A challenge during the development of an application is how to add new functions without compromising existing ones. Using the Cake Pattern, the application can be structured into logical components, thus minimizing the coupling between them and controlling the effects of changes. You will learn what this pattern is, and how to introduce it step by step in a Play Application. You will be shown how an application designed that way is easy to test, especially with the Play testing API. Finally, the talk will describe the common pitfalls of the Cake Pattern and how to avoid them. Video of the talk: http://www.ustream.tv/recorded/42775808 Sources: https://github.com/yanns/TPA Sources of the final version: https://github.com/yanns/TPA/tree/master/frontend/TBA_05_final

1. Structure your Play
application with
the cake pattern
and test it

2. Structure a Play application like a cake

3. We'll build a website for
Toys Basketball Association

4. Architecture

5. Player Backend
GET http://localhost:9001/players/1
HTTP/1.1 200 OK
Content-Type: application/json; charset=utf-8
Content-Length: 91
{"id":1,"name":"James P.
Sullivan","height":"34 cm","weight":"370
g","team":"Monstropolis"}

6. Player Backend
GET http://localhost:9001/players/1/photo
HTTP/1.1 200 OK
Last-Modified: Thu, 12 Dec 2013 13:11:02 GMT
Etag: "4911f28b55213..."
Content-Length: 1753583
Cache-Control: max-age=3600
Content-Type: image/jpeg
Date: Mon, 23 Dec 2013 10:01:15 GMT
<binary>

7. Video Backend
GET http://localhost:9002/videos/top
HTTP/1.1 200 OK
Content-Type: application/json; charset=utf-8
Content-Length: 83
[{"id":1,"summary":"dunk","players":[1]},...]

8. Video Backend
GET http://localhost:9002/videos/1/stream.mp4
range: bytes=200-280
HTTP/1.1 206 Partial Content
Content-Range: bytes 200-280/2154777
Accept-Ranges: bytes
Connection: keep-alive
Content-Length: 81
Content-Type: video/mp4
http:////www.videolan.org/x264.html - options:
cabac=1 ref=3 deblock=1:0:0 analyse=0x3:0

9. Let's code
•
first version

10. Proud of ourselves?
let's refactor

11. Tests with the new version?

12. Test pyramid
unit tests
def f(input): output
assert output

13. Test pyramid
component tests

14. Test pyramid
integration tests

15. Which tests are possible?
with the actual version

16. Component hierarchy
how to avoid that?

17. Introducing components
•
Live coding

18. Dependencies between components
Dependencies
class TopVideoService {
val videoGateway =
new VideoGateway
val playerGateway =
new PlayerGateway
trait TopVideoServiceComp
extends PlayerGatewayComp
with VideoGatewayComp {
val topVideoService =
new TopVideoService
def topVideos(): [...] = {
videoGateway.top() [...]
}
class TopVideoService {
def topVideos(): [...] = {
videoGateway.top() [...]
}
}
}
Provided service
}

19. Introducing components

20. Introducing components
✔
✔
Components expose services only to
other components
One component must depend from
another one to use the exposed service

21. Testing with components
unit tests

22. Testing with components
component
tests

23. Testing with components
component
tests

24. components as traits
✔
Components with exposed services and
dependencies
✔
Unit tests
✔
Component tests
✔
Integration tests

25. But...
some drawbacks
•
Testing is not optimal
•
Which dependency should be mocked?
•
The compiler can check that for us

26. Exposed service abstract
Dependencies
trait TopVideoServiceComp
extends PlayerGatewayComp
with VideoGatewayComp {
trait TopVideoServiceComp
extends PlayerGatewayComp
with VideoGatewayComp {
val topVideoService =
new TopVideoService
def topVideoService:
TopVideoService
[...]
[...]
}
}
Provided service

27. Defining the desired service
trait TopVideoServiceComp
extends PlayerGatewayComp
with VideoGatewayComp {
trait Application
extends Controller
with TopVideoServiceComp {
def index = [...]
def topVideoService: TopVideoService
}
[...]
}
object Application
extends Application

28. Defining the desired service
trait TopVideoServiceComp
extends PlayerGatewayComp
with VideoGatewayComp {
trait Application
extends Controller
with TopVideoServiceComp {
def index = [...]
def topVideoService: TopVideoService
}
[...]
}
ro r
r
object Applicationn e
atio
extends lApplication
pi
om
c

29. Defining the desired service
trait TopVideoServiceComp
extends PlayerGatewayComp
with VideoGatewayComp {
trait Application
extends Controller
with TopVideoServiceComp {
def index = [...]
def topVideoService: TopVideoService
}
[...]
}
ro r
r
object Applicationn e
atio
extends lApplication
pi
om
c
object Application
extends Application {
override val topVideoService =
new TopVideoService
}

30. Defining the desired service
trait TopVideoServiceComp
extends PlayerGatewayComp
with VideoGatewayComp {
trait Application
extends Controller
with TopVideoServiceComp {
def index = [...]
def topVideoService: TopVideoService
}
[...]
}
trait TopVideoServiceCompImpl
extends TopVideoServiceComp {
override val topVideoService =
new TopVideoService
}
ro r
r
object Applicationn e
atio
extends lApplication
pi
om
c
object Application
extends Application {
override val topVideoService =
new TopVideoService
}
object Application
extends Application
with TopVideoServiceCompImpl

31. Introducing Registry
object Application
extends Application
with PlayerGatewayCompImpl
with VideoGatewayCompImpl
with HttpClientCompImpl
with TopVideoServiceCompImpl
object Players
extends Players
with PlayerGatewayCompImpl
with VideoGatewayCompImpl
with HttpClientCompImpl
with TopVideoServiceCompImpl
trait Registry
extends HttpClientComp
with PlayerGatewayComp
with VideoGatewayComp
with TopVideoServiceComp
trait RuntimeEnvironment
extends Registry
with VideoGatewayCompImpl
with HttpClientCompImpl
with PlayerGatewayCompImpl
with TopVideoServiceCompImpl
object Application
extends Application
with RuntimeEnvironment
object Players
extends Players
with RuntimeEnvironment

32. Runtime and Test Registries
trait Registry
extends HttpClientComp
with PlayerGatewayComp
with VideoGatewayComp
with TopVideoServiceComp
trait RuntimeEnvironment extends Registry
with VideoGatewayCompImpl
with HttpClientCompImpl
with PlayerGatewayCompImpl
with TopVideoServiceCompImpl
trait MockEnvironment extends Registry with Mockito {
override val httpClient = mock[HttpClient]
override val playerGateway = mock[PlayerGateway]
override val videoGateway = mock[VideoGateway]
override val topVideoService = mock[TopVideoService]
}

33. Traits with abstract methods
✔
Components with exposed services and
dependencies
✔
Dependencies checked by compiler
✔
Unit tests
✔
Component tests
✔
Integration tests

34. drawback of traits inheritance

35. Introducing self type
Dependencies
trait TopVideoServiceComp
extends PlayerGatewayComp
with VideoGatewayComp {
trait TopVideoServiceComp {
self: PlayerGatewayComp
with VideoGatewayComp =>
def topVideoService:
TopVideoService
def topVideoService:
TopVideoService
[...]
[...]
}
}
Provided service

36. self type annotation
„Any concrete class that mixed in the
trait must ensure that its type conforms to
the trait's self type“
source: http://docs.scala-lang.org/glossary/#self_type

37. Traits with self types
✔
Components with exposed services and
only explicit dependencies
✔
Unit tests
✔
Component tests
✔
Integration tests

38. Parallel @Inject / traits
public class HttpClient {
...
}
trait HttpClientComp {
def httpClient: HttpClient
class HttpClient {
...
}
public class PlayerGateway {
@Inject
private HttpClient httpClient;
<use httpClient>
}
trait PlayerGatewayComp {
self: HttpClientComp =>
}
<use httpClient>
public class VideoGateway {
@Inject
private HttpClient httpClient;
<use httpClient>
}
}
trait VideoGatewayComp {
self: HttpClientComp =>
<use httpClient>
}

39. Dependencies Injection
•
„side effect“ of Cake pattern
•
dependencies checked by compiler

40. Alternatives for DI
•
Spring, Guice...
•
DI with macros: macwire
http://typesafe.com/activator/template/macwire-activator

41. Resolving dependencies at runtime
source: http://apmblog.compuware.com/2013/12/18/the-hidden-class-loading-performance-impact-of-the-spring-framework/

42. Resolving dependencies at runtime
source: http://apmblog.compuware.com/2013/12/18/the-hidden-class-loading-performance-impact-of-the-spring-framework/

43. Resolving dependencies at runtime
source: http://apmblog.compuware.com/2013/12/18/the-hidden-class-loading-performance-impact-of-the-spring-framework/

44. Resolving dependencies at runtime
source: http://apmblog.compuware.com/2013/12/18/the-hidden-class-loading-performance-impact-of-the-spring-framework/

45. DI with cake pattern
•
dependencies are resolved at compile time
•
no surprise at runtime

46. traits with self type and implementation
some drawbacks
•
mix interface / implementation
•
difficult to provide alternative runtime
implementation
•
cannot provide component dependency
only for one implementation

47. traits with self type and implementation
ex with top videos

48. traits with self type and implementation
Let's fix it
•
decouple interface / implementation

49. Decouple service definition / impl
trait TopVideoServiceComp {
trait TopVideoServiceComp {
def topVideoService: TopVideoService
self: PlayerGatewayComp
with VideoGatewayComp =>
def topVideoService:
TopVideoService
[impl of TopVideoService]
}
trait TopVideoService {
def topVideos(): Future[Option[Seq[TopVideo]]]
}
}
trait TopVideoServiceCompImpl
extends TopVideoServiceComp {
trait TopVideoServiceCompImpl
extends TopVideoServiceComp {
self: PlayerGatewayComp
with VideoGatewayComp =>
self: PlayerGatewayComp
with VideoGatewayComp =>
override val topVideoService =
new TopVideoServiceImpl
override val topVideoService
= new TopVideoService
class TopVideoServiceImpl
extends TopVideoService {
def topVideos(): Future[Option[Seq[TopVideo]]]
= videoGateway.top() [...]
}
}
}

50. Decouple service definition / impl
trait TopVideoServiceComp {
def topVideoService: TopVideoService
trait TopVideoService {
def topVideos(): Future[Option[Seq[TopVideo]]]
}
}
Provided service
definition
Dependencies
specific to impl
trait TopVideoServiceCompImpl extends TopVideoServiceComp {
self: PlayerGatewayComp with VideoGatewayComp =>
override val topVideoService = new TopVideoServiceImpl
class TopVideoServiceImpl extends TopVideoService {
def topVideos(): Future[Option[Seq[TopVideo]]] = {
videoGateway.top() [...]
}
}
}
service impl

51. comparison of all variants
1st version
✔
✗
✗
simple
alternative impl very difficult
forget what to override (in tests)
trait VideoGatewayComp extends HttpClientComp {
val videoGateway = new VideoGateway
sealed trait TopVideosResponse
[...]
class VideoGateway {
def top(): Future[TopVideosResponse] = [...]
}
}

52. comparison of all variants
2nd version
✔
✗
dependencies checked by compiler
invisible inheritance of other
dependencies
trait VideoGatewayComp extends HttpClientComp {
def videoGateway: VideoGateway
sealed trait TopVideosResponse
[...]
class VideoGateway {
def top(): Future[TopVideosResponse] = [...]
}
}
trait VideoGatewayCompImpl extends VideoGatewayComp {
override val videoGateway = new VideoGateway
}

53. comparison of all variants
3rd version
✔
✗
✗
explicit dependent components
no separation interface / impl
boilerplate
trait VideoGatewayComp {
self: HttpClientComp =>
def videoGateway: VideoGateway
sealed trait TopVideosResponse
[...]
class VideoGateway {
def top(): Future[TopVideosResponse] = [...]
}
}
trait VideoGatewayCompImpl extends VideoGatewayComp {
self: HttpClientComp =>
override val videoGateway = new VideoGateway
}

54. comparison of all variants
4th version
✔
✔
trait VideoGatewayComp {
✗
separation interface / impl
flexibility
boilerplate ++
def videoGateway: VideoGateway
sealed trait TopVideosResponse
[...]
trait VideoGateway {
def top(): Future[TopVideosResponse]
}
}
trait VideoGatewayCompImpl extends VideoGatewayComp {
self: HttpClientComp =>
override val videoGateway: VideoGateway = new VideoGatewayImpl
class VideoGatewayImpl extends VideoGateway {
def top(): Future[TopVideosResponse] = [...]
}
}

55. Number of traits in app
components with
abstract methods
components with
self type
annotations
components with
self type annotations
and real separation
interface /
implementation
16
16
20

56. Downside of Cake pattern (1)

57. What do you need?
•
only DI?
•
multiple alternative implementations of
same service?

58. Downside of Cake pattern (2)
•
compiler error
•
let's minimize it

59. Reducing # of compiler errors
trait RuntimeEnvironment extends Registry
with HttpClientCompImpl
with VideoGatewayCompImpl
with PlayerGatewayCompImpl
with TopVideoServiceCompImpl
class RuntimeEnvironment extends Registry
with HttpClientCompImpl
with VideoGatewayCompImpl
with PlayerGatewayCompImpl
with TopVideoServiceCompImpl

60. Downside of Cake pattern (3)
•
compilation speed
✔
minimize it with (abstract) class
✔
let's remove some traits

61. Removing some traits
class RuntimeEnvironment
extends Registry
with HttpClientCompImpl
with VideoGatewayCompImpl
with PlayerGatewayCompImpl
with TopVideoServiceCompImpl
class RuntimeEnvironment
extends Registry {
override val httpClient =
new HttpClient
override val playerGateway =
new PlayerGateway
override val videoGateway =
new VideoGateway
override val topVideoService =
new TopVideoService
trait HttpClientCompImpl
extends HttpClientComp {
override val httpClient = new HttpClient
}
trait VideoGatewayCompImpl
extends VideoGatewayComp {
self: HttpClientComp =>
override val videoGateway = new VideoGateway
}
trait PlayerGatewayCompImpl
extends PlayerGatewayComp {
[...]
}
trait TopVideoServiceCompImpl
extends TopVideoServiceComp {
[...]
}
}

62. Number of traits
components with
abstract methods
components with
self type
annotations
components with
self type
annotations and
real separation
interface /
implementation
16
12
20

63. About testing
different strategies

64. About testing

65. About testing

66. DI ease unit testing
Do no over-use it!

67. further discussion
•
make cake pattern more manageable with
https://github.com/sullivan-/congeal
trait UService extends hasDependency[URepository] {
...
}

68. Questions?
source: https://github.com/yanns/TPA/

69. Yann Simon
Software Engineer
Blücherstr. 22
10961 Berlin
yann.simon@leanovate.de
twitter: @simon_yann

70. Credits
•
•
•
•
•
•
http://www.flickr.com/photos/cefeida/2306611187/
62/366: Cake, redux (Magic Madzik)
http://www.flickr.com/photos/jason_burmeister/2125022193
Iced Tree (Jason)
http://www.flickr.com/photos/leandrociuffo/6270204821
Berlin skyline (Leandro Neumann Ciuffo)
http://www.flickr.com/photos/8047705@N02/5668841148/
Slow and steady (John Liu)
http://www.flickr.com/photos/jcapaldi/4201550567/
Bon Appetit (Jim, the Photographer)
http://www.epicfail.com/2012/07/17/about-to-fail-26/