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@crichardsonNodeJS: the good parts?A skeptic’s viewChris RichardsonAuthor of POJOs in ActionFounder of the original CloudFoundry.com@crichardsonchris.richardson@springsource.comhttp://plainoldobjects.com

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@crichardsonPresentation goalHow a curmudgeonlyserver-side Java developerdiscovered an appreciationfor NodeJS and JavaScript

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@crichardsonAbout Chris

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@crichardson(About Chris)

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@crichardsonAbout Chris()

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@crichardsonAbout Chris

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@crichardsonAbout Chrishttp://www.theregister.co.uk/2009/08/19/springsource_cloud_foundry/

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@crichardsonvmc push About-ChrisDeveloper AdvocateSignup at http://cloudfoundry.com

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@crichardsonAgendaOverview of NodeJSBuilding a front-end server with NodeJSTaming tangled asynchronous code with promises

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@crichardsonWhat’s NodeJS?Designed for DIRTy apps

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@crichardsonSmall but growing rapidlyBusy!

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@crichardsonNodeJS Hello Worldapp.js$ node app.js$ curl http://localhost:1337http://nodejs.org/Load a modulerequest handler

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@crichardsonNodeJSJavaScriptReactorpatternModules

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@crichardsonNodeJSJavaScriptReactorpatternModules

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@crichardsonDynamic, weakly-typedDynamic:Types are associated with values - not variablesDefine new program elements at runtimeWeakly typed:Leave out arguments to methodsAccess non-existent object propertiesWeird implicit conversions: 99 == “99”!truthy and falsy valuesComprehensive tests are essential

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@crichardsonJavaScript is object-oriented> var fred = {name: “Fred”, gender: “Male”};undefined> fred.name“Fred”> console.log("reading age=" + fred.age);reading age=undefinedundefined> fred.age = 99;99> fred{ name: Fred,gender: Male,age: 99 }> delete fred.agetrue> fred{ name: Fred, gender: Male }Unordered key-value pairsKeys = propertiesAdd propertyDelete property

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@crichardsonJavaScript “classes”function Person(name) {this.name = name;}Person.prototype.sayHello = function () { console.log("Hello " + this.name); };var chris = new Person("Chris");chris.sayHello();Looks like aconstructor?!?What’s that?!?!This Java-like syntax is a messbecause JavaScript isn’t classbasedLooks familiar

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@crichardsonoverridesJavaScript is a prototypallanguage__proto__a 99__proto__a 100b 200inheritedPrototypeXY

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@crichardsonPrototypal code$ node> var person = {};undefined> person.sayHello = function () { console.log("Hello " + this.name); };[Function]> var chris = Object.create(person, {name: {value: "Chris"}});undefined> var sarah = Object.create(person, {name: {value: "Sarah"}});undefined> chris.sayHello();Hello Chrisundefined> sarah.sayHello();Hello Sarahundefined> chris.sayHello = function () { console.log("Hello mate: " + this.name); };[Function]> chris.sayHello();Hello mate: ChrisundefinedNot definedhereprototype properties

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@crichardsonJavaScript is Functionalfunction makeGenerator(nextFunction) {var value = 0;return function() {var current = value;value = nextFunction(value);return current;};}var inc = makeGenerator(function (x) {return x + 1; });> inc()0> inc()1Pass function as anargumentReturn a functionclosure

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@crichardsonPartial function application> var join = require("path").join;undefined> join("/x", "y")/x/y> var withinx = join.bind(undefined, "/x");undefined> withinx("y");/x/y>partially apply join

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@crichardsonCreated in a hurry with thegoal of looking like JavaThe ‘Java...’ name creates expectations that it can’t satisfyFake classes: Hides prototypes BUT still seems weirdglobal namespacescope of vars is confusingMissing return statement = confusion‘function’ is really verbose‘this’ is dynamically scoped

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@crichardsonJavaScript is the only way toget things done in thebrowser

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@crichardsonStockholm syndrome“Stockholm syndrome ... is a psychologicalphenomenon in which hostages ... havepositive feelings toward their captors,sometimes to the point of defending them...”http://en.wikipedia.org/wiki/Stockholm_syndrome

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@crichardsonMartin Fowler once said:"...Im one of those who despairs that alanguage with such deep flaws plays such animportant role in computation. Still theconsequence of this is that we must takejavascript seriously as a first-class languageand concentrate on how to limit the damageits flaws cause. ...."http://martinfowler.com/bliki/gotoAarhus2012.html

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@crichardsonUse just the good parts

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@crichardsonUse a better language thatcompiles to JavaScriptTypeScriptTyped parameters and fieldsClasses and interfaces (dynamic structural typing)DartClass-based OOOptional static typingBidirectional binding with DOM elements

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@crichardsonCoffeeScript Hello Worldhttp = require(http)class HttpRequestHandlerconstructor: (@message) ->handle: (req, res) =>res.writeHead(200, {Content-Type: text/plain})res.end(@message + n)handler = new HttpRequestHandler "Hi There from CoffeeScript"server = http.createServer(handler.handle)server.listen(1338, 127.0.0.1)console.log(Server running at http://127.0.0.1:1338/)Classes :-)Bound method

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@crichardsonNodeJSJavaScriptReactorpatternModules

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@crichardsonAbout the Reactor patternDefined by Doug Schmidt in 1995Pattern for writing scalable serversAlternative to thread-per-connection modelSingle threaded event loop dispatches events onhandles (e.g. sockets, file descriptors) to event handlers

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@crichardsonReactor pattern structureEvent Handlerhandle_event(type)get_handle()Initiation Dispatcherhandle_events()register_handler(h)select(handlers)for each h in handlersh.handle_event(type)end loophandleSynchronous EventDemultiplexerselect()ownsnotifiesuseshandlers

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@crichardsonBenefits:Separation of concerns - event handlers separatedfrom low-level mechanismMore efficient - no thread context switchingSimplified concurrency - single threaded

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@crichardsonDrawbacks:Non-pre-emptive - handlers can’t block/take a longtimeDifficult to understand and debug - inverted flow ofcontrol

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@crichardsonNodeJS event loop implementsthe Reactor pattern

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@crichardsonApplication codeEvent-driven architectureNodeJS event loopBasic networking/file-system/etc.HTTP DB driver ...EventlistenerCallbackfunctionOnetimeeventsRecurringevents

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@crichardsonGetting notified: Callbackexamplevar fs = require("fs");function statFile(path) {fs.stat(path, function (err, stat) {if (err) {console.log("Stat failed: " + path, err);throw err;}console.log("stat result=" + path, stat);});};By convention: firstparam is error objectBy convention: Lastarg is a callbackCallbacks aregood for onetimenotifications

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@crichardsonGetting notified: eventlistenersEventEmitter class - inherit or useListener registration methods:on(eventName, listener)once(eventName, listener)Emitting eventsemit(eventName, args...)‘error’ event = special case: no listener print stack trace andexit!Good forrecurringevents

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@crichardsonEvent listener examplevar fs = require("fs");var readStream = fs.createReadStream("events.js", {encoding: "utf-8"});// ReadStream << ReadableStream << EventEmitterreadStream.on(open, function (fd) {console.log("opened with fd=", fd);});// Node v0.10 has readable instead: this is deprecatedreadStream.on(data, function (data) {console.log("data=", data);});Register listenerRegister listener

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@crichardsonCallback hellfunction times2(x, callback) {setTimeout(function () {callback(x * 2)}, 500);}function plus3(x, callback) {setTimeout(function (){callback(x + 3)}, 500);}function displayResult(z) {console.log("The result is=", z);}function plus3AndThenTimes2(x, callback){plus3(x, function (y) {times2(y, callback)})}plus3AndThenTimes2(10, displayResults);function sum(a, b, callback) {setTimeout(function () {callback(a + b);}, 500);}function plus3PlusTimes2(x, callback) {var p3, t2;function perhapsDone() {if (p3 & t2)sum(p3, t2, callback);};plus3(x, function (y) {p3 = y;perhapsDone();});times2(x, function (y) {t2 = y;perhapsDone();});}plus3PlusTimes2(10, displayResult);times2(plus3(x)) times2(x) + plus3(x)

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@crichardsonLong running computationsLong running computation blocks event loop forother requestsNeed to run outside of main event loopOptions:Community: web workers threadsBuilt-in: NodeJS child processes

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@crichardsonUsing child processesvar child = require(child_process).fork(child.js);function sayHelloToChild() {child.send({hello: "child"});}setTimeout(sayHelloToChild, 1000);child.on(message, function(m) {console.log(parent received:, m);});function kill() {child.kill();}setTimeout(kill, 2000);process.on(message, function (m) {console.log("child received message=", m);process.send({ihateyou: "you ruined my life"})});parent.jschild.jsCreate child processSend message to child

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@crichardsonNodeJSJavaScriptReactorpatternModules

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Core built-in modulesBasic networkingHTTP(S)FilesystemEventsTimers...

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@crichardsonThousands of communitydeveloped moduleshttps://npmjs.org/web frameworks, SQL/NoSQLdatabase, drivers, messaging, utilities...

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@crichardsonWhat’s a module?One or more JavaScript filesOptional native code:Compiled during installationJavaScript != systems programming languagePackage.json - metadata including dependenciesexports.sayHello = function () {console.log(“Hello”);}foo.js

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@crichardsonEasy to install$ npm install package-name

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@crichardsonEasy to usevar http = require(“http”)var server = http.createServer...Core module ORPath to file ORmodule in node_modulesModule’s exports

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@crichardsonDeveloping with NodeJSmodulesCore modulesCommunity modulesYour modulesApplication code

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@crichardsonLots of modules BUT...Variable qualityMultiple incomplete MySQL drivers, e.g. withoutconnection pooling!Often abandoned...

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@crichardsonTo summarizeNodeJSJavaScriptReactor patternModulesFlawed andmisunderstoodScalable yetcostly andoftenunnecessaryRich butvariable quality

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@crichardsonAlternative technologiesAtmosphere - portable event deliveryNetty - asynchronous I/OVert.xSpringSource’s Reactor project...

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@crichardsonSo why care aboutNodeJS?Easy to write scalable network servicesEasy to push events to the browserEasy to get (small) stuff doneIt has a role to play in modernapplication architecture

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@crichardsonEvolving from a monolithicarchitecture....WARShippingServiceAccountingServiceInventoryServiceStoreFrontUI

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@crichardson... to a micro-service architectureStore front web applicationshipping web applicationinventory web applicationAccountingServiceStoreFrontUIaccounting web applicationShippingServiceInventoryService

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@crichardsonBrowserWARStoreFrontUIModelView ControllerPresentation layer evolution....HTML / HTTP+JavaScript

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@crichardsonBrowser Web applicationRESTfulEndpointsModelView Controller...Presentation layer evolutionJSON-RESTHTML 5 -JavaScriptNo elaborate, server-side web frameworkrequiredEventpublisherEventsStaticcontent

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@crichardsonNodeJS as an API gatewayBrowserService 1Service 2MessageBusHTML 5/JavaScriptSocket.ioclientEventsRESTful WSServerapplicationSocket.ioserverNode JSService 3RESTfulWS

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@crichardsonAgendaOverview of NodeJSBuilding a front-end server with NodeJSTaming tangled asynchronous code with promises

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@crichardsonServing static content

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@crichardsonUsing low-level APIsvar http = require(http), path = require(path), mime = require(mime), fs = require("fs");var server = http.createServer(function (req, res) {var filePath;if (req.url == /) {filePath = public/index.html;} else {filePath = public + req.url;}fs.exists(filePath, function (exists) {if (exists) {res.writeHead(200, {"content-type": mime.lookup(path.basename(filePath)) });fs.createReadStream(filePath).pipe(res);} else {res.writeHead(404, {Content-Type: text/plain});res.write(Error 404: resource not found.);res.end();}});});server.listen(1337, 127.0.0.1);console.log(Server running at http://127.0.0.1:1337/);Uses fileextensionCool!

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@crichardsonUsing the express webframeworkvar express = require(express), http = require(http), app = express(), server = http.createServer(app);app.configure(function(){app.use(express.static(__dirname + /public));});server.listen(8081);

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@crichardsonRESTful web services

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@crichardsonImplementing RESTful WSwith Expressvar express = require(express),http = require(http),path = require(path);var app = express();var server = http.createServer(app);app.get(/portfolio/:userId, function (req, res) {var portfolio = retrievePortfolio(req.params.userId);res.json(portfolio);});Easy URLrouting anddestructuring

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@crichardsonProxying to backend serverexpress = require(express)request = require(request)app = express.createServer()proxyToBackend = (baseUrl) ->(req, res) ->callback = (error, response, body) -> console.log("error=", error)originRequest = request(baseUrl + req.url, callback)req.pipe(originRequest)originRequest.pipe(res)app.get(/restaurant/*, proxyToBackend(http://available-restaurant....com))app.post(/orders, proxyToBackend(http://restaurant-management...com))app.get(/orders, proxyToBackend(http://restaurant-management...com))Returns a request handlerthat proxies to baseUrl

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@crichardsonDelivering events to thebrowser

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@crichardsonNodeJS clock exampleIncrements everysecond

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@crichardsonSocket.io - Server sidevar express = require(express), http = require(http), app = express(), server = http.createServer(app), io = require(socket.io).listen(server);app.configure(function(){app.use(express.static(__dirname + /public));});server.listen(8081);io.sockets.on(connection, function (socket) { var counter = 0; function tick() { counter = counter + 1; socket.emit(tick, counter); }; setInterval(tick, 1000);});handle newconnectionSend tick event tobrowser every 1 secinitializes socket.io

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@crichardsonSocket.io - client side using theknockout.js MVVM frameworkvar socket = io.connect(location.hostname);function ClockModel() {self.ticker = ko.observable(1);socket.on(tick, function (data) {self.ticker(data);});};ko.applyBindings(new ClockModel());<html><body>The event is <span data-bind="text: ticker"></span><script src="/socket.io/socket.io.js"></script><script src="/knockout-2.0.0.js"></script><script src="/clock.js"></script></body></html>clock.jsConnect tosocket.ioSubscribeto tick eventBind to modelUpdatemodel

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@crichardsonDistributed NodeJS clockexampleBrowser NodeJS NodeJSRabbitMQProducerConsumersocket.iosocket.io

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@crichardsonAMQP Publishervar amqp = require(amqp),amqpConnection = amqp.createConnection(...),tickTockExchange;function tick() {var message = { tick: Date.now() };tickTockExchange.publish("tickTock", message, {mandatory: true,contentType: "text/plain"});};amqpConnection.on(ready,function () {tickTockExchange =amqpConnection.exchange("tickTock",options = {passive: false, type: fanout});tickTockExchange.on(open, function () { setInterval(tick, 1000) });});Connect toAMQPEnsureexchangeexistsPublishmessageInitialize timer

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@crichardsonAMQP socket.iovar express = require(express), http = require(http), amqp = require(‘amqp’)....;server.listen(8081);...var amqpCon = amqp.createConnection(...);io.sockets.on(connection, function (socket) {function amqpMessageHandler(message, headers, deliveryInfo) {var m = JSON.parse(message.data.toString());socket.emit(‘tick’, m);};amqpCon.queue(“”, {},function(queue) {queue.bind(“tickTock”, “”);queue.subscribe(amqpMessageHandler);});});Connect toAMQP queueSubscribe toAMQP queueRepublishas socket.ioeventhttps://github.com/cer/nodejs-clock

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@crichardsonAgendaOverview of NodeJSBuilding a front-end server with NodeJSTaming tangled asynchronous code with promises

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@crichardsonAsync code = callback hellScenarios:Sequential: A B CFork and join: A and B CCode quickly becomes very messy

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@crichardsonSimplifying code withPromises (a.k.a. Futures)Functions return a promise - no callback parameterA promise represents an eventual outcomeUse a library of functions for transforming andcomposing promisesPromises/A+ specification - http://promises-aplus.github.io/promises-specwhen.js (part of cujo.js by SpringSource) is a popularimplementation

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@crichardsonTaming callback hell 1function times2(x) {var deferred = when.defer();setTimeout(function () {deferred.resolve(x * 2)}, 500);return deferred.promise;}times2(plus3(x)) Create a deferredReturn a promiseEventually supply a valuefunction plus3AndThenTimes2(x) {return plus3(x).then(times2);}plus3AndThenTimes2(10).then(displayResult);Transform value inpromisefunction plus3(x) {var deferred = when.defer();setTimeout(function () {deferred.resolve(x + 3) }, 500);return deferred.promise;}Simpler, almostsynchronous-style code

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@crichardsonTaming callback hell 2function sum(a, b) {var deferred = when.defer();setTimeout(function () {deferred.resolve(a + b);}, 500);return deferred.promise;}function plus3PlusTimes2(x) {var p3 = plus3(x),t2 = times2(x);return when.join(p3, t2).spread(sum);}plus3PlusTimes2(10).then(displayResult);times2(x) + plus3(x)Combine resultsof two promisesCall with arrayelements asarguments

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@crichardsonCalling non-promise codevar deferred = when.defer();fs.stat(path, function (err, statInfo) {if (err)deferred.reject(err);elsedeferred.resolve(statInfo);}var promise = deferred.promise;var nodefn = require("when/node/function");var promise = nodefn.call(fs.stat, path)Hidesboilerplatecode

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@crichardsonFilesystem scanner exampleRead contents of directoryUse stat to determine if directory or fileRecurse on directoriesMerge the results

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@crichardsonRead contents of directoryfunction findFilesInDir(dir) {var directoryContents =nodefn.call(self.fs.readdir, dir);...}Returns promise containingan array of file names

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@crichardsonCreate absolute pathsfunction findFilesInDir(dir) {var directoryContents = ...var toAbsolute = join.bind(undefined, dir)var absolutePaths = when.map(directoryContents, toAbsolute);...}Partially applyjoin()

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@crichardsonUse stat to determine ifdirectory or filefunction findFilesInDir(dir) {var directoryContents = ...var absolutePaths = ...var statTasks = when.map(absolutePaths, makeStatTask);var statResults = parallel(statTasks);...}function makeStatTask(path) {return function () {function makeStatInfo(stats) {return {path: path, isdir: stats.isDirectory(),ctime: stats.ctime};}return nodefn.call(self.fs.stat, path).then(makeStatInfo);};}Execute stats inparallel

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@crichardsonRecurse on directoriesfunction findFilesInDir(dir) {...var statResults = ...;var listOfListsOfFiles =when.map(statResults, processStatInfo);...}function processStatInfo(statInfo) {if (statInfo.isdir) {return findFilesInDir(statInfo.path);} else {return [statInfo.path];}}Map each statresult to a list offiles

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@crichardsonFlatten array of arrays of filepathsfunction findFilesInDir(dir) {...var listOfListsOfFiles = ...;return when.reduce(listOfListsOfFiles,concatenateLists, []);}function concatenateLists(currentResult, value, index, total){return currentResult.concat(value);}

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@crichardsonSummaryJavaScript is a very flawed languageThe asynchronous model is often unnecessary; veryconstraining; and adds complexityBUT despite those problemsToday, NodeJS is remarkably useful for building network-focussed components

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@crichardsonQuestions?@crichardson chris.richardson@springsource.comhttp://plainoldobjects.com - code and slideswww.cloudfoundry.com