
var tress = require('tress');
var q = tress(function(job, done){/*...*/});

Every code using caolan/async.queue must work with tress. If it does not work exactly the same way, please start the issue.

All documentation of caolan/async.queue is right for tress, but it doesn't describe it completely. Any way, you can use tress only with this documentation (instead of referencs below) and don't even think about any extra features. Only exception - tress require node >=8 and doesn't work in browsers.

Main difference between tress and caolan/async.queue is that in tress job not disappear after worker finished. It moves to failed or finished (depends of done first argument) and can be used later. Second difference is that in tress fields of queue object are more safe. They are readable/writable only in correct way. Also tress has some new fields in queue object (see in reference).


npm install tress


var tress = require('tress');
// create a queue object with worker and concurrency 2
var q = tress(function(job, done){
    console.log('hello ' +;
    someAsyncFunction(job, function(err, data){
        if (err) {
            done(err, 'some message');
        } else {
            done(null, 'anything');
}, 2);
// assign a callbacks
q.drain = function(){
q.error = function(err) {
    console.log('Job ' + this + ' failed with error ' + err);
q.success = function(data) {
    console.log('Job ' + this + ' successfully finished. Result is ' + data);
// add some items to the queue
q.push({name: 'Bob'});
q.push({name: 'Alice'});
// add some items to the queue (batch-wise)
// and specify callback only for that items (not for all queue)
q.push([{name: 'Good'}, {name: 'Bad'}, {name: 'Ugly'}], function (err) {
    console.log('finished processing item');
// add some items to the front of the queue
q.unshift({name: 'Cristobal Jose Junta'});


tress(worker, [concurrency]) creates queue object that will store jobs and process them with worker function in parallel (up to the concurrency limit).


  • worker(job, done) - An asynchronous function for processing a queued data.
    • job - data for processing. It may be primitive or object.
    • done(err, ...args) - callback function. worker must call done when finished. done may take various argumens, but first argument must be error (if job failed), null/undefined (if job successfully finished) or boolean (if job need to be returned to queue head (if true) or to queue tail (if false) for retry).
  • concurrency - An integer for determining how many worker functions should be run in parallel. Defaults to 1. If negative - no parallel and delay between worker calls (concurrency -1000 sets 1 second delay).

Queue object properties

  • started - still false till any items have been pushed and processed by the queue. Than became true and never change in queue lifecycle (Not writable).
  • concurrency - This property for alter the concurrency/delay on-the-fly.
  • buffer A minimum threshold buffer in order to say that the queue is unsaturated. Defaults to concurrency / 4.
  • paused - a boolean for determining whether the queue is in a paused state. Not writable (use pause() and resume() instead).
  • waiting (new) - array of queued jobs.
  • active (new) - array of jobs currently being processed.
  • failed (new) - array of failed jobs (done callback was called from worker with error in first argument).
  • finished (new) - array of correctly finished jobs (done callback was called from worker with null or undefined in first argument).

Note, that properties waiting, active, failed and finished are not writable, but they point to arrays, that you can cahge manually. Do it carefully.

Queue object methods

Note, that in tress you can't rewrite methods.

  • push(job, [callback]) or unshift(job, [callback]) - add a new job to the queue. push adds job to queue tail and unshift to queue head.
    • job - Job to add to queue. Instead of a single job, array of jobs can be submitted.
    • callback(err, ...args) - optional callback. tress calls this callback once the worker has finished processing the job. With same arguments as in done callback.
  • pause() - a function that pauses the processing of new jobs until resume() is called.
  • resume() - a function that resumes the processing of queued jobs paused by pause.
  • kill() - a function that removes the drain callback and empties queued jobs.
  • remove() a function that removes job from queue.
  • length() - a function returning the number of items waiting to be processed.
  • running() - a function returning the number of items currently being processed.
  • workersList() - a function returning the array of items currently being processed.
  • idle() - a function returning false if there are items waiting or being processed, or true if not.
  • save(callback) (new) - a function that runs a callback with object, that contains arrays of waiting, failed, and finished jobs. If there are any active jobs at the moment, they will be concatenated to waiting array.
  • load(data) (new) - a function that loads new arrays from data object to waiting, failed, and finished arrays and sets active to empty array. Rise error if started is true.
  • status(job) (new) a function returning the status of job ('waiting', 'running', 'finished', 'pending' or 'missing').

Queue objects callbacks

You can assign callback function to this six properties. Note, you can't call any of that calbacks manually.

  • saturated() - a callback that is called when the number of running workers hits the concurrency limit, and further jobs will be queued.
  • unsaturated() - a callback that is called when the number of running workers is less than the concurrency & buffer limits, and further jobs will not be queued.
  • empty() - a callback that is called when the last item from the queue is given to a worker.
  • drain() - a callback that is called when the last item from the queue has been returned from the worker.
  • error(...args) (new) - a callback that is called when job failed (worker call done with error as first argument).
  • success(err, ...args) (new) - a callback that is called when job correctly finished (worker call done with null or undefined as first argument).
  • retry(err, ...args) (new) - a callback that is called when job returned to queue for retry (worker call done with boolean as first argument).

Job lifecycle

After pushing/unshifting to queue every job passes through the following steps:

  • processed in worker
  • sent to done (as this or explicitly as one of parameters if you prefere arraw-callbacks)
  • moved from active to failed/finished/waiting (wether first parameter of done is error, null or boolean)
  • sent to job callback (from push/unshift)
  • sent to error/success/retry callback (wether first parameter of done is error, null or boolean)





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