Flow-JS

Overview

Flow-JS provides a continuation-esque construct that makes it much easier to express multi-step asynchronous logic in non-blocking callback-heavy environments like Node.js or javascript in the web browser.

The concept is best explained with an example. The following code uses a simple asynchronous key-store to look-up a user's ID from his username and then sets his email address, first name, and last name.

In this example, the dbGet and dbSet functions are assumed to rely on asynchronous I/O and both take a callback that is called upon completion.

dbGet('userIdOf:bobvance', function(userId) {
	dbSet('user:' + userId + ':email', '[email protected]', function() {
		dbSet('user:' + userId + ':firstName', 'Bob', function() {
			dbSet('user:' + userId + ':lastName', 'Vance', function() {
				okWeAreDone();
			});
		});
	});
});

Notice how every single step requires another nested function definition. A four-step process like the one shown here is fairly awkward. Imagine how painful a 10-step process would be!

One might point out that there is no reason to wait for one dbSet to complete before calling the next, but, assuming we don't want okWeAreDone to be called until all three calls to dbSet are finished, we'd need some logic to manage that:

dbGet('userIdOf:bobvance', function(userId) {
	var completeCount = 0;
	var complete = function() {
		completeCount += 1;
		if (completeCount == 3) {
			okWeAreDone();
		}
	}
	
	dbSet('user:' + userId + ':email', '[email protected]', complete);
	dbSet('user:' + userId + ':firstName', 'Bob', complete);
	dbSet('user:' + userId + ':lastName', 'Vance', complete);
});

Now look at the same example using Flow-JS:

flow.exec(
	function() {
		dbGet('userIdOf:bobvance', this);
		
	},function(userId) {
		dbSet('user:' + userId + ':email', '[email protected]', this.MULTI());
		dbSet('user:' + userId + ':firstName', 'Bob', this.MULTI());
		dbSet('user:' + userId + ':lastName', 'Vance', this.MULTI());
	
	},function() {
		okWeAreDone()
	}
);

A flow consists of a series of functions, each of which is applied with a special this object which serves as a callback to the next function in the series. In cases like our second step, this.MULTI() can be used to generate a callback that won't call the next function until all such callbacks have been called.

Installing

Flow-JS is a CommonJS compatible module. Place the "flow.js" file in any directory listed in your require.paths array and require it like this:

var flow = require('flow')

Or you can just put "flow.js" next to your script and do this:

var flow = require('./flow')

Defining a Flow

flow.define defines a flow given any number of functions as parameters. It returns a function that can be used to execute that flow more than once. Whatever parameters are passed each time that flow is called are passed as the parameters to the first function in the flow.

Each function in the flow is called with a special this object which maintains the state of the flow's execution, acts as a container for saving values for use between functions in the flow, and acts as a callback to the next function in the flow.

Here is an example to make this clear:

// define a flow for renaming a file and then printing its stats
var renameAndStat = flow.define(

	function(fromName, toName) {
		// arguments passed to renameAndStat() will pass through to this first function
		
		this.toName = toName; // save to be used in the next function
		fs.rename(fromName, toName, this);
	
	},function(err) {
		// when fs.rename calls the special "this" callback above, this function will be called
		// whatever arguments fs.rename chooses to pass to the callback will pass through to this function
	
		if (err) throw err;
		
		// the "this" here is the same as in the function above, so this.toName is available
		fs.stat(this.toName, this);
	
	},function(err, stats) {
		// when fs.stat calls the "this" callback above, this function will be called
		// whatever arguments fs.stat chooses to pass to the callback will pass through to this function
		
		if (err) throw err;
		
		sys.puts("stats: " + JSON.stringify(stats));
	}
);

// now renameAndStat can be used more than once
renameAndStat("/tmp/hello1", "/tmp/world1");
renameAndStat("/tmp/hello2", "/tmp/world2");

Executing a Flow Just Once

flow.exec is a convenience function that defines a flow and executes it immediately, passing no arguments to the first function.

Here's a simple example very similar to the one above:

flow.exec(
	function() {
		fs.rename("/tmp/hello", "/tmp/world", this);
	},function(err) {
		if (err) throw err;
		fs.stat("/tmp/world", this)
	},function(err, stats) {
		if (err) throw err;
		sys.puts("stats: " + JSON.stringify(stats));
	}
);

Multiplexing

Sometimes, it makes sense for a step in a flow to initiate several asynchronous tasks and then wait for all of those tasks to finish before continuing to the next step in the flow. This can be accomplished by passing this.MULTI() as the callback rather than just this.

Here is an example of this.MULTI() in action (repeated from the overview):

flow.exec(
	function() {
		dbGet('userIdOf:bobvance', this);
		
	},function(userId) {
		dbSet('user:' + userId + ':email', '[email protected]', this.MULTI());
		dbSet('user:' + userId + ':firstName', 'Bob', this.MULTI());
		dbSet('user:' + userId + ':lastName', 'Vance', this.MULTI());
	
	},function() {
		okWeAreDone()
	}
);

You can identify the results of a function by passing a result identifier to MULTI. The results of a function can retrieved using this key in the final step. The result will be a single value if callback receives 0 or 1 argument, otherwise it will be an array of arguments passed to the callback.

Example:

flow.exec(
	function() {
		dbGet('userIdOf:bobvance', this.MULTI('bob'));
		dbGet('userIdOf:joohndoe', this.MULTI('john'));
	},function(results) {
	  dbSet('user:' + results['bob'] + ':email', '[email protected]');
	  dbSet('user:' + results['john'] + ':email', '[email protected]');
	  okWeAreDone();
	}
);

In many cases, you may simply discard the arguments passed to each of the callbacks generated by this.MULTI(), but if you need them, they are accessible as an array of arguments objects passed as the first argument of the next function. Each arguments object will be appended to the array as it is received, so the order will be unpredictable for most asynchronous APIs.

Here's a quick example that checks for errors:

flow.exec(
	function() {
		fs.rename("/tmp/a", "/tmp/1", this.MULTI());
		fs.rename("/tmp/b", "/tmp/2", this.MULTI());
		fs.rename("/tmp/c", "/tmp/3", this.MULTI());
	
	},function(argsArray) {
		argsArray.forEach(function(args){
			if (args[0]) then throw args[0];
		});
	}
);

serialForEach

Flow-JS comes with a convience function called flow.serialForEach which can be used to apply an asynchronous function to each element in an array of values serially:

flow.serialForEach([1, 2, 3, 4], function(val) {
	keystore.increment("counter", val, this);
},function(error, newVal) {
	if (error) throw error;
	sys.puts('newVal: ' + newVal);
},function() {
	sys.puts('This is the end!');
});

flow.serialForEach takes an array-like object, a function to be called for each item in the array, a function that receives the callback values after each iteration, and a function that is called after the entire process is finished. Both of the second two functions are optional.

flow.serialForEach is actually implemented with flow.define.

Thanks to John Wright for suggesting the idea! (http://github.com/mrjjwright)