Scala in practice

@donatasm

About me

Originally C# developer @adform
Deploying Scala to production for the last 2 years

How did that happen?

Or why would a C# developer search for something beyond .NET?

Online advertising

Scaling is hard

Running everything on Windows and .NET

Everything is possible, if you try hard
Usually, what you need is already done by someone, but:
- works only on Linux, or
- "use at your own risk on Windows", or
- is not implemented in .NET

..., for example

Deployment*
- Puppet, Docker
Fast, scalable and awesome data stores*
- Cassandra, Aerospike, Vertica, Redis, Kafka
High performance libraries and frameworks*
- clients for the data stores, networking, data processing

* may not be true anymore

Why JVM?

Runs everywhere
Huge ecosystem of open-source libraries and frameworks

Why not _?

Java
Clojure
Groovy, JRuby, Jython

Quick facts about Scala

A scalable language, released in 2004
Object-oriented and functional
Statically typed with very strong type system
Current version 2.11.8

Hello, world!

C#

public static class Hello
{
  public static void Main(string[] args)
  {
    Console.WriteLine("Hello, world!");
  }
}

Scala

object Hello {
  def main(args: Array[String]) {
    println("Hello, world!")
  }
}

Syntax

C#

interface I {}
class B<T> {}
class A : B<String>, I
{
  int MyMethod() { return 42; }
}

Scala

trait T
class B[T]
class A extends B[String] with I {
  def myMethod = 42
}

Variables

Scala supports two types of variable declarations:

var declares a variable like in C#

var name = "world"
name = name + "!"

val declares a read-only value

val name = "world"
name = name + 1 // ERROR

Lazy vals

Scala

lazy val resource = resourceExpensiveToCreate()

C#

Lazy<Resource> resource =
  new Lazy<Resource>(
    () => ResourceExpensiveToCreate(),
    isThreadSafe = true);

Methods

class Hello {
  def sayTo(name: String): String = {
    return s"Hello, $name"
  }
}

class Hello {
  def sayTo(name: String) = s"Hello, $name"
}

Methods

val hello = new Hello
hello.sayTo("world")

val hello = new Hello
hello sayTo "world"

Defining data transfer objects in C#

public sealed class Product
{
  private readonly string _name;
  private readonly string _category;
  private readonly int _price;

  public Product(string name, string category, int price)
  {
    _name = name;
    _category = category;
    _price = price;
  }

  public string Name { get { return _name; }}
  public string Category { get { return _category; }}
  public int Price { get { return _price; }}

  public override bool Equals(object other)
  {
    if (ReferenceEquals(null, other)) return false;
    if (ReferenceEquals(this, other)) return true;
    var o = (Product)other;
    return o._name == _name
      && o._category == _category
      && o._price == _price;
  }

  public override int GetHashCode()
  {
    // magic using number 37, usually generated by tool
  }
}

Defining data transfer objects (DTO) in Scala

case class Product(name: String, category: String, price: Int)

val p1 = Product("product1", "category1", 99)
val p2 = Product("product1", "category1", 99)
val p3 = Product("product2", "category2", 100)
println(p1)
println(p1 == p2)
println(p2 == p3)
println(p1.price)

Product(product1, category1, 99)
true
false
99

Transforming DTO in C#

public static class ProductJson
{
  public static JsonObject ToJson(this Product product)
  {
    return new JsonObject(
      new JsonProperty("name", product.Name),
      new JsonProperty("category", product.Category),
      new JsonProperty("price", product.Price)
    );
  }
}

And then:

var json = new Product("name", "category", 101).ToJson();

Scala implicit conversions

object ProductJson
{
  implicit def toJson(product: Product): JsonObject =
    JsonObject(
      JsonProperty("name", product.Name),
      JsonProperty("category", product.Category),
      JsonProperty("price", product.Price)
    )
}

And then:

val json: JsonObject = Product("name", "category", 101);

Scala implicit parameters

case class User(id: Int)

class UserRepository {
  def getUserById(id: Int): User = {
    // call to database
  }
}

class UserService {
  def getUserById(id: Int, repository: UserRepository): User = {
    repository.getUserById(id)
  }
}

And then:

val repository = new UserRepository()
val service = new UserService()

service.getUserById(1, repository)

Scala implicit parameters

class UserService {
  def getUserById(id: Int)(implicit repository: UserRepository): User = {
    repository.getUserById(id)
  }
}

And then:

implicit val repository = new UserRepository()
val service = new UserService()

service.getUserById(1)

Functions, Actions

C#

Action a1 = () => {};
Action<int> a2 = (int i) => {};
Func<int> f1 = () => 0;
Func<int, int> f2 = (int i) => 0;

Scala

val a1 = () => {}
val a2 = (i: Int) => {}
val f1 = () => 0
val f2 = (i: Int) => 0

LINQ

C#

var n = Enumerable.Range(1, 10);
n.Select(i => i*i);
n.Where(i => i%2 == 0);
n.Take(3);
n.Count();
n.Distinct();
n.Sum();

Scala

val n = 1 to 10
n.map(i => i*i)
n.filter(i => i%2 == 0)
n.take(3)
n.size
n.distinct
n.sum

That's (almost) it

Convert Java or C# code line by line to Scala, and...
you'll have beautiful code. Win!

Tests, for example

test("four bids with the same price, winner is randomized") {
  val adSlot = AdSlot(id = "1", minPrice = Money(1))
  val adSlots = Set(adSlot)
  val bidResponses = List(BidResponse("bidder", bids()))

  val results = (1 to 1000)
    .map(_ => auctioneer.winningBids(adSlots, bidResponses).head)
    .groupBy(_.bid)
    .map { case (_, bids) =>  bids.toSeq.length / 1000.0 }

  all(results) should equal(0.25 +- 0.01)
}

Tests, for example

it should "return 400 when request headers " +
  "too large" in withNginxBidder { bidder =>

  val headers = (1 to 8192)
    .map(i => RawHeader(s"X-Header-$i", i.toString))

  val response = bidder
    .client
    .send(Get(s"http://localhost:$nginxPort/")
    .withHeaders(headers: _*))

  response should equal (HttpResponse(400, HttpEntity.Empty,
    List(`Content-Length`(0), Connection("close"))
  ))
}

Or DSL for rendiring html

def statsPresentation(stats: Stats) = HttpResponse(
  entity = HttpEntity(`text/html`,
  <html>
    <body>
    <h1>HttpServer Stats</h1>
    <table>
      <tr><td>days:</td><td>{stats.uptime.toDays}</td></tr>
      <tr><td>hours:</td><td>{stats.uptime.toHours}</td></tr>
      <tr><td>minutes:</td><td>{stats.uptime.toMinutes}</td></tr>
      <tr><td>totalRequests:</td><td>{stats.totalRequests}</td></tr>
    </table>
    </body>
  </html>.toString))

Or abuse it

When you see this...

					
val result = (0 /: |)(_ + _)

					
val | = List(1, 2, 3, 4, 5, 6, 7, 8, 9)

					
val list = List(1, 2, 3, 4, 5, 6, 7, 8, 9)
val result = list.foldLeft(0)(x, y => x + y)
// result = 45

Patterns

Cake Pattern

The famous pattern of doing DI in Scala

Classical example

case class Size(width: Int, height: Int)
case class Ad(id: Int, size: Size)

trait AdRepository {
  def get(id: Int): Ad
  def find(size: Size): Ad
}

Wrap it into component

trait AdRepositoryComponent {
  def adRepository: AdRepository

  trait AdRepository {
    def get(id: Int): Ad
    def find(size: Size): Ad
  }
}

Dependency defined as self-type annotation

// self-type annotation
trait Bidder { this: AdRepositoryComponent =>

  def getAd(id: Int): Ad =
    adRepository.get(id)

  def matchAd(size: Size): Ad =
    adRepository.find(size)
}

Concrete implementation

trait MySqlAdRepositoryComponent extends AdRepositoryComponent {
  def adRepository = new MySqlAdRepository

  class MySqlAdRepository extends AdRepository {
    override def get(id: Int): Ad = {
      // TODO: get from database
    }

    override def find(size: Size): Ad = {
      // TODO: get from database
    }
  }
}

Finally, wiring it up

class DefaultBidder extends Bidder with MySqlAdRepositoryComponent

object BidderApplication {
  def main(args: Array[String]): Unit = {
    val bidder = new DefaultBidder
    // ...
  }
}

Testing

class BidderTest extends Bidder with AdRepositoryComponent {
  lazy val adRepository: AdRepository = mock[AdRepository]

  // TODO: write some tests
}

Classical constructor injection, pimped with implicits

class DefaultBidder(implicit val repository: AdRepository)

object BidderApplication {
  def main(args: Array[String]): Unit = {
    implicit val repository = new MySqlAdRepository
    val bidder = new DefaultBidder
    // ...
  }
}

Scala in practice

donatasm

Scala in practice

0 0 (function() { var po = document.createElement('script'); po.type = 'text/javascript'; po.async = true; po.src = 'https://apis.google.com/js/platform.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(po, s); })();

scala-in-practice

Scala in practice

About me

How did that happen?

Or why would a C# developer search for something beyond .NET?

Online advertising

Scaling is hard

Running everything on Windows and .NET

..., for example

Why JVM?

Why not _?

Quick facts about Scala

Hello, world!

C#

Scala

Syntax

C#

Scala

Variables

Lazy vals

Scala

C#

Methods

Methods

Defining data transfer objects in C#

Defining data transfer objects (DTO) in Scala

Transforming DTO in C#

Scala implicit conversions

Scala implicit parameters

Scala implicit parameters

Functions, Actions

C#

Scala

LINQ

C#

Scala

That's (almost) it

Tests, for example

Tests, for example

Or DSL for rendiring html

Or abuse it

When you see this...

Patterns

Cake Pattern

The famous pattern of doing DI in Scala

Classical example

Wrap it into component

Dependency defined as self-type annotation

Concrete implementation

Finally, wiring it up

Testing

Classical constructor injection, pimped with implicits

Yes, but this is Java'ish code!

Scala is also great for OOP

Thank you!

QA

0 0