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Create tasks in node.js that run in parallel, or sequences that run tasks one after another.
JavaScript
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lib/parallel
README.md
package.json
test.js

README.md

Usage

var Task = require('parallel').Task;

// Here is your typical async function
var myAsyncFunction = function myAsyncFunction(id, callback) {
  database.getEntry(id, function (data) {
    callback(data);
  });
};

// We want to make a task that executes `myAsyncFunction` and passes
// 1 & 3 as the first argument. Note: You do not need to supply a
// callback argument

// Every 'task entry' needs a unique id so it can be identified later.
var myTask = new Task({
  1: [myAsyncFunction, 1], // This task has the id `1`, calls myAsyncFunction
                           // and passes `1` as the first argument
  2: [myAsyncFunction, 3]  // This task has the id `2`, calls myAsyncFunction
                           // and passes `3` as the first argument
});

// We can then add 'listeners' to task entries using `bind`, and
// referencing the entry id

// We pass a callback as the last argument, which is called like
// it was passed directly to the async function.
myTask.bind(1, function (data) {
  doSomethingWith(data);
});

// We can also bind to multiple entries
myTask.bind(1, 2, function (data) {
  doSomethingElseWith(data);
});

// We then run the task with `run`. The only argument is a callback
// which is passed the entry id as the first argument, and the additional
// returned arguments are suffixed. The task id is `null` when everything
// is finished.
myTask.run(function (entry_id, data) {
  // This will be true when everything is done
  if (entry_id === null) {
    moveOnAndDoSomethingElse();
  }

  // This is true when entry 1 is done
  else if (entry_id === 1) {
    doSomethingWith(data);
  }

  // This is true when entry 2 is done
  else if (entry_id === 2) {
    doSomethingElseWith(data);
  }
});

// You can also add tasks dynamically
// This will create 2 task entries that have the id's 0 & 1, and call
// `myAsyncFunction` with the first argument as 0 & 2 respectivly
myTask = new Task();

for (var i = 0; i < 2; i++) {
  myTask.add(i, [myAsyncFunction, i * 2]);
}

// We can then bind and run the task like before

Also some usage of Sequence:

var Sequence = require('parallel').Sequence,
    fs       = require('fs'),
    path     = require('path'),
    sys      = require('sys');

// Create a new sequence, pass in an array of funcions, or
// pass them as arguments
var task = new Sequence([
  function (next) {
    // First argument is always a function that gets called on completion
    sys.puts('Hello world. We are starting!');
    fs.readFile(__filename, next);
  },
  function (next, err, buffer) {
    // The extra args gets suffixed onto the arguments
    sys.puts('The size of this example is: ' + buffer.length);

    // next.parallel is a variable you can pass which
    // enables you to do  multiple tasks at once.
    // It should only be used in cases where the expected
    // arguments passed to the callback are: error, result
    fs.readdir(__dirname, next.parallel);
    fs.readdir(path.join(__dirname, 'lib'), next.parallel);
  }
]);

// You can also add tasks with the add method. This also opens
// up dynamic task creation as an option
task.add(function (next, err, results, first, second) {
  // The returned arguments from a parallel sequence
  // are as follows:
  // * Callback for next task (as usual)
  // * Error is there was one, otherwise will be null
  // * All the results as an array in order
  // * The result of the first task
  // * Result of second task
  // etc.
  sys.puts('This dir had ' + results[0].length + ' items.');
  sys.puts('The lib directory has ' + second.length + ' items.');

  // You can either make the last changes here, or call next
  // to run the callback passed to run(). You can pass arguments
  // as usual.
  next('Done.');
});

task.run(function (text) {
  // The callback to run is optional, and only gets called if the last
  // entry called next, or next.parallel.
  // Only difference is that callback function will not be the first
  // arguments.
  sys.puts(text);
});
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