Scala in practice
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
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
// ...
}
}