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.

Working with Node's asynchronous, callback-based model is much easier with a handful of simple control-flow abstractions, like:

• pipelines (which invoke a list of asynchronous callbacks sequentially)
• parallel pipelines (which invoke a list of asynchronous callbacks in parallel and invoke a top-level callback when the last one completes).
• queues
• barriers

But these structures also introduce new types of programming errors: failing to invoke the callback can cause the program to hang, and inadvertently invoking it twice can cause all kinds of mayhem that's very difficult to debug.

The facilities in this module keep track of what's going on so that you can figure out what happened when your program goes wrong. They generally return an object describing details of the current state. If your program goes wrong, you have several ways of getting at this state:

• On illumos-based systems, use MDB to find the status object and then print it out.
• Provide an HTTP API (or AMQP, or whatever) that returns these pending status objects as JSON (see kang).
• Incorporate a REPL into your program and print out the status object.
• Use the Node debugger to print out the status object.

This module implements the following utilities:

• parallel(args, callback): invoke N functions in parallel (and merge the results)
• forEachParallel(args, callback): invoke the same function on N inputs in parallel
• pipeline(args, callback): invoke N functions in series (and stop on failure)
• barrier([args]): coordinate multiple concurrent operations
• queuev(args): fixed-size worker queue

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.

This function returns the same "result" object it passes to the callback, and you can use the fields in this object to debug or observe progress:

• operations: array corresponding to the input functions, with
  • func: input function,
  • status: "pending", "ok", or "fail",
  • err: returned "err" value, if any, and
  • result: returned "result" value, if any
• successes: "result" field for each of "operations" where "status" == "ok" (in no particular order)
• ndone: number of input operations that have completed
• nerrors: number of input operations that have failed

This status object lets you see in a debugger exactly which functions have completed, what they returned, and which ones are outstanding.

All errors are combined into a single "err" parameter to the final callback (see below).

Example usage:

console.log(mod_vasync.parallel({
    'funcs': [
        function f1 (callback) { mod_dns.resolve('joyent.com', callback); },
        function f2 (callback) { mod_dns.resolve('github.com', callback); },
        function f3 (callback) { mod_dns.resolve('asdfaqsdfj.com', callback); }
    ]
}, function (err, results) {
        console.log('error: %s', err.message);
        console.log('results: %s', mod_util.inspect(results, 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' } ],
  successes: [],
  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: queryA ENOTFOUND

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],
       funcname: 'f1',
       status: 'ok',
       err: null,
       result: [ '165.225.132.33' ] },
     { func: [Function: f2],
       funcname: 'f2',
       status: 'ok',
       err: null,
       result: [ '207.97.227.239' ] },
     { func: [Function: f3],
       funcname: 'f3',
       status: 'fail',
       err: { [Error: queryA ENOTFOUND] code: 'ENOTFOUND',
          errno: 'ENOTFOUND', syscall: 'queryA' },
       result: undefined } ],
  successes: [ [ '165.225.132.33' ], [ '207.97.227.239' ] ],
  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.

Note that "successes" is provided as a convenience and the order of items in that array may not correspond to the order of the inputs. To consume output in an ordered manner, you should iterate over "operations" and pick out the result from each item.

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:

console.log(mod_vasync.forEachParallel({
    'func': mod_dns.resolve,
    'inputs': [ 'joyent.com', 'github.com', 'asdfaqsdfj.com' ]
}, function (err, results) {
    console.log('error: %s', err.message);
    console.log('results: %s', mod_util.inspect(results, null, 3));
}));

The named arguments (that go inside args) 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' } ],
  successes: [],
  ndone: 0,
  nerrors: 0 }

Note that the second and third stages are now "waiting", rather than "pending" in the parallel case. The error and complete result look just like the parallel case.

Returns a new barrier object. Like parallel, barriers are useful for coordinating several concurrent operations, but instead of specifying a list of functions to invoke, you just say how many (and optionally which ones) are outstanding, and this object emits 'drain' when they've all completed. This is syntactically lighter-weight, and more flexible.

• Methods:

  • start(name): Indicates that the named operation began. The name must not match an operation which is already ongoing.
  • done(name): Indicates that the named operation ended.

• Read-only public properties (for debugging):

  • pending: Set of pending operations. Keys are names passed to "start", and values are timestamps when the operation began.
  • recent: Array of recent completed operations. Each element is an object with a "name", "start", and "done" field. By default, 10 operations are remembered.

• Options:

  • nrecent: number of recent operations to remember (for debugging)

Example: printing sizes of files in a directory

var mod_fs = require('fs');
var mod_path = require('path');
var mod_vasync = require('../lib/vasync');

var barrier = mod_vasync.barrier();

barrier.on('drain', function () {
	console.log('all files checked');
});

barrier.start('readdir');

mod_fs.readdir(__dirname, function (err, files) {
	barrier.done('readdir');

	if (err)
		throw (err);

	files.forEach(function (file) {
		barrier.start('stat ' + file);

		var path = mod_path.join(__dirname, file);

		mod_fs.stat(path, function (err2, stat) {
			barrier.done('stat ' + file);
			console.log('%s: %d bytes', file, stat['size']);
		});
	});
});

This emits:

barrier-readdir.js: 602 bytes
foreach-parallel.js: 358 bytes
barrier-basic.js: 552 bytes
nofail.js: 384 bytes
pipeline.js: 490 bytes
parallel.js: 481 bytes
queue-serializer.js: 441 bytes
queue-stat.js: 529 bytes
all files checked

This function returns an object that allows up to a fixed number of tasks to be dispatched at any given time. The interface is compatible with that provided by the "async" Node library, except that the returned object's fields represent a public interface you can use to introspect what's going on.

• Arguments

  • worker: a function invoked as worker(task, callback), where task is a task dispatched to this queue and callback should be invoked when the task completes.
  • concurrency: a positive integer indicating the maximum number of tasks that may be dispatched at any time. With concurrency = 1, the queue serializes all operations.

• Methods

  • push(task, [callback]): add a task (or array of tasks) to the queue, with an optional callback to be invoked when each task completes. If a list of tasks are added, the callback is invoked for each one.
  • length(): for compatibility with node-async.

• Read-only public properties (for debugging):

  • concurrency: for compatibility with node-async
  • worker: worker function, as passed into "queue"/"queuev"
  • worker_name: worker function's "name" field
  • npending: the number of tasks currently being processed
  • pending: an object (not an array) describing the tasks currently being processed
  • queued: array of tasks currently queued for processing

• Hooks (for compatibility with node-async):

  • saturated
  • empty
  • drain

If the tasks are themselves simple objects, then the entire queue may be serialized (as via JSON.stringify) for debugging and monitoring tools. Using the above fields, you can see what this queue is doing (worker_name), which tasks are queued, which tasks are being processed, and so on.

Here's an example demonstrating the queue:

var mod_fs = require('fs');
var mod_vasync = require('../lib/vasync');

var queue;

function doneOne()
{
	console.log('task completed; queue state:\n%s\n',
	    JSON.stringify(queue, null, 4));
}

queue = mod_vasync.queue(mod_fs.stat, 2);

console.log('initial queue state:\n%s\n', JSON.stringify(queue, null, 4));

queue.push('/tmp/file1', doneOne);
queue.push('/tmp/file2', doneOne);
queue.push('/tmp/file3', doneOne);
queue.push('/tmp/file4', doneOne);

console.log('all tasks dispatched:\n%s\n', JSON.stringify(queue, null, 4));

The initial queue state looks like this:

initial queue state: 
{
    "nextid": 0,
    "worker_name": "anon",
    "npending": 0,
    "pending": {},
    "queued": [],
    "concurrency": 2
}

After four tasks have been pushed, we see that two of them have been dispatched and the remaining two are queued up:

all tasks pushed:
{
    "nextid": 4,
    "worker_name": "anon",
    "npending": 2,
    "pending": {
        "1": {
            "id": 1,
            "task": "/tmp/file1"
        },
        "2": {
            "id": 2,
            "task": "/tmp/file2"
        }
    },
    "queued": [
        {
            "id": 3,
            "task": "/tmp/file3"
        },
        {
            "id": 4,
            "task": "/tmp/file4"
        }
    ],
    "concurrency": 2
}

As they complete, we see tasks moving from "queued" to "pending", and completed tasks disappear:

task completed; queue state:
{
    "nextid": 4,
    "worker_name": "anon",
    "npending": 1,
    "pending": {
        "3": {
            "id": 3,
            "task": "/tmp/file3"
        }
    },
    "queued": [
        {
            "id": 4,
            "task": "/tmp/file4"
        }
    ],
    "concurrency": 2
}

When all tasks have completed, the queue state looks like it started:

task completed; queue state:
{
    "nextid": 4,
    "worker_name": "anon",
    "npending": 0,
    "pending": {},
    "queued": [],
    "concurrency": 2
}

You can use a queue with concurrency 1 and where the tasks are themselves functions to ensure that an arbitrary asynchronous function never runs concurrently with another one, no matter what each one does. Since the tasks are the actual functions to be invoked, the worker function just invokes each one:

var mod_vasync = require('../lib/vasync');

var queue = mod_vasync.queue(
    function (task, callback) { task(callback); }, 1);

queue.push(function (callback) {
	console.log('first task begins');
	setTimeout(function () {
		console.log('first task ends');
		callback();
	}, 500);
});

queue.push(function (callback) {
	console.log('second task begins');
	process.nextTick(function () {
		console.log('second task ends');
		callback();
	});
});

This example outputs:

$ node examples/queue-serializer.js 
first task begins
first task ends
second task begins
second task ends