Multi-paradigm programming in Scala and Akka

Why not both?

You really don’t have to choose just one.

Agenda for tonight

⊸ Demonstrate a blend of OOP and FP in a hybrid language - Scala

⊸ Use Scala with Akka to easily build scalable Actor Systems

⊸ Disclaimer: ▫ This is not a talk about Scala as a language, its features or its

comparison with Java.

Don’t want to do this =>

▫ As polyglot programmers, we acknowledge that some languages are better than others for the specific kind of problems that they are trying to solve.

▫ Scala and Java are just being used as tools to understand and help in embracing functional programming paradigm.

➢ Why should you consider another programming paradigm?

➢ What is the Best code?

➢ What is the Best code?

○ The following =>

➢ What is the Best code?○ The best code is the one that doesn't exists!

Why?○ Code rots.○ needs maintenance.○ needs to be tested.○ needs to be read and understood again and again

over time.

Conciseness matters!

➢ Get all products from the orders

➢ Get all products from the orders

Or

➢ Sort the products by Id

Or

➢ Why Scala?

○ Because Scala smoothens the transition from OOP to FP.

○ Supplements Java and adds more features and reduces the verbosity.

○ It is not new - first introduced in 2003.

➢ Why learn Functional Programming ?

➢ Why learn Functional Programming ?

○ Write high-performing code that's easier to understand and reason about.

○ Write programs that can utilize multiple cores more efficiently.

○ It would evolve your way of thinking about programs and eventually make you a better programmer.

➢ How does FP improve code?

➢ How does FP improve code?

○ Scalable and efficient code that can utilize multiple cores better - Map/Reduce engines.

○ Better to reason about code. No need to create mental model or remember gradual state changes.

○ By emphasizing on the evaluation of expressions instead of evaluation of statements

➢ What is functional programming ?

○ Immutability.○ Referential transparency.○ Higher order functions.○ Tail call recursion, mapping, reducing, currying,

lazy evaluation,...

➢ What is functional programming ?

○ Immutability.○ Referential transparency.○ Higher order functions.○ Tail call recursion, mapping, reducing, currying,

lazy evaluation,...

➢ Writing code without side effects.

It is a restriction on how we write programs and not on what programs we can express.

➢ What makes Scala functional ?

➢ Functions are 1st class citizens =>

○ Instead =>

➢ Functions are 1st class citizens =>

○ Instead =>

➢ Immutability (preferred)

○ Preventing reference reassignment through val.○ Avoiding mutable state through immutable

collections.■ Eg : 2 threads referring to same tree.

➢ After one thread adds a node.

○ Immutability has benefits in concurrency as synchronization is not needed.

➢ Having referential transparency.○ Should hold substitution model.

➢ No side effects

➢ Having referential transparency.○ Should hold substitution model.

➢ No side effects

StringBuilder.append has side effects!

String is referentially transparent and holds substitution model.

➢ Similarly,

add() here is also without side-effects and referentially transparent.

➢ Function with Side Effect○ A function has side effects if it does something

other than simply return a result like ■ Modifying a variable/data structure.■ Throwing exception or halting abruptly.■ Reading/writing to file/database■ …

➢ In a pure function○ Input is obtained only by parameters.○ Output is calculated and returned.

➢ Just enough Scala for Akka.

➢ Case classes○ Regular classes which export their constructor

parameters and which provide a recursive decomposition mechanism via pattern matching.

○ The params can be accessed with .(dot) notation.

➢ Pattern matching○ Switch case on steroids

➢ Pattern matching○ A pattern match includes a sequence of

alternatives, each starting with the keyword ‘case’.

○ Each alternative case includes a pattern and one or more expressions, which will be evaluated if the pattern matches.

○ An arrow symbol => separates the pattern from the expressions.

Example =>

➢ Pattern matching

http://www.youtube.com/watch?v=e-DXJT8VnAA

Questions?

The Actor Model

⊸ Avoiding shared mutable state

▫ Functional Programming - all state is immutable

▫ Actor Model - state is mutable but encapsulated in actors

What’s an actor?

⊸ An actor has state⊸ An actor has behaviour⊸ ….

Sounds similar to something else, doesn’t it?

Actors are what “objects” were supposed to be

⊸ Actors are what objects are supposed to be (Smalltalk objects)

⊸

What’s an actor?

⊸ An actor has state⊸ An actor has behaviour

Similar to OOP objects but with one difference

⊸ Actors communicate by sending immutable messages to each other

Akka - actors on the JVM

⊸ Akka actors ▫ are light-weight JVM objects▫ are guaranteed to run on a single thread (no

locks/synchronization required)▫ process messages sent to them in order and one at

a time▫ do not share their state▫ can be addressed irrespective of location▫ asynchronous and non-blocking

What is Akka?

Akka is a toolkit and runtime for building highly concurrent, distributed, and resilient message-driven applications on the JVM.

Anatomy of an actor

An akka actor must have⊸ State⊸ Behavior

▫ Message processing

State

Processing messages

Let’s look at an actor

DEMO

Scaling out in clusters

⊸ Location transparency with ActorRef

⊸ Akka’s cluster and cluster-sharding modules

⊸ Resilience with Supervision strategies

Where to go from here

⊸ Take Martin Odersky’s course on Coursera

⊸ Try out some Lightbend Activator projects from Github

⊸ Reach out the Akka team on Gitter and Google Groups

Start thinking in Hybrids

Questions?