vasync

What is vasync?

The vasync npm package provides utilities for managing asynchronous control flow in JavaScript. It is particularly useful for handling series, parallel, and queue-based asynchronous operations, making it easier to manage complex asynchronous code.

What are vasync's main functionalities?

Series

The `series` function allows you to run asynchronous tasks in series, meaning one after the other. Each task waits for the previous one to complete before starting.

const vasync = require('vasync');

vasync.series([
  function (callback) {
    setTimeout(() => {
      console.log('First task');
      callback(null, 'one');
    }, 1000);
  },
  function (callback) {
    setTimeout(() => {
      console.log('Second task');
      callback(null, 'two');
    }, 500);
  }
], function (err, results) {
  console.log('All tasks completed', results);
});

Parallel

The `parallel` function allows you to run asynchronous tasks in parallel, meaning they all start at the same time and the final callback is called when all tasks have completed.

const vasync = require('vasync');

vasync.parallel([
  function (callback) {
    setTimeout(() => {
      console.log('First task');
      callback(null, 'one');
    }, 1000);
  },
  function (callback) {
    setTimeout(() => {
      console.log('Second task');
      callback(null, 'two');
    }, 500);
  }
], function (err, results) {
  console.log('All tasks completed', results);
});

Queue

The `queue` function allows you to create a queue for processing tasks with a specified concurrency. Tasks are processed in the order they are added to the queue, but no more than the specified number of tasks will be processed at the same time.

const vasync = require('vasync');

const queue = vasync.queue(function (task, callback) {
  console.log('Processing task:', task.name);
  setTimeout(callback, 1000);
}, 2);

queue.push({name: 'task1'}, function (err) {
  console.log('Finished processing task1');
});
queue.push({name: 'task2'}, function (err) {
  console.log('Finished processing task2');
});
queue.push({name: 'task3'}, function (err) {
  console.log('Finished processing task3');
});

Other packages similar to vasync

async

The `async` package is a widely-used library for managing asynchronous control flow in JavaScript. It provides similar functionalities to vasync, such as series, parallel, and queue operations, but with a larger community and more extensive documentation.

bluebird

The `bluebird` package is a fully-featured Promise library for JavaScript. While it focuses on Promises rather than callback-based control flow, it provides utilities for managing asynchronous operations, including series and parallel execution.

p-queue

The `p-queue` package is a promise-based queue implementation for JavaScript. It allows you to manage task concurrency and provides a more modern, promise-based API compared to vasync's callback-based approach.

README

vasync: utilities for observable asynchronous control flow

This module provides facilities for asynchronous control flow. There are many modules that do this already (notably async.js). This one's claim to fame is aided debuggability: each of the contained functions return a "status" object with the following fields:

operations          array corresponding to the input functions, with

        func            input function

        status          "pending", "ok", or "fail"

        err             returned "err" value, if any

        result          returned "result" value, if any

ndone               number of input operations that have completed

nerrors             number of input operations that have failed

You can use this from a debugger (or your own monitoring code) to understand the state of an ongoing asynchronous operation. For example, you could see how far into a pipeline some particular operation is.

parallel(args, callback): invoke N functions in parallel and merge the results

This function takes a list of input functions (specified by the "funcs" property of "args") and runs them all. These input functions are expected to be asynchronous: they get a "callback" argument and should invoke it as callback(err, result). The error and result will be saved and made available to the original caller when all of these functions complete.

All errors are combined into a single "err" parameter to the final callback (see below). You can also observe the progress of the operation as it goes by examining the object returned synchronously by this function.

Example usage:

status = mod_vasync.parallel({
    'funcs': [
         function f1 (callback) { mod_fs.stat('/tmp', callback); },
         function f2 (callback) { mod_fs.stat('/noexist', callback); },
         function f3 (callback) { mod_fs.stat('/var', callback); }
    ]
}, function (err, results) {
        console.log('error: %s', err.message);
        console.log('results: %s', mod_util.inspect(results, null, 3));
});

console.log('status: %s', mod_sys.inspect(status, null, 3));

In the first tick, this outputs:

status: { operations: 
   [ { func: [Function: f1], status: 'pending' },
     { func: [Function: f2], status: 'pending' },
     { func: [Function: f3], status: 'pending' } ],
  ndone: 0,
  nerrors: 0 }

showing that there are three operations pending and none has yet been started. When the program finishes, it outputs this error:

error: first of 1 error: ENOENT, no such file or directory '/noexist'

which encapsulates all of the intermediate failures. This model allows you to write the final callback like you normally would:

if (err)
        return (callback(err));

and still propagate useful information to callers that don't deal with multiple errors (i.e. most callers).

The example also prints out the detailed final status, including all of the errors and return values:

results: { operations: 
   [ { func: [Function: f1],
       status: 'ok',
       err: null,
       result: 
        { dev: 140247096,
          ino: 879368309,
          mode: 17407,
          nlink: 9,
          uid: 0,
          gid: 3,
          rdev: 0,
          size: 754,
          blksize: 4096,
          blocks: 8,
          atime: Thu, 12 Apr 2012 23:18:57 GMT,
          mtime: Tue, 17 Apr 2012 23:56:34 GMT,
          ctime: Tue, 17 Apr 2012 23:56:34 GMT } },
     { func: [Function: f2],
       status: 'fail',
       err: { [Error: ENOENT, no such file or directory '/noexist'] errno: 34, code: 'ENOENT', path: '/noexist' },
       result: undefined },
     { func: [Function: f3],
       status: 'ok',
       err: null,
       result: 
        { dev: 23658528,
          ino: 5,
          mode: 16877,
          nlink: 27,
          uid: 0,
          gid: 0,
          rdev: -1,
          size: 27,
          blksize: 2560,
          blocks: 3,
          atime: Fri, 09 Sep 2011 14:28:55 GMT,
          mtime: Wed, 04 Apr 2012 17:51:20 GMT,
          ctime: Wed, 04 Apr 2012 17:51:20 GMT } } ],
  ndone: 3,
  nerrors: 1 }

You can use this if you want to handle all of the errors individually or to get at all of the individual return values.

forEachParallel(args, callback): invoke the same function on N inputs in parallel

This function is exactly like parallel, except that the input is specified as a single function ("func") and a list of inputs ("inputs"). The function is invoked on each input in parallel.

This example is exactly equivalent to the one above:

mod_vasync.forEachParallel({
    'func': mod_fs.stat,
    'inputs': [ '/var', '/nonexistent', '/tmp' ]
}, function (err, results) {
    console.log('error: %s', err.message);
    console.log('results: %s', mod_util.inspect(results, null, 3));
});

pipeline(args, callback): invoke N functions in series (and stop on failure)

The arguments for this function are:

• funcs: input functions, to be invoked in series
• arg: arbitrary argument that will be passed to each function

The functions are invoked in order as func(arg, callback), where "arg" is the user-supplied argument from "args" and "callback" should be invoked in the usual way. If any function emits an error, the whole pipeline stops.

The return value and the arguments to the final callback are exactly the same as for parallel. The error object for the final callback is just the error returned by whatever pipeline function failed (if any).

This example is similar to the one above, except that it runs the steps in sequence and stops early because pipeline stops on the first error:

console.log(mod_vasync.pipeline({
    'funcs': [
        function f1 (_, callback) { mod_fs.stat('/tmp', callback); },
        function f2 (_, callback) { mod_fs.stat('/noexist', callback); },
        function f3 (_, callback) { mod_fs.stat('/var', callback); }
    ]
}, function (err, results) {
        console.log('error: %s', err.message);
        console.log('results: %s', mod_util.inspect(results, null, 3));
}));

As a result, the status after the first tick looks like this:

{ operations: 
   [ { func: [Function: f1], status: 'pending' },
     { func: [Function: f2], status: 'waiting' },
     { func: [Function: f3], status: 'waiting' } ],
  ndone: 0,
  nerrors: 0 }

(Note that the second and third stages are now "waiting", rather than "pending" in the parallel case.) The error reported is:

error: ENOENT, no such file or directory '/noexist'

and the complete result is:

results: { operations: 
   [ { func: [Function: f1],
       status: 'ok',
       err: null,
       result: 
        { dev: 140247096,
          ino: 879368309,
          mode: 17407,
          nlink: 9,
          uid: 0,
          gid: 3,
          rdev: 0,
          size: 754,
          blksize: 4096,
          blocks: 8,
          atime: Thu, 12 Apr 2012 23:18:57 GMT,
          mtime: Tue, 17 Apr 2012 23:56:34 GMT,
          ctime: Tue, 17 Apr 2012 23:56:34 GMT } },
     { func: [Function: f2],
       status: 'fail',
       err: { [Error: ENOENT, no such file or directory '/noexist'] errno: 34, code: 'ENOENT', path: '/noexist' },
       result: undefined },
     { func: [Function: f3], status: 'waiting' } ],
  ndone: 2,
  nerrors: 1 }