The document discusses asynchronous programming in Node.js. It explains that Node.js uses an asynchronous and non-blocking model that requires using callbacks and event handlers. It compares synchronous and asynchronous code examples. It also discusses challenges with asynchronous code like the "pyramid of doom" and error handling. Finally, it introduces techniques for dealing with these challenges, like named callbacks, control flow libraries like Async.js, and promises.

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Put On Your Asynchronous
Hat and Node
Marc Fasel @marcfasel
Shine Technologies http://blog.shinetech.com
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What is an Asynchronous Hat?
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Node.js
Server-side JavaScript platform
Event-driven, non-blocking I/O
All I/O is asynchronous
Asynchrononous everywhere
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Different Mindsets
Synchronous coding comes natural
Natural execution order
Asynchronous coding a bit harder
Serial execution leads to nesting
New: parallel execution
Single Callback functions and Event Emitters
Explicit error handling
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Synchronous Code
var filenames = fs.readdirSync('/tmp/');
for (var i = 0; i < filenames.length; i++) {
console.log(filenames[i]);
}
console.log('Done.');
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Asynchronous Code
fs.readdir('/tmp/', function (err, filenames) {
for (var i = 0; i < filenames.length; i++) {
console.log(filenames[i]);
}
console.log('Done.');
});
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Some More Synchronous Code
var totalBytes = 0;
for (var i = 0; i < filenames.length; i ++) {
var stats = fs.statSync('/tmp/' + filenames[i]);
totalBytes += stats.size;
}
console.log('Total bytes: ' + totalBytes);
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Asynchronous Parallel Execution
var count = filenames.length;
for (var i = 0; i < filenames.length; i++) {
! fs.stat('/tmp/' + filenames[i], function (err, stats) {
! ! totalBytes += stats.size;
! ! count--;
! ! if (count === 0) {
! ! ! // We’re done
! ! ! console.log('Total bytes: ' + totalBytes);
! ! }
! });
};
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fs.readFile(dirFrom+filename,'utf-8', function (err, data){
! ! if (err) return callback(err);
! ! fs.writeFile(dirTo+filename,data,'utf-8', function (err){
! ! ! ! if (err) return callback(err);
! ! ! ! fs.chmod(dirTo+filename,0777, function (err){
! ! ! ! ! ! if (err) return callback(err);
! ! ! ! ! ! anotherAsyncFunction(param1, function (err){
! ! ! ! ! ! ! ! if (err) return callback(err);
! ! ! ! ! ! ! ! anotherAsyncFunction(param1, function (err){
! ! ! ! ! ! ! ! })
! ! ! ! ! ! })
! ! ! ! })
! ! })
});
Pyramid of Doom
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Event Mechanisms
Events used for asynchronous communication
Two types of asynchronous functions:
Single Callback: one ‘done’ event, single callback function
Event Emitter: more than one event
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Single Callback Functions
fs.readdir(dir, function (err, filenames) {
// Failure
! if (err) return callback(err);
// Success
! console.log(dir + ' has ' + filenames.length + 'files.');
});!
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Event Emitter
var readStream = fs.createReadStream(filename);
readStream.on('open', function () {
! readStream.pipe(res);
});
readStream.on('error', function(err) {
! // Failure
! res.end(err);
});
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Error Handling
Synchronous
Exceptions give us lots of goodies
Asynchronous
Explicit error code everywhere
Lots of boilerplate
More Pyramid of Doom
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Synchronous Error Handling
We are blessed with Exceptions
Stop execution immediately
Automatically bubble up call hierarchy
Error code logically separated from application code
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World With Exceptions
function readFile(fileName) {
! var file = openFile(fileName);
! var data = readContent(file);
! closeFile(file);
! return data;
}
function readContent(file) {
! throw new Error("Exception!");
}
try {
! var myData = readFile(fileName);
} catch (e) {
! // Handle error!
}
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World Before Exceptions
function readFile(err, filePath) {
! var file = openFile(err, filePath);
! if (err) { return err; }
! var data = readContent(err, file);
! if (err) { return err;}
! closeFile(err, file);
! if (err) { return err; }
! return data;
}
var myFileContent = readFile(err,filePath);
if (err) { // Handle error };
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Asynchronous Error Handling
try {
fs.readFile('/tmp/test.txt', 'utf-8', function (err, data) {
!if (err) {
!! throw err;
!}
console.log(data);
});!
} catch (e) {
! console.log(e);
}
Exceptions possible
try/catch does not have desired result
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Callback Error Handling
fs.readFile('/tmp/test.txt', 'utf-8', function (err, data) {
! if (err) return callback(err);
! // Success
! console.log(data);
});!
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Event Emitter Error Handling
var readStream = fs.createReadStream(filename);
readStream.on('open', function () {
! readStream.pipe(res);
});
readStream.on('error', function(err) {
res.end(err);
});
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Pyramid of Doom With Error Handling
fs.readFile(dirFrom+filename,'utf-8', function (err, data){
! ! if (err) return callback(err);
! ! fs.writeFile(dirTo+filename,data,'utf-8', function (err){
! ! ! ! if (err) return callback(err);
! ! ! ! fs.chmod(dirTo+filename,0777, function (err){
! ! ! ! ! ! if (err) return callback(err);
! ! ! ! ! ! anotherAsyncFunction(param1, function (err){
! ! ! ! ! ! ! ! if (err) return callback(err);
! ! ! ! ! ! ! ! anotherAsyncFunction(param1, function (err){
! ! ! ! ! ! ! ! ! if (err) return callback(err);
! ! ! ! ! ! ! ! ! callback();
! ! ! ! ! ! ! ! })
! ! ! ! ! ! })
! ! ! ! })
! ! })
});
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Asynchronous Error Handling
Can’t catch exceptions that happen in callbacks
Explicit code to handle error
Simple Callback: error code gets mixed with application code
Event Emitter: error code separate
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Help is Available
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Named Callbacks
fs.readdir('/tmp/', readDirCallback);
function readDirCallback (err, filenames) {
for (var i = 0; i < filenames.length; i++) {
console.log(filenames[i]);
}
console.log('Done.');
};
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Nesting Named Callbacks
function copyFile(filePathFrom, filePathTo, callback){
! fs.readFile(filePathFrom,'utf-8', readFileCallback);
! function readFileCallback(err, data){
! ! if (err) return callback(err);
! ! fs.writeFile(filePathTo, data, 'utf-8', writeFileCallback);
! ! function writeFileCallback(err){
! ! ! if (err) return callback(err);
! ! ! callback();
! ! };
! };
};
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Control Flow libraries
Fix problems with asynchronous Control Flow
Control flow first thing people want to fix
Many control flow libraries available
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Async.js
The Underscore.js of asynchronous code
Control flow constructs
Functional constructs
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Series
async.series([
! function(callback) {
! ! fs.chmod(file1,0777, function (err){
! ! ! if (err) return callback(err);
! ! ! callback ();
! ! });
! },
! function(callback) {
! ! fs.chmod(file2,0777, function (err){
! ! ! if (err) return callback(err);
! ! ! callback();
! ! });
! }],
! function done(err) {
! ! if (err) return console.log(err);
! ! console.log ('Done.');
! }
);
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Parallel
async.parallel([
! function(callback) {
! ! fs.chmod(file1,0777, function (err){
! ! ! if (err) return callback(err);
! ! ! callback ();
! ! });
! },
! function(callback) {
! ! fs.chmod(file2,0777, function (err){
! ! ! if (err) return callback(err);
! ! ! callback();
! ! });
! }],
! function done(err) {
! ! if (err) return console.log(err);
! ! console.log ('Done.');
! }
);
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Functional Constructs
async.map(['file1','file2','file3'], fs.stat, function(err, results){
// results is now an array of stats for each file
});
async.filter(['file1','file2','file3'], path.exists, function(results){
// results now equals an array of the existing files
});
async.reduce(...);
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Promises
Pattern for asynchronous control flow
Promises are objects representing a future outcome of an
asynchronous call
Outcome will be either ‘resolved’ or ‘rejected’
Attach callbacks to ‘resolved’, ‘rejected’
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Callback -> Promise
function getReadFilePromise(){
! var deferred = q.defer();
! fs.readFile('/tmp/test.txt', 'utf-8', function (err, data) {
! ! if (err) { deferred.reject(err); }
! ! deferred.resolve(data);
! });! !
! return deferred.promise;
}
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Promises
var readFilePromise = getReadFilePromise();
readFilePromise.then(
function(data){ console.log('Resolved:' + data); },
function(err){ console.log('Rejected:' + err); });
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Promises
Cool Stuff: Promise Chaining
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Promise Chaining
readFilePromise(filePathFrom)
! .then(writeFilePromise(filePathTo))
! .then(changeFilePermissionPromise(filePathTo,'777'))
! .then(oneMorePromise())
! .then(oneMorePromise());
var promises = [
! readFilePromise,
! writeFilePromise,
! changeFilePermissionPromise,
! oneMorePromise,
! oneMorePromise
];
var allPromise = Q.all(promises);
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Promise Error Handling
readFilePromise(filePathFrom)
! .then(writeFilePromise(filePathTo))
! .then(changeFilePermissionPromise(filePathTo,'777'),
! ! errorCallback);
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Promises
Promises help with asynchronous control flow
Avoid the Pyramid of Doom
Exception style error bubbling
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Conclusion
Asynchronous programming needs an asynchronous hat
New things
Callbacks
Event Emitters
Explicit error handling
For the more difficult stuff
Named callbacks
Async.js
Promises
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Thank you.
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References
Trevor Burnham - Async JavaScript: Build More Responsive Apps with
Less Code
Pedro Teixeira - Professional Node.js: Building Javascript-Based
Scalable Software
You’re Missing the Point of Promises - http://domenic.me/2012/10/14/
youre-missing-the-point-of-promises/
Popular Control Flow Libraries - http://dailyjs.com/2011/11/14/popular-
control-flow/
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