Domain-Driven Design & REST

Slides of the talk I gave at Spring I/O 2016.

@springcentral

Transcript

1. DDD & REST
   Domain-Driven APIs for the web
   / olivergierke
   Oliver Gierke
   

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3. Background
   3
   

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4. 4
   Spring Data
   Repositories &

   Aggregates
   Spring HATEOAS
   Hypermedia

   for Spring MVC
   Spring Data REST
   

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5. REST ≠ 

   CRUD via HTTP
   5
   

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6. What does it take to
   bridge the worlds of
   DDD & REST?
   “
   6
   

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8. 8
   http://www.infoq.com/minibooks/domain-driven-design-quickly
   

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11. Implementing
    Domain-Driven
    Design
    10
    

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12. Value objects
    11
    

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13. Stringly typed code
    12
    public class Customer {
    private Long id;
    private String firstname, lastname, email;
    …
    }
    

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14. Stringly typed code
    13
    public class SomeService {
    public void createUser(String firstname,

    String lastname, String email) {
    …
    }
    }
    

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15. 14
    public class Customer {
    private Long id;
    private Firstname firstname;
    private Lastname lastname;
    private EmailAddress emailAddress;
    …
    }
    

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16. Value Objects are a

    PITA to build in

    some languages.
    15
    

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17. Still, they’re worth it.
    16
    See „Power Use of Value Objects in DDD“ by Dan Bergh Johnsson.
    

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18. Lombok — putting the
    spice back into Java.
    17
    

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19. 18
    @Value
    public class Customer {
    UUID id = UUID.randomUUID();
    Firstname firstname;
    Lastname lastname;
    EmailAddress email;
    @Value
    static class EmailAddress {
    String value;
    }
    }
    

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20. AutoValue
    19
    Project website on GitHub.
    

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21. Key opponents:
    Mapping libraries

    that need to

    (de)serialize them.
    20
    

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22. Entities &
    Repositories
    21
    

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23. 22
    Order
    LineItem
    Product
    Invoice
    Customer
    Payment Address
    Email
    

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24. 22
    Order
    LineItem
    Product
    Invoice
    Customer
    Payment Address
    Email
    

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25. Entity +

    Repository = 

    Aggregate
    23
    

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26. Aggregates form nice
    representation
    boundaries.
    24
    

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27. Aggregates become

    the key things

    to refer to.
    25
    

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28. Don’t get trapped by
    datastore thinking.
    26
    

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29. Try to avoid 

    bi-directional
    relationships.
    27
    

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30. Bounded Context
    28
    

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31. Order
    LineItem
    Product
    Invoice
    Customer
    Payment Address
    

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32. 30
    Shipping
    Accounting
    Catalog
    Orders
    User

    Registration
    

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33. 30
    Shipping
    Accounting
    Catalog
    Orders
    User

    Registration
    Accounting
    Payment information
    Billing Address
    Shipping
    Shipping address
    Customer
    Product
    

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34. Domain Events
    31
    

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35. 32
    Level 0: No events at all
    

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36. 32
    Level 0: No events at all
    Level 1: Explicit operations
    

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37. If you’re calling two
    setters in a row, you’re
    missing a concept.
    33
    

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38. 34
    Level 0: No events at all
    Level 1: Explicit operations
    

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39. 34
    Level 0: No events at all
    Level 1: Explicit operations
    Level 2: Some operations as events
    

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40. Domain events as

    state transitions.
    35
    

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41. Expose important

    events to interested
    parties via feeds.
    36
    

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42. 37
    Level 0: No events at all
    Level 1: Explicit operations
    Level 2: Some operations as events
    

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43. 37
    Level 0: No events at all
    Level 1: Explicit operations
    Level 2: Some operations as events
    Level 3: Event Sourcing
    

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44. REST
    38
    

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45. Representation
    design matters
    39
    

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46. Aggregates
    Identifiable

    Referable

    Scope of consistency
    40
    

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47. Resources
    Identifiable

    Referable

    Scope of consistency
    41
    

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48. Hypermedia
    42
    

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49. Serving data and
    navigation information

    at the same time.
    43
    

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50. Trading domain
    knowledge with protocol
    complexity in clients.
    44
    

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51. Reducing decisions in
    clients to whether a

    link is present or not.
    45
    

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52. Prefer explicit 

    state transitions over
    poking at your resources
    using PATCH.
    46
    

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53. Translate domain
    concepts into web-
    appropriate ones.
    47
    

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54. 48
    Aggregate Root /
    Repository
    Collection / Item
    Resources
    IDs URIs
    @Version ETags
    Last Modified
    Property
    Last Modified
    Header
    Relations Links
    

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55. RESTBucks
    payment
    expected
    preparing
    cancelled
    ready completed
    1
    2
    3
    4
    5 6
    

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56. 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
    

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57. 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
    

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58. 52
    GET /orders
    {
    „_links“ : {
    „cancel“ : { „href“ : … },
    …
    „createdDate“ : …,
    „status“ : „Payment expected“
    …
    }
    

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59. 53
    GET /orders
    {
    „_links“ : {
    „cancel“ : { „href“ : … },
    …
    „createdDate“ : …,
    „status“ : „Payment expected“
    …
    }
    

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60. 54
    “Internationalize

    all user facing text.
    

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61. Aaaargh!
    55
    

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62. 56
    GET /orders
    {
    „_links“ : {
    „cancel“ : { „href“ : … },
    …
    „createdDate“ : …,
    „status“ : „Payment expected“
    …
    }
    

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63. Spring
    RESTBucks
    57
    

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64. Web
    Service
    Repository
    -
    Orders
    Spring Data
    Spring Data

    REST
    Payment
    Spring Data
    Manual

    implementation
    Manual

    implementation
    

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65. JacksonCustomizations
    Externalize tweaks to the
    general JSON design
    59
    

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66. Spring Data REST

    for the CRUDdy parts.
    60
    

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67. ResourceProcessor
    To conditionally sneak
    links into the default
    representation.
    61
    

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68. Resources
    62
    Spring RESTBucks

    https://github.com/olivergierke/spring-restbucks
    Benefits of Hypermedia APIs

    http://olivergierke.de/2016/04/benefits-of-hypermedia/index.html
    

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69. Questions?
    63
    

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