DDD & REST — Domain-Driven APIs for the web

Slides of the talk I gave a Java Forum Nord 2016.

@springcentral

Transcript

1. DDD & REST Domain-Driven APIs for the web / olivergierke Oliver Gierke ogierke@pivotal.io
4. Background 4
5. 5 Spring Data Repositories &
 Aggregates Spring HATEOAS Hypermedia
 for Spring MVC Spring Data REST
6. When designing REST APIs, what can we learn from DDD? “ 6
7. What does it take to bridge the worlds of DDD & REST? “ 7
9. 9 http://www.infoq.com/minibooks/domain-driven-design-quickly
11. Implementing Domain-Driven Design 11
12. Value objects 12
13. Stringly typed code 13 public class Customer { private Long id; private String firstname, lastname, email; … }
14. Stringly typed code 14 public class SomeService { public void createUser(String firstname,
 String lastname, String email) { … } }
15. 15 public class Customer { private Long id; private Firstname firstname; private Lastname lastname; private EmailAddress emailAddress; … }
16. Value Objects are a
 PITA to build in
 some languages. 16
17. Still, they’re worth it. 17 See „Power Use of Value Objects in DDD“ by Dan Bergh Johnsson.
18. Lombok — putting the spice back into Java. 18
19. 19 @Value public class Customer { UUID id = UUID.randomUUID(); Firstname firstname; Lastname lastname; EmailAddress email; @Value static class EmailAddress { String value; } }
20. Entities & Repositories 21
21. 22 Order LineItem Product Invoice Customer Payment Address Email
22. 22 Order LineItem Product Invoice Customer Payment Address Email
23. Don’t get trapped by datastore thinking. 23
24. 24 Order LineItem Product Invoice Customer Payment Address Email
25. Entity +
 Repository = 
 Aggregate 25
26. Aggregates.
 Scopes of consistency. 26
27. 27 Order LineItem Product Invoice Customer Payment Address Email
28. Aggregates.
 The key things
 to refer to. 28
29. 29 Order LineItem Product Invoice Customer Payment Address Email
30. Bounded Context 30
31. Order LineItem Product Invoice Customer Payment Address
32. 32 Shipping Accounting Catalog Orders User
 Registration
33. 32 Shipping Accounting Catalog Orders User
 Registration Accounting Payment information Billing Address Shipping Shipping address Customer Product
34. Domain Events 33
35. 34 Level 0: No events at all
36. 34 Level 0: No events at all Level 1: Explicit operations
37. If you’re calling two setters in a row, you’re missing a concept. 35
38. 36 Level 0: No events at all Level 1: Explicit operations Level 2: Some operations as events
39. Domain events as
 state transitions. 37
40. Expose important
 events to interested parties via feeds. 38
41. 39 Level 0: No events at all Level 1: Explicit operations Level 2: Some operations as events Level 3: Event Sourcing
42. REST 40
43. REST ≠ 
 CRUD via HTTP 41
44. Representation design matters 42
45. Value objects
 need custom 
 serializers. 43
46. Aggregates Identifiable
 Referable
 Scope of consistency 44
47. Resources Identifiable
 Referable
 Scope of consistency 45
48. Hypermedia 46
49. Serving data and navigation information
 at the same time. 47
50. Trading domain knowledge with protocol complexity in clients. 48
51. 49 Amount of domain knowledge in the client Amount of protocol knowledge in the client Coupling to the server Non-hypermedia
 based systems Hypermedia
 based systems
52. API evolvability is key
 in a system of systems 50 See „Evolving Distributed Systems“
53. 51 GET /orders { „_links“ : { „cancel“ : { „href“ : … }, … „createdDate“ : …, „status“ : „Payment expected“ … }
54. How does the client
 know if a state
 transition is allowed? 52
55. Option 1:
 Inspecting the payload 53
56. 54 GET /order/42 { „_links“ : { „cancel“ : { „href“ : … }, … „createdDate“ : …, „status“ : „Payment expected“ … }
57. 55 “Internationalize
 all user facing text.
58. Aaaargh! 56
59. Option 2:
 Inspecting
 hypermedia elements 57
60. 58 GET /order/42 { „_links“ : { „cancel“ : { „href“ : … }, … „createdDate“ : …, „status“ : „Payment expected“ … }
61. Reducing decisions in clients to whether a
 link is present or not. 59
62. Prefer explicit 
 state transitions over poking at your resources using PATCH. 60
63. Translate backend concepts into web- appropriate ones. 61
64. 62 Aggregate Root / Repository Collection / Item Resources IDs URIs @Version ETags Last Modified Property Last Modified Header Relations Links
65. RESTBucks payment expected preparing cancelled ready completed 1 2 3 4 5 6
66. Method URI Action Step POST /orders Create new order 1 POST/PATCH /orders/{id} Update the order (only if "payment expected") 2 DELETE /orders/{id} Cancel order (only if "payment expected") 3 PUT /orders/{id}/payment Pay order (only if "payment expected") 4 Barista preparing the order GET /orders/{id} Poll order state 5 GET /orders/{id}/receipt Access receipt DELETE /orders/{id}/receipt Conclude the order process 6
67. Method Resource type Action Step POST orders Create new order 1 POST/PATCH update Update the order 2 DELETE cancel Cancel order 3 PUT payment Pay order 4 Barista preparing the order GET order Poll order state 5 GET receipt Access receipt DELETE receipt Conclude the order process 6
68. Spring RESTBucks 66
69. Web Service Repository - Orders Spring Data Spring Data
 REST Payment Spring Data Manual
 implementation Manual
 implementation
70. Resources 68 Spring RESTBucks
 https://github.com/olivergierke/spring-restbucks Benefits of Hypermedia APIs
 http://olivergierke.de/2016/04/benefits-of-hypermedia/ Evolving Distributed Systems http://olivergierke.de/2016/10/evolving-distributed-systems/
71. Questions? 69