NodeJS: the good parts? A skeptic’s view (jmaghreb, jmaghreb2013)
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JavaScript used to be confined to the browser. But these days, it becoming increasingly popular in server-side applications in the form of NodeJS. NodeJS provides event-driven, non-blocking I/O model that supposedly makes it easy to build scalable network application. In this talk you will learn about the consequences of combining the event-driven programming model with a prototype-based, weakly typed, dynamic language. We will share our perspective as a server-side Java developer who wasn’t entirely happy about JavaScript in the browser, let alone on the server. You will learn how to use NodeJS effectively in modern, polyglot applications.
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NodeJS: the good parts? A skeptic’s view (jmaghreb, jmaghreb2013)
- 1. NodeJS: the good parts? A skeptic’s view Chris Richardson Author of POJOs in Action Founder of the original CloudFoundry.com @crichardson chris@chrisrichardson.net http://plainoldobjects.com @crichardson
- 2. Presentation goal How a grumpy, gray-haired server-side Java developer discovered an appreciation for NodeJS and JavaScript @crichardson
- 3. ! WARNING VIEWER DISCRETION IS ADVISED @crichardson
- 4. About Chris LispWorks 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 Z80 RPG 3 BCPL Pascal 6502 C Assembler Basic @crichardson
- 5. EJB 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 C++ @crichardson
- 6. CloudFoundry.com 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 @crichardson
- 7. About Chris ? 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 @crichardson
- 8. Agenda Overview of NodeJS JavaScript: Warts and all The Reactor pattern: an event-driven architecture NodeJS: There is a module for that Building a front-end server with NodeJS @crichardson
- 9. What’s NodeJS? Designed for DIRTy apps @crichardson
- 11. NodeJS Hello World app.js Load a module request handler $ node app.js $ curl http://localhost:1337 http://nodejs.org/ No complex configuration: simple! @crichardson
- 14. Dynamic and weakly-typed Dynamic: Types are associated with values - not variables Define new program elements at runtime Weakly typed: Leave out arguments to methods Read non-existent object properties Add new properties by simply setting them @crichardson
- 15. JavaScript is object-oriented > var fred = {name: “Fred”, gender: “Male”}; undefined > fred.name “Fred” > console.log("reading age=" + fred.age); reading age=undefined undefined > fred.age = 99; 99 > fred { name: 'Fred', gender: 'Male', age: 99 } > delete fred.age true > fred { name: 'Fred', gender: 'Male' } Unordered key-value pairs Keys = properties Add property Delete property @crichardson
- 16. JavaScript is a prototypal language Person __proto__ sayHello ... Prototype function ... inherited Chris __proto__ name nickname “Chris” “CER” overrides object specific @crichardson
- 17. Prototypal code Not defined here $ node > var 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 Chris create using undefined properties > sarah.sayHello(); prototype Hello Sarah undefined > chris.sayHello = function () { console.log("Hello mate: " + this.name); }; [Function] > chris.sayHello(); Hello mate: Chris undefined @crichardson
- 18. JavaScript is Functional function makeGenerator(nextFunction) { Return a function closure var value = 0; return function() { var current = value; value = nextFunction(value); return current; }; Pass function as an argument } var inc = makeGenerator(function (x) {return x + 1; }); > inc() 0 > inc() 1 @crichardson
- 19. But JavaScript was created in a hurry The ‘Java...’ name creates expectations that it can’t satisfy Fake classes: Hides prototypes BUT still seems weird global namespace scope of vars is confusing Missing return statement = confusion ‘function’ is really verbose ‘this’ is dynamically scoped Unexpected implicit conversions: 99 == “99”! truthy and falsy values 52-bit ints @crichardson
- 20. Dynamic + weakly-typed (+ event-driven) code + misspelt property names lots of time spent in the abyss Essential: Use IDE integrated with JSLint/JSHint + tests
- 21. Prototypal languages have benefits BUT Developers really like classes JavaScript prototypes lack the powerful features from the Self language e.g. Multiple (and dynamic) inheritance http://www.cs.ucsb.edu/~urs/oocsb/self/papers/papers.html @crichardson
- 22. Verbose function syntax > var numbers = [1,2,3,4,5] > numbers.filter(function (n) { return n % 2 == 0; } ).map(function (n) { return n * n; }) [ 4, 16 ] > Versus Prelude> let numbers = [1,2,3,4,5] Prelude> map (n -> n * n) (filter (n -> mod n 2 == 0) numbers) [4,16] Or scala> val numbers = 1..5 scala> numbers filter { _ % 2 == 0} map { n => n * n } Vector(4, 16) @crichardson
- 23. Verbose DSLs describe('SomeEntity', function () { beforeEach(function () { ... some initialization ... }); Jasmine it('should do something', function () { ... expect(someExpression).toBe(someValue); }); }); Versus class SomeScalaTest ...{ Scalatest before { ... some initialization ... } it should "do something" in { ... someExpression should be(someValue) } @crichardson
- 24. JavaScript is the language of the web “You have to use the programming language you have, not the one that you might want” @crichardson
- 25. It works but the result is lost opportunities and impeded progress @crichardson
- 26. But if you think that this isn’t a problem then perhaps .... “Stockholm syndrome ... is a psychological phenomenon in which hostages ... have positive feelings toward their captors, sometimes to the point of defending them...” http://en.wikipedia.org/wiki/Stockholm_syndrome @crichardson
- 27. Martin Fowler once said: "...I'm one of those who despairs that a language with such deep flaws plays such an important role in computation. Still the consequence of this is that we must take javascript seriously as a first-class language and concentrate on how to limit the damage its flaws cause. ...." http://martinfowler.com/bliki/gotoAarhus2012.html @crichardson
- 28. Use just the good parts Douglas Crockford http://www.crockford.com/ @crichardson
- 29. Use a language that compiles to JavaScript TypeScript Classes and interfaces (dynamic structural typing) Typed parameters and fields Dart Class-based OO Optional static typing Bidirectional binding with DOM elements Less backwards compatibility with JavaScript Also has it’s own VM @crichardson
- 30. CoffeeScript Hello World Classes :-) http = require('http') Concise class HttpRequestHandler constructor: (@message) -> Bound method 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/') @crichardson
- 33. About the Reactor pattern Defined by Doug Schmidt in 1995 Pattern for writing scalable servers Alternative to thread-per-connection model Single threaded event loop dispatches events on handles (e.g. sockets, file descriptors) to event handlers @crichardson
- 34. Reactor pattern structure Application register_handler(h1) register_handler(h2) handle_events() Initiation Dispatcher handle_events() handlers register_handler(h) uses select(handlers) for each h in handlers h.handle_event(type) end loop Event Handler handle_event(type) get_handle() owns Synchronous Event Demultiplexer select() notifies handle @crichardson
- 35. Benefits Separation of concerns - event handlers separated from low-level mechanism More efficient - no thread context switching Simplified concurrency - single threaded = no possibility of concurrent access to shared state @crichardson
- 36. Drawbacks Non-pre-emptive - handlers must not take a long time Difficult to understand and debug: Inverted flow of control Can’t single step through code easily Limited stack traces No stack-based context, e.g. thread locals, exception handlers How to enforce try {} finally {} behavior? @crichardson
- 37. NodeJS app = layers of event handlers Recurring events from Event Emitters Application code Event listener HTTP Callback function DB driver ... One time events: async operation completion Basic networking/file-system/etc. NodeJS event loop @crichardson
- 38. Async code = callback hell Scenarios: Sequential: A B C Scatter/Gather: A and B C Code quickly becomes very messy @crichardson
- 39. Messy callback code The result of getProductDetails getProductDetails = (productId, callback) -> productId = req.params.productId result = {productId: productId} Propagate error makeCallbackFor = (key) -> (error, x) -> if error callback(error) else result[key] = x if (result.productInfo and result.recommendations and result.reviews) callback(undefined, result) Update result Gather getProductInfo(productId, makeCallbackFor('productInfo')) getRecommendations(productId, makeCallbackFor('recommendations')) getReviews(makeCallbackFor('reviews')) Scatter @crichardson
- 40. Simplifying code with Promises (a.k.a. Futures) Functions return a promise - no callback parameter A promise represents an eventual outcome Use a library of functions for transforming and composing promises Promises/A+ specification - http://promises-aplus.github.io/promises-spec when.js (part of cujo.js by SpringSource) is a popular implementation Crockford’s RQ library is another option @crichardson
- 41. Simpler promise-based code class ProductDetailsService getProductDetails: (productId) -> makeProductDetails = (productInfo, recommendations, reviews) -> productId: productId productDetails: productInfo.entity recommendations: recommendations.entity reviews: reviews.entity responses = [getProductInfo(productId), getRecommendations(productId), getReviews(productId)] getReviews(productId)] all(responses).spread(makeProductDetails) all(responses) spread(makeProductDetails) @crichardson
- 42. Not bad but lacks Scala’s syntactic sugar class ProductDetailsService .... { def getProductDetails(productId: Long) = { for (((productInfo, recommendations), reviews) <getProductInfo(productId) zip getProductInfo(productId) zip getRecommendations(productId) zip getRecommendations(productId) zip getReviews(productId) getReviews(productId)) yield ProductDetails(productInfo, recommendations, reviews) } } @crichardson
- 43. Long running computations Long running computation blocks event loop for other requests Need to run outside of main event loop Options: Community: web workers threads Built-in: NodeJS child processes @crichardson
- 44. Using child processes parent.js var child = require('child_process').fork('child.js'); function sayHelloToChild() { child.send({hello: "child"}); } Create child process Send message to child setTimeout(sayHelloToChild, 1000); child.on('message', function(m) { console.log('parent received:', m); }); function kill() { child.kill(); } setTimeout(kill, 2000); child.js process.on('message', function (m) { console.log("child received message=", m); process.send({ihateyou: "you ruined my life"}) }); @crichardson
- 45. Modern multi-core machines vs. single-threaded runtime Many components of many applications Don’t need the scalability of the Reactor pattern Request-level thread-based parallelism works fine There are other concurrency options Actors, Software transactional memory, ... Go goroutines, Erlang processes, ... Imposing a single-threaded complexity tax on the entire application is questionable @crichardson
- 47. Core built-in modules Basic networking HTTP(S) Filesystem Events Timers ...
- 48. Thousands of community developed modules https://npmjs.org/ web frameworks, SQL/NoSQL database drivers, messaging, utilities... @crichardson
- 49. What’s a module? foo.js One or more JavaScript files exports.sayHello = function () { console.log(“Hello”); } Optional native code: Compiled during installation JavaScript != systems programming language Package.json - metadata including dependencies @crichardson
- 50. Easy to install $ npm install package-name --save @crichardson
- 51. Easy to use Core module OR Path to file OR module in node_modules Module’s exports var http = require(“http”) var server = http.createServer... @crichardson
- 52. Developing with NodeJS modules Application code Your modules Community modules Core modules @crichardson
- 53. There is a module for that... Modules + glue code = rapid/easy application development AWESOME!... @crichardson
- 54. ... BUT Variable quality Multiple incomplete/competing modules, e.g. MySQL drivers without connection pooling! Often abandoned No notion of a Maven-style local repository/cache = repeated downloads ... @crichardson
- 55. To summarize Flawed and misunderstood JavaScript Rich but variable quality NodeJS Modules Scalable yet costly and often unnecessary Reactor pattern @crichardson
- 56. How will future history view NodeJS? C++ ? EJB @crichardson
- 57. Agenda Overview of NodeJS JavaScript: Warts and all The Reactor pattern: an event-driven architecture NodeJS: There is a module for that Building a front-end server with NodeJS @crichardson
- 58. So why care about NodeJS? Easy to write scalable network services Easy to push events to the browser Easy to get (small) stuff done It has a role to play in modern application architecture @crichardson
- 59. Evolving from a monolithic architecture.... WAR StoreFrontUI Product Info Service Recommendation Service Review Service Order Service @crichardson
- 60. ... to a micro-service architecture product info application Product Info Service recommendations application Store front application StoreFrontUI Recommendation Service reviews application Review Service orders application Order Service @crichardson
- 61. Presentation layer evolution.... WAR StoreFrontUI HTML / HTTP View Controller Browser + JavaScript Model @crichardson
- 62. ...Presentation layer evolution Browser View Web application Static content Controller JSON-REST Model HTML 5/JavaScript IOS/Android clients Events RESTful Endpoints Event publisher @crichardson
- 63. Directly connecting the front-end to the backend Chatty API View REST Product Info service REST Recommendation Service AMQP Controller Review service Model Traditional web application View Controller Model Browser/Native App Web unfriendly protocols @crichardson
- 64. NodeJS as an API gateway Single entry point Browser View Controller NodeJS Model HTML 5 - JavaScript Product Info service REST Recommendation Service AMQP Review service REST proxy Native App View API Gateway REST Controller Model Event publishing Optimized Client specific APIs Protocol translation @crichardson
- 65. Serving static content with the Express web framework var express = require('express') , http = require('http') , app = express() , server = http.createServer(app) ; From public sub directory app.configure(function(){ ... app.use(express.static(__dirname + '/public')); }); server.listen(8081); @crichardson
- 67. Proxying to backend server express = require('express') request = require('request') Returns a request handler that proxies to baseUrl 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('/productinfo/*', proxyToBackend('http://productinfo....')) app.get('/recommendations/*', proxyToBackend(''http://recommendations...')) app.get('/reviews/*', proxyToBackend('http://reviews...')) @crichardson
- 68. Implementing coarsegrained mobile API var express = require('express'), ...; app.get('/productdetails/:productId', function (req, res) { getProductDetails(req.params. productId).then( function (productDetails) { res.json(productDetails); } }); @crichardson
- 69. Delivering events to the browser @crichardson
- 70. Socket.io server-side var express = require('express') , http = require('http') , amqp = require(‘amqp’) ....; server.listen(8081); ... var amqpCon = amqp.createConnection(...); Handle socket.io connection io.sockets.on('connection', function (socket) { function amqpMessageHandler(message, headers, deliveryInfo) { Republish var m = JSON.parse(message.data.toString()); socket.emit(‘tick’, m); as socket.io }; event amqpCon.queue(“”, {}, function(queue) { queue.bind(“myExchange”, “”); queue.subscribe(amqpMessageHandler); Subscribe to }); }); AMQP queue https://github.com/cer/nodejs-clock @crichardson
- 71. Socket.io - client side <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> Bind to model Connect to socket.io clock.js var socket = io.connect(location.hostname); function ClockModel() { self.ticker = ko.observable(1); socket.on('tick', function (data) { self.ticker(data); }); }; ko.applyBindings(new ClockModel()); Subscribe to tick event Update model @crichardson
- 72. NodeJS is also great for writing backend micro-services “Network elements” Simply ‘route, filter and transform packets’ Have minimal business logic @crichardson
- 73. NodeJS-powered home security FTP Server Log file FTP Server Upload directory Upload2S3 S3 SQS Queue UploadQueue Processor DynamoDB @crichardson
- 74. Summary JavaScript is a very flawed language The asynchronous model is often unnecessary; very constraining; and adds complexity BUT despite those problems Today, NodeJS is remarkably useful for building networkfocussed components @crichardson