miniq

miniq is yet another tiny async control flow library. It implements parallelLimit, with the ability to share the concurrency-limited queue.

Changelog

v1.0.1: when given an empty tasks array, empty was not emitted.

v1.0.0: reduced the overall size by using ondone for the "wait to complete" functionality. Deprecated the removeTasks() and concurrency() methods. Added the isFull and isEmpty methods.

Features

• small: miniq only implements parallelLimit
• can be used for all three basic control flow patterns
  • series = parallel(1, tasks, onDone)
  • parallel without a concurrency limit = parallel(Infinity, tasks, onDone)
  • parallel with a concurrency = default behavior
• no result passing: Many control flow libraries have a dozen variants which simply pass the result around in slightly different ways (e.g. chain vs. map). I'd rather just use JavaScript's scope rules to handle all those variants rather than have specialized functions for each thing.
• Node 10.x compatibility

miniq has one advanced feature, which is the ability to share the concurrency-limited queue among multiple different tasks. In other words, many different sets of operations can share the same queue and run limit. Each set of tasks can have it's own onDone function, but they share the same concurrency limit.

For example, if you are writing something that does a recursive directory traversal and does various (file system) operations, you can push all the operations into the same queue. This will allow you to limit (file system) concurrency across multiple operations.

Installation

npm install --save miniq

API

parallel(limit, tasks, [onDone]):

• limit is a number which controls the maximum number of concurrent tasks. Set limit = 1 for serial execution and limit = Infinity for unlimited parallelism.
• onDone is a callback function(err) { ... }; it is called when the tasks it is associated with have run
• tasks are callbacks function(done) { ... } which should call done() when they are complete.

The return value is an object with the following API:

• .exec(tasks, [onDone]): appends the new set of tasks and queues the onDone function once all of those tasks have completed.
• isEmpty: returns true if the queue is empty.
• isFull: returns true if the queue is currently running the maximum number of tasks.

Some notes on Node 0.10.x (supported since 0.1.x)

miniq uses setImmediate when available to break call stacks.

This is done by default in order to prevent stack overflows from occurring when executing in a tight loop. However, if your workload is already asynchronous, then you will never run into a call stack overflow since async calls break up the call stack.

The .maxStack property on the queue controls when a setImmediate / nextTick call is inserted. It is set to 50 by default, which seems to retain a good balance between call stack size and avoiding scheduling overhead.

You should disable maxStack by setting it to Infinity if you know in advance that the work payloads are async and hence you will not need to occasionally break out of the call stack.

To set the maxStack, set it on the return value. For example:

var queue = parallel(10, [ ... ], onDone);
queue.maxStack = Infinity;

For maximum performance when operations are cheap (e.g. stat calls), set the queue limit to Infinity and the maxStack property to Infinity. This skips a lot of management overhead as all tasks are launched immediately and no stack breaks are inserted.

Example: replacement for parallelLimit

var parallel = require('miniq');

parallel(10, [
  function(done) {
    fs.readFile(function(err, result) {
      if(err) {
        return done(err); // done takes one argument: the error
      }
    }
  },
], function(err) {
  // err is sent if any of the tasks returned an error
});

Example: replacement for parallel

var parallel = require('miniq');

parallel(Infinity, [
  function(done) { ... },
], function(err) {
  // err is sent if any of the tasks returned an error
});

Example: replacement for series

var parallel = require('miniq');

parallel(1, [
  function(done) { ... },
], function(err) {
  // err is sent if any of the tasks returned an error
});

Example: using miniq as a shared maximum-concurrency limited queue

var parallel = require('miniq');

function Foo() {
  this.queue = parallel(12);
}

Foo.prototype.bar = function() {
  this.queue.exec(tasks, function(err) { ... });
};

Foo.prototype.all = function() {
  // when the queue is empty
  this.queue.once('empty', function() {
    console.log('All done!');
  });

  this.queue.exec(tasks);
};