3 years ago, Springer decided to use Scala on a large, strategic project. This talk is about the journey the development teams made. Why did they choose Scala in the first place? Did they get what they hoped for? What challenges and surprises did they encounter along the way? And, most importantly, are they still happy with their choice?

1. in PracticeScala
@patforna
patric.fornasier@springer.com
3 years later…

2. Context

3. Context
Pat
Springer
Software
Engineer
Technical
Principal
10+ years
Academic
Publisher
Pretty Large
Springer
Link
Strategic
95 % Online
Revenue
66 % Total
Revenue
9 Mio items
2TB XML
Re-built
3 years ago
Content
Delivery
Platform
52 Mio
PageViews / Month
99.9%
Availability
Scala NoSql
Co-Sourced
Team
TW
Ex-TW
Springer
Global
CD

4. SpringerLink

5. Timeline
Start
development
Cycling
Trip
Re-joined
Product -> platform
Paying back tech debt
Today
04/11 04/12 08/13 04/14
Inception
05/12 09/12
Bookfair
Release
RD
Live
User migration,
enhancements
Link
Live
Smart
Books
Live
Enhancements

6. Looking back: why scala?
• Increase productivity
• Be more attractive employer
• Team decision

7. Looking back: good vs bad
Good
• Functional programming
• Terse syntax
• JVM ecosystem
• Gentle learning curve
• DSL friendly syntax
• Motivated team
Bad
• Tool support
• Compilation times
• Language complexity #moreRope

8. Fast-forward

9. Fast-forward (Aug 2013)
• 2.5 years into project
• 1.5 years of weekly live releases
• 100k LOC
• >10k commits
• >90 committers

10. Fast-forward (Aug 2013)
• 2.5 years into project
• 1.5 years of weekly live releases
• 100k LOC
• >10k commits
• >90 committers
not all related to Scala - to be fair
• Poor feedback loops
• Lots of accidental complexity

11. Trend (2 years)
LOC
~ 100k

12. Trend (2 years)
build time
LOC

13. Trend (2 years)
build time
LOC
• 1:34 min src/main
• 6:44 min src/test
• 8:18 min total

14. What did we do?

15. What did we do
• Reduced build time
• Improved feedback loops
• Reduced accidental complexity

16. Build time
• Reduced size of codebase (broke off vertical slices, pulled out APIs, pulled out libraries,
removed unused features, removed low-value tests, etc.)
• Reduced usage of certain language features (esp. traits and implicits)

17. Trend (Dec 2013)
LOC

18. Trend (Dec 2013)
build time
# traits
LOC
unable to compile on
13” macbook

19. Trend (Dec 2013)
build time
# traits
LOC
unable to compile on
13” macbook

20. The problem with traits
• Will re-compile on every class the trait is mixed in
• Slows down dev-build cycle
• Will result in byte code bloat
• Will compile *a lot* slower
!
For faster compile times:
• Use pure traits
• Use old-school composition for code re-use
• Use pure functions via imports (e.g. import Foo._)
• If unavoidable, use inheritance for code re-use

21. Build time

22. Build time
• 1:34 min src/main
• 6:44 min src/test
• 8:18 min total

23. Build time
• 1:34 min src/main
• 6:44 min src/test
• 8:18 min total
• 0:24 min src/main
• 3:11 min src/test
• 3:35 min total

24. Build time (on CI server)
• Incremental compilation on CI
• Only one dedicated CI agent
• Physical build servers
• CPUs with higher clock speed

25. Build time (on CI server)
• Incremental compilation on CI
• Only one dedicated CI agent
• Physical build servers
• CPUs with higher clock speed

26. Complexity

27. Complexity
• There’s still a lot of code in our codebase that is hard to read
• It seems to be very easy to shoot yourself in the foot with Scala
• Scala *is* complex (and that’s why scalac will never be as fast as javac)

28. Complexity
• There’s still a lot of code in our codebase that is hard to read
• It seems to be very easy to shoot yourself in the foot with Scala
• Scala *is* complex (and that’s why scalac will never be as fast as javac)
Invariant/covariant/contravariant types (T, +T and -T)
Refined types (new Foo {...})
Structural types (x: {def y: Int})
Path dependant types (a.B)
Specialized types (@specialized)
Self types (this =>)
Projection types (A#B)
Existential types (M[_])
Type bounds (<:, >:)
Type constraints (=:=, <:< and <%<)
Type members (type T)
Type aliases (type T = Int)
Type classes ( (implicit ...) )
View bounds (<%)
Higher kinded types (* => *)
F-Bounded type polymorphism (M[T <: M[T]])
http://nurkiewicz.github.io/talks/2014/scalar/#/16

29. Not opinionated
For example: http://twitter.github.io/effectivescala/
• Many ways to do the same thing
• Coding conventions help, but only so much

30. Not opinionated
def foo() = "foo"
def bar = "bar"
!
foo
foo()
bar
bar() // won't compile
For example: http://twitter.github.io/effectivescala/
• Many ways to do the same thing
• Coding conventions help, but only so much

31. Not opinionated
def foo() = "foo"
def bar = "bar"
!
foo
foo()
bar
bar() // won't compile
For example: http://twitter.github.io/effectivescala/
• Many ways to do the same thing
• Coding conventions help, but only so much
def baz(x: String) = x
“x”.charAt(0)
“x” charAt(0) // won't compile
“x”.charAt 0 // won't compile
“x” charAt 0
baz("x")
baz “x" // won't compile

32. Not opinionated
def foo() = "foo"
def bar = "bar"
!
foo
foo()
bar
bar() // won't compile
list.foreach { x => println(x) }
list.foreach ( x => println(x) )
list.foreach { println(_) }
list.foreach ( println(_) )
list foreach { x => println(x) }
list foreach ( x => println(x) )
list foreach { println(_) }
list foreach ( println(_) )
For example: http://twitter.github.io/effectivescala/
• Many ways to do the same thing
• Coding conventions help, but only so much
def baz(x: String) = x
“x”.charAt(0)
“x” charAt(0) // won't compile
“x”.charAt 0 // won't compile
“x” charAt 0
baz("x")
baz “x" // won't compile

33. Not opinionated
def foo() = "foo"
def bar = "bar"
!
foo
foo()
bar
bar() // won't compile
list.foreach { x => println(x) }
list.foreach ( x => println(x) )
list.foreach { println(_) }
list.foreach ( println(_) )
list foreach { x => println(x) }
list foreach ( x => println(x) )
list foreach { println(_) }
list foreach ( println(_) )
if (foo) "x" else "y"
foo match {
case true => "x"
case _ => "y"
}
For example: http://twitter.github.io/effectivescala/
• Many ways to do the same thing
• Coding conventions help, but only so much
def baz(x: String) = x
“x”.charAt(0)
“x” charAt(0) // won't compile
“x”.charAt 0 // won't compile
“x” charAt 0
baz("x")
baz “x" // won't compile

34. Surprises
http://dan.bodar.com/2013/12/04/wat-scala/

35. Surprises
List(1, 2, 3).toSet
http://dan.bodar.com/2013/12/04/wat-scala/

36. Surprises
List(1, 2, 3).toSet
scala.collection.immutable.Set[Int] = Set(1, 2, 3)
http://dan.bodar.com/2013/12/04/wat-scala/

37. Surprises
List(1, 2, 3).toSet
scala.collection.immutable.Set[Int] = Set(1, 2, 3)
List(1, 2, 3).toSet()
http://dan.bodar.com/2013/12/04/wat-scala/

38. Surprises
List(1, 2, 3).toSet
scala.collection.immutable.Set[Int] = Set(1, 2, 3)
List(1, 2, 3).toSet()
Boolean = false
http://dan.bodar.com/2013/12/04/wat-scala/

39. Implicits
• Can make it very hard to read code
• Tool support is very bad
• Impacts compilation time
• Surprising behaviour (esp. when used with overloaded methods or optional params)

40. Tooling
def handle(response: HttpResponse, request: HttpRequest)
• Tool support is still very basic
• Makes it hard to continuously refactor (which means people are less likely to do it)

41. Tooling
def handle(response: HttpResponse, request: HttpRequest)
• Tool support is still very basic
• Makes it hard to continuously refactor (which means people are less likely to do it)
no luck with “change signature”
refactoring support

42. Trait entanglements
• Makes it difficult to reason about behaviour

43. Trait entanglements
• Makes it difficult to reason about behaviour
trait A {
def foo = "a"
}

44. Trait entanglements
• Makes it difficult to reason about behaviour
trait A {
def foo = "a"
}
trait B extends A {
override def foo = "b"
}

45. Trait entanglements
• Makes it difficult to reason about behaviour
trait A {
def foo = "a"
}
trait B extends A {
override def foo = "b"
}
class C extends A with B
new C().foo

46. Trait entanglements
• Makes it difficult to reason about behaviour
trait A {
def foo = "a"
}
trait B extends A {
override def foo = "b"
}
class C extends A with B
new C().foo
"b"

47. Trait entanglements
• Makes it difficult to reason about behaviour
trait A {
def foo = "a"
}
trait B extends A {
override def foo = "b"
}
class C extends A with B
new C().foo
"b"
class D extends B with A
new D().foo

48. Trait entanglements
• Makes it difficult to reason about behaviour
trait A {
def foo = "a"
}
trait B extends A {
override def foo = "b"
}
class C extends A with B
new C().foo
"b"
class D extends B with A
new D().foo
"b"

49. Trait entanglements (2)
ArticlePageSteps
WebDriverSupport
AuthorStepsCoverImageSteps
SummarySection
Waiter SectionPageSteps
CommonPageSteps
WebElementSupport
Assertions
Uris TripleEquals
OnHost TripleEqualsSupport
MachineNames

50. Trait entanglements (3)
ArticlePageSteps_0
WebDriverSupport_1 AuthorSteps_1 CoverImageSteps_1 SummarySection_1Waiter_1 SectionPageSteps_1 CommonPageSteps_1
CommonPageSteps_2 WebElementSupport_2Assertions_2 WebDriverSupport_2Uris_2
WebElementSupport_3 WebDriverSupport_3Uris_3 TripleEquals_3 OnHost_3
OnHost_4 TripleEqualsSupport_4 MachineNames_4
MachineNames_5

51. Trait entanglements (3)
ArticlePageSteps_0
WebDriverSupport_1 AuthorSteps_1 CoverImageSteps_1 SummarySection_1Waiter_1 SectionPageSteps_1 CommonPageSteps_1
CommonPageSteps_2 WebElementSupport_2Assertions_2 WebDriverSupport_2Uris_2
WebElementSupport_3 WebDriverSupport_3Uris_3 TripleEquals_3 OnHost_3
OnHost_4 TripleEqualsSupport_4 MachineNames_4
MachineNames_5

52. Trait entanglements (4)
ArticlePageTests_0
ArticlePageSteps_1 AboutSectionSteps_1 SearchResultsPageSteps_1 Uris_1 ArticleTestFixture_1 JavascriptSupport_1 IssuePageSteps_1 CommonAbstractSteps_1 GoogleAnalyticsSteps_1 FakeEntitlementSteps_1 ExportCitationPageSteps_1 FullTextPageSteps_1 OtherActionsSectionSteps_1
WebDriverSupport_2 AuthorSteps_2 CoverImageSteps_2 SummarySection_2Waiter_2 SectionPageSteps_2 CommonPageSteps_2
CommonPageSteps_3 WebElementSupport_3Assertions_3 WebDriverSupport_3Uris_3
WebElementSupport_4 WebDriverSupport_4Uris_4 TripleEquals_4 OnHost_4
OnHost_5 TripleEqualsSupport_5 MachineNames_5
MachineNames_6

53. Trait entanglements (4)
ArticlePageTests_0
ArticlePageSteps_1 AboutSectionSteps_1 SearchResultsPageSteps_1 Uris_1 ArticleTestFixture_1 JavascriptSupport_1 IssuePageSteps_1 CommonAbstractSteps_1 GoogleAnalyticsSteps_1 FakeEntitlementSteps_1 ExportCitationPageSteps_1 FullTextPageSteps_1 OtherActionsSectionSteps_1
WebDriverSupport_2 AuthorSteps_2 CoverImageSteps_2 SummarySection_2Waiter_2 SectionPageSteps_2 CommonPageSteps_2
CommonPageSteps_3 WebElementSupport_3Assertions_3 WebDriverSupport_3Uris_3
WebElementSupport_4 WebDriverSupport_4Uris_4 TripleEquals_4 OnHost_4
OnHost_5 TripleEqualsSupport_5 MachineNames_5
MachineNames_6
Imagine many more circle here

54. So, what’s next?

55. Today
• We’ve delivered successfully using Scala
• Don’t think we’re more productive (pure gut feeling, though)
• We try to stick to the good parts (conventions, functional programming, pattern matching, etc.)
• Complexity, slow compilation and lack of tool support are real problems

56. The future
• No urgency to move away from Scala or re-write existing systems
• Java 8 is an alternative
• Smaller teams and apps will probably lead to more polyglotism (and less Scala)

57. Thanks
@patforna
patric.fornasier@springer.com
http://joinit.springer.com