Structure a Play application with the cake pattern (and test it)

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

Transcript

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 class TopVideoService { val videoGateway = new VideoGateway val playerGateway = new PlayerGateway def topVideos(): [...] = { videoGateway.top() [...] } } trait TopVideoServiceComp extends PlayerGatewayComp with VideoGatewayComp { val topVideoService = new TopVideoService class TopVideoService { def topVideos(): [...] = { videoGateway.top() [...] } } } Dependencies 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... • Testing is not optimal • Which dependency should be mocked? • The compiler can check that for us some drawbacks
26. Exposed service abstract trait TopVideoServiceComp extends PlayerGatewayComp with VideoGatewayComp { def topVideoService: TopVideoService [...] } trait TopVideoServiceComp extends PlayerGatewayComp with VideoGatewayComp { val topVideoService = new TopVideoService [...] } Dependencies Provided service
27. Defining the desired service trait TopVideoServiceComp extends PlayerGatewayComp with VideoGatewayComp { def topVideoService: TopVideoService [...] } trait Application extends Controller with TopVideoServiceComp { def index = [...] } object Application extends Application
28. Defining the desired service trait TopVideoServiceComp extends PlayerGatewayComp with VideoGatewayComp { def topVideoService: TopVideoService [...] } trait Application extends Controller with TopVideoServiceComp { def index = [...] } object Application extends Application compilation error compilation error
29. Defining the desired service trait TopVideoServiceComp extends PlayerGatewayComp with VideoGatewayComp { def topVideoService: TopVideoService [...] } trait Application extends Controller with TopVideoServiceComp { def index = [...] } object Application extends Application compilation error compilation error object Application extends Application { override val topVideoService = new TopVideoService }
30. Defining the desired service trait TopVideoServiceComp extends PlayerGatewayComp with VideoGatewayComp { def topVideoService: TopVideoService [...] } trait Application extends Controller with TopVideoServiceComp { def index = [...] } object Application extends Application compilation error compilation error object Application extends Application { override val topVideoService = new TopVideoService } trait TopVideoServiceCompImpl extends TopVideoServiceComp { 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 trait TopVideoServiceComp extends PlayerGatewayComp with VideoGatewayComp { def topVideoService: TopVideoService [...] } trait TopVideoServiceComp { self: PlayerGatewayComp with VideoGatewayComp => def topVideoService: TopVideoService [...] } Dependencies 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 trait HttpClientComp { def httpClient: HttpClient class HttpClient { ... } } trait PlayerGatewayComp { self: HttpClientComp => <use httpClient> } trait VideoGatewayComp { self: HttpClientComp => <use httpClient> } public class HttpClient { ... } public class PlayerGateway { @Inject private HttpClient httpClient; <use httpClient> } public class VideoGateway { @Inject private HttpClient httpClient; <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 • mix interface / implementation • difficult to provide alternative runtime implementation • cannot provide component dependency only for one implementation some drawbacks
47. traits with self type and implementation ex with top videos
48. traits with self type and implementation • decouple interface / implementation Let's fix it
49. Decouple service definition / impl trait TopVideoServiceComp { def topVideoService: TopVideoService trait TopVideoService { def topVideos(): Future[Option[Seq[TopVideo]]] } } 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() [...] } } trait TopVideoServiceComp { self: PlayerGatewayComp with VideoGatewayComp => def topVideoService: TopVideoService [impl of TopVideoService] } trait TopVideoServiceCompImpl extends TopVideoServiceComp { self: PlayerGatewayComp with VideoGatewayComp => override val topVideoService = new TopVideoService }
50. Decouple service definition / impl trait TopVideoServiceComp { def topVideoService: TopVideoService trait TopVideoService { def topVideos(): Future[Option[Seq[TopVideo]]] } } 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() [...] } } } Dependencies specific to impl Provided service definition service impl
51. comparison of all variants trait VideoGatewayComp extends HttpClientComp { val videoGateway = new VideoGateway sealed trait TopVideosResponse [...] class VideoGateway { def top(): Future[TopVideosResponse] = [...] } } ✔ simple ✗ alternative impl very difficult ✗ forget what to override (in tests) 1st version
52. comparison of all variants 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 } ✔ dependencies checked by compiler ✗ invisible inheritance of other dependencies 2nd version
53. comparison of all variants 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 } ✔ explicit dependent components ✗ no separation interface / impl ✗ boilerplate 3rd version
54. comparison of all variants trait VideoGatewayComp { 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] = [...] } } ✔ separation interface / impl ✔ flexibility ✗ boilerplate ++ 4th version
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 class RuntimeEnvironment extends Registry with HttpClientCompImpl with VideoGatewayCompImpl with PlayerGatewayCompImpl with TopVideoServiceCompImpl trait 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 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 { [...] } class RuntimeEnvironment extends Registry { override val httpClient = new HttpClient override val playerGateway = new PlayerGateway override val videoGateway = new VideoGateway override val topVideoService = new TopVideoService }
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. Do no over-use it! DI ease unit testing
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/