Reactive Web Applications with Scala & Liftweb - CodeWeek 2015

Reactive Web Applications using Scala & Liftweb Andrea Zanda CTO & Co-founder andrea.zanda@guidemeright.com Riccardo Sirigu Full Stack Dev riccardo.sirigu@guidemeright.com

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Reactive Manifesto ● The techniques and technologies traditionally used in a class of Reactive and Interactive Systems is gaining quite a bit of attention from the mainstream developers community ● Recently, the Reactive Manifesto was defined to summarise the key traits of “Reactive Applications”

Reactive Manifesto ● Responsive: Timely react to stimuli ● Event-Driven: modular, asynchronous ● Resilent: Tolerate failures functionally and ideally temporally ● Scalable: Gracefully scale up and down depending on load demands

https://github.com/zaza81/lift-codeweek @MarioBot /codeweek banana luca gmr-codeweek.cloudapp.net

Scala ● Functional (style) programming ● Everything is an Object ● Run on JVM ● Practical expressivity ○ Expressive Power ○ List(1,2,3,4).map(x => x + 1) ○ What vs How ● N(Bugs) ∝ N(LOC) ○ Fewer LOC fewer bugs ○ Halstead complexity measures

Scala vs Java ● Every operator is a method call ○ 12 + 4 (12).+(4) ○ “#name” #> “david” ○ “codeWeek” indexOf “w” ● Object Equality ○ List(56,13,75) == List(56,13,75) ● Optional Types ○ NullPointerException ○ val secret: Option[String] = Some(“zbmdfbmknnaprzvds=”) ○ val secret: Option[String] = None

Scala vs Java ● Immutable data structures scala> val even = List(2,4) scala> 6 :: 8 :: even res0: List[Int] = List(6,8,2,4) ● Type inference scala> Map('a' -> 1, 'b' -> 2) res2: scala.collection.immutable.Map[Char,Int] = Map(a -> 1, b -> 2)

Scala vs Java class User(val email: String) class User{ private String email; public User(String email){ this.email = email } public String getEmail(){ return email; } }

Scala (1st class) Functions var botMe = (name: String) => “@” + name + ”Bot” botMe: String => String = <function1> val names = “Riccardo” :: “Andrea” :: “Mario” :: Nil names: List[String] = List(Riccardo, Andrea, Mario) names.map(botMe) res0: List[String] = List(@RiccardoBot, @AndreaBot, @MarioBot)

Objects Everywhere Functions are Objects (x: Int) => x * x * x ..is expanded to.. new Function1[Int, Int] { def apply(x: Int) = x * x * x }

Objects Everywhere val f = (x: Int) => x * x * x f(2) val f = new Function1[Int, Int] { def apply(x: Int) = x * x * x } f.apply(2)

Companion Objects • Same name & same source file as the class class User extends MegaProtoUser[User] { def getSingleton = User // reference to the companion object below def allAccounts : List[Account] = Account.findAll(By(Account.owner, this.id)) } // Create a "companion object" to the User class (above). // The companion object is a "singleton" object that shares the same // name as its companion class. object User extends User with MetaMegaProtoUser[User] { override def dbTableName = "users" }

Pattern Matching • Switch case on steroids User.findById(“000001”) match { case Some(user) => println(user.name) case Empty => println(“No User Found!”) }

Scala Actors ● Manual threading and locking is difficult Actor abstraction ● Concurrent processes that communicate by exchanging messages ● Actor encapsulates both state and behavior ● Fire&Forget chatserver ! msg Actor Model

Introducing Lift Why? ● Based on Scala ● Comet and Ajax, js abstractions ● Designer friendly templates View First ● Security ○ injections, XSS, CSRF... ○ OWASP Top 10 http://seventhings.liftweb.net/ http://lift.la/blog/lift-state-and-scaling

MVVM in concrete View Model HTML page Scala Class (ORM) Snippet (MV)

CSS Selectors “#id_element” #> <p> Element Content</p>

SHtml “#id_text” #> SHtml.text(“text”, textVar = _) & “#id_submit” #> SHtml.submit(“Submit”, processResult()) def processResult(): JsCmd = { //do something S.notice(“Work Done”) }

Comet • Server Push • Real time event-based systems • Thread per request, long polling

Comet One thread per request leads to…….. SCALABILITY ISSUES Web container request suspension or continuation

follow up: andrea.zanda@guidemeright.com riccardo.sirigu@guidemeright.com

Reactive Web Applications with Scala & Liftweb - CodeWeek 2015

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