Galaxy brings async/await semantics to JavaScript with a minimal API, thanks to EcmaScript 6 generators.

async/await in JavaScript

Galaxy lets you write async code as if JavaScript had async/await keywords.

For example, here is how you would write an async function that counts lines in a file:

function* countLines(path) {
	var names = yield fs.readdir(path);
	var total = 0;
	for (var i = 0; i < names.length; i++) {
		var fullname = path + '/' + names[i];
		if ((yield fs.stat(fullname)).isDirectory()) {
			total += yield countLines(fullname);
		} else {
			var count = (yield fs.readFile(fullname, 'utf8')).split('\n').length;
			console.log(fullname + ': ' + count);
			total += count;
		}
	}
	return total;
}

Just think of the * in function* as an async keyword and of yield as an await keyword.

You can write another async function that calls countLines:

function* projectLineCounts() {
	var total = 0;
	total += yield countLines(__dirname + '/../examples');
	total += yield countLines(__dirname + '/../lib');
	total += yield countLines(__dirname + '/../test');
	console.log('TOTAL: ' + total);
	return total;
}

Note: don't forget to prefix all the async calls with a yield. Otherwise you will be adding generator objects to total and you'll just get a NaN.

Cool! But where does galaxy come into play? This is just plain JavaScript (with ES6 generators) and there are no calls to any galaxy API. Pretty mysterious!

That's the whole point behind galaxy: you hardly see it. It lets you write async functions that call other async functions with function* and yield. There are no extra API calls. Exactly like you would write your code if the language had async/await keywords.

star/unstar

The magic happens in two places: when you call node.js async functions, and when node.js calls your async functions.

The node.js functions that we called in the example functions are fs.readdir and fs.readFile. Part of the magic is that fs is not your usual require('fs'). It is initialized as:

var galaxy = require('galaxy');
var fs = galaxy.star(require('fs'));

The galaxy.star call returns a module in which all the asynchronous functions have been starred, i.e. promoted from function to function*. When you call these functions with yield you get await semantics.

Note that galaxy.star can also be applied to individual functions. So, instead of starring the entire fs module, we could have starred individual functions:

var galaxy = require('galaxy');
var fs = require('fs');
var readdir = galaxy.star(fs.readdir);
var readFile = galaxy.star(fs.readFile);

---

The other side of the magic happens when node.js calls your function* APIs. In our example, this happens when we call projectLineCounts, our main function. Here is the code:

var projectLineCountsCb = galaxy.unstar(projectLineCounts);

projectLineCountsCb(function(err, result) {
	if (err) throw err;
	console.log('CALLBACK RESULT: ' + result);
});

The galaxy.unstar call converts our function* into a regular node.js function that we then call with a callback.

galaxy.unstar can also be applied to a whole module, in which case it unstars all the functions of the module. This is handy if you have written a library with galaxy and you want to make it available to developers who write their code in callback style. Just create another module that exports the unstarred version of your functions:

var galaxy = require('galaxy');
module.exports = galaxy.unstar(require('my-starred-functions'));

Together, galaxy.star and galaxy.unstar take care of all the ugly work to make */yield behave like async/await.

Parallelizing

Fine. But all the code that we have seen above is completely sequential. Would be nice if we could parallelize some calls.

This is actually not very difficult: instead of yielding on a generator returned by a starred function you can spin on it. This runs the generator in parallel with your other code and it gives you back a future. The future that you obtain is just another starred function on which you can yield later to get the result of the computation.

So, for example, you can parallelize the projectLineCounts operation by rewriting it as:

function* projectLineCountsParallel() {
 	var future1 = galaxy.spin(countLines(__dirname + '/../examples'));
 	var future2 = galaxy.spin(countLines(__dirname + '/../lib'));
	var future3 = galaxy.spin(countLines(__dirname + '/../test'));
 	var total = (yield future1()) + (yield future2()) + (yield future3());
	console.log('TOTAL: ' + total);
	return total; 
}

Note: this is not true parallelism; the futures only move forwards when execution reaches yield keywords in your code.

Galaxy also provides a funnel call that you can use to limit the level of parallelism on a given block of code. By setting the funnel's size to 1 you can set up critical sections. See the galaxy API for details.

Array utilities

Galaxy provides async variants of the EcmaScript 5 array functions (forEach, map, filter, ...). These variants give you the choice between sequential and parallel execution when relevant.

See API documentation for details.

Exception Handling

The usual exception handling keywords (try/catch/finally/throw) work as you would expect them to.

If an exception is thrown during the excution of a future, it is thrown when you yield on the future, not when you create it with galaxy.spin.

Long stacktrace

Galaxy provides long stacktraces. Here is a typical stacktrace:

Error: getaddrinfo ENOTFOUND
    <<< yield stack >>>
    at googleSearch (/Users/bruno/dev/syracuse/node_modules/galaxy/tutorial/tuto6-mongo.js:43:11)
    at search (/Users/bruno/dev/syracuse/node_modules/galaxy/tutorial/tuto6-mongo.js:30:34)
    at  (/Users/bruno/dev/syracuse/node_modules/galaxy/tutorial/tuto6-mongo.js:22:29)
    <<< raw stack >>>
    at errnoException (dns.js:37:11)
    at Object.onanswer [as oncomplete] (dns.js:124:16)

The <<< yield stack >>> part is a stack which has been reconstructed by the galaxy library and which reflects the stack of yield calls in your code.

The <<< raw stack >>> part gives you the low level callback stack that triggered the exception. It is usually a lot less helpful than the yield stack because it does not give you much context about the error.

This feature requires that you install the galaxy-stack addon module:

npm install galaxy-stack

Stable context

Global variables are evil. Everyone knows that!

But there are a few cases where they can be helpful. The main one is to track information about who is executing the current request: security context, locale, etc. This kind of information is usually very stable (for a given request) and it would be very heavy to pass it explicitly down to all the low level APIs that need it. So the best way is to pass it implicitly through some kind of global context.

But you need a special global which is preserved across yield points. If you set it at the beginning of a request it should remain the same throughout the request (unless you change it explicitly). It should not change under your feet because other requests with different contexts get interleaved.

Galaxy exposes a context property that is guaranteed to be stable across yield points. If you assign an object to galaxy.context at the beginning of a request, you can retrieve it later.

Note: this functionality is more or less equivalent to Thread Local Storage (TLS) in threaded systems.

Odd callbacks

Galaxy is designed to work with functions that have the usual node.js callback signature: callback(err, result). It also works with functions that return several results through their callback. In this case the results are returned as an array. For example:

var request = require('request');
// request.get calls its callback as callback(err, response, body)

var get = galaxy.star(request.get);
var r = yield get(url);
// the starred version returns [response, body]
console.log("status=" + r[0].statusCode);
console.log("body=" + r[1]);

On the other hand, galaxy cannot deal directly with functions that have an odd callback signature. The best example is fs.exists which does not have any error parameter in its callback. You need a special wrapper to deal with such calls:

// the wrapper
function existsWrapper(path, cb) {
	fs.exists(path, function(result) { cb(null, result); })
}

var exists = galaxy.star(existsWrapper);
var found = yield exists(__dirname + '/README.md');

Interrupting a future

Futures can be made interruptible by passing an interrupt option to the galaxy.spin call:

var fut = galaxy.spin(asyncFn(), {
	interrupt: function() {
		if (interruptRequested) return true;
	});

// somewhere else
// this yield will never return if fut is interrupted (see note below)
var result = yield fut();

Note: this feature is very experimental. It could be enhanced to throw an exception into the code which is yielding on the future but it is not clear whether this should bypass or not the catch/finally clauses that may be active on the future's stack.

Streams

Galaxy works with ez-streams, a simple streaming API for node.js.

Asynchronous constructor

Galaxy also lets you invoke constructors that contain asynchronous calls but this is one of the rare cases where you cannot just use the usual JavaScript keyword. Instead of the new keyword you use the special galaxy.new helper. Here is an example:

// asynchronous constructor
function* MyClass(name) {
	this.name = name;
	yield myAsyncFn();
}

// create an instance of MyClass
var myObj = (yield galaxy.new(MyClass)("obj1"));
console.log(myObj.name);

API

See API.md

See also the tutorial and the examples.

Installation

$ npm install galaxy
$ npm install galaxy-stack

galaxy-stack is an optional module that you should install to get long stacktraces.

Then you can try the examples:

$ cd node_modules/galaxy
$ node --harmony examples/countLines
... some output ...
$ node --harmony examples/countLinesParallel
... slightly different output  ...

Running in the browser

Galaxy also runs browser side but you need a recent browser like Firefox 31 or Google Chrome 39 that you can download from https://www.google.com/chrome.

You're all set and you can open the examples/hello-browser.html page to see Galaxy in action. Pretty boring demo but at least it works!

Gotchas

Generators have been added very recently to V8. To use them you need to:

• Install node.js version 0.11.2 (unstable) or higher.
• Run node with the --harmony flag.

For example, to run the example above:

$ node -v
v0.11.2
$ node --harmony examples/countLines

The yield keyword can be tricky because it has a very low precedence. For example you cannot write:

var sum1 = yield a() + yield b();
var sum2 = yield c() + 3;

because they get interpreted as:

var sum1 = yield (a() + yield b()); // compile error
var sum2 = yield (c() + 3); // galaxy gives runtime error

You have to write:

var sum = (yield a()) + (yield b());
var sum2 = (yield c()) + 3;

This brings a little lispish flavor to your JS code.

More info

This design is strongly inspired from bits and pieces of streamline.js. The following blog articles (some a bit old) give background information on this design:

• bringing async/await to life in JavaScript
• an early experiment with generators.
• futures = currying the callback
• stream API: events or callbacks

The streamline.js tool has been adapted to generate galaxy code in --generators mode. So, you can also use streamline.js as a preprocessor to generate galaxy code.

License

This work is licensed under the MIT license.