Domain-Driven Design & REST

Slides of the talk I gave at JAX 2016, Mayence.

@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?
   “
   

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10. Value objects
    9
    

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11. Stringly typed code
    10
    public class Customer {
    private Long id;
    private String firstname, lastname, email;
    …
    }
    

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12. Stringly typed code
    11
    public class SomeService {
    public void createUser(String firstname,

    String lastname, String email) {
    …
    }
    }
    

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13. 12
    public class Customer {
    private Long id;
    private Firstname firstname;
    private Lastname lastname;
    private EmailAddress emailAddress;
    …
    }
    

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14. Value Objects are a

    PITA to build in

    some languages.
    13
    

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15. Still, they’re worth it.
    14
    See „Power Use of Value Objects in DDD“ by Dan Bergh Johnsson.
    

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16. Lombok — putting the
    spice back into Java.
    15
    

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17. 16
    @Value
    public class Customer {
    UUID id = UUID.randomUUID();
    Firstname firstname;
    Lastname lastname;
    EmailAddress email;
    @Value
    static class EmailAddress {
    String value;
    }
    }
    

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18. AutoValue
    17
    Project website on GitHub.
    

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19. Key opponents:
    Mapping libraries

    that need to

    (de)serialize them.
    18
    

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20. Entities &
    Repositories
    19
    

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21. 20
    Order
    LineItem
    Product
    Invoice
    Customer
    Payment Address
    Email
    

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22. 20
    Order
    LineItem
    Product
    Invoice
    Customer
    Payment Address
    Email
    

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23. Entity +

    Repository = 

    Aggregate
    21
    

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24. Aggregates form nice
    representation
    boundaries.
    22
    

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25. Aggregates become

    the key things

    to refer to.
    23
    

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26. Don’t get trapped by
    datastore thinking.
    24
    

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27. Try to avoid 

    bi-directional
    relationships.
    25
    

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28. Domain Events
    26
    

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29. 27
    Level 0: No events at all
    

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30. 27
    Level 0: No events at all
    Level 1: Explicit operations
    

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31. If you’re calling two
    setters in a row, you’re
    missing a concept.
    28
    

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32. 29
    Level 0: No events at all
    Level 1: Explicit operations
    

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33. 29
    Level 0: No events at all
    Level 1: Explicit operations
    Level 2: Some operations as events
    

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34. Domain events as

    state transitions.
    30
    

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35. Expose important

    events to interested
    parties via feeds.
    31
    

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36. 32
    Level 0: No events at all
    Level 1: Explicit operations
    Level 2: Some operations as events
    

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37. 32
    Level 0: No events at all
    Level 1: Explicit operations
    Level 2: Some operations as events
    Level 3: Event Sourcing
    

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38. REST
    33
    

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39. Representation
    design matters
    34
    

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40. Aggregates
    Identifiable

    Referable

    Scope of consistency
    35
    

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41. Resources
    Identifiable

    Referable

    Scope of consistency
    36
    

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42. Hypermedia
    37
    

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43. Serving data and
    navigation information

    at the same time.
    38
    

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44. Trading domain
    knowledge with protocol
    complexity in clients.
    39
    

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45. Reducing decisions in
    clients to whether a

    link is present or not.
    40
    

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46. Prefer explicit 

    state transitions over
    poking at your resources
    using PATCH.
    41
    

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47. Translate domain
    concepts into web-
    appropriate ones.
    42
    

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48. 43
    Aggregate Root /
    Repository
    Collection / Item
    Resources
    IDs URIs
    @Version ETags
    Last Modified
    Property
    Last Modified
    Header
    Relations Links
    

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

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50. 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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51. 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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52. Spring
    RESTBucks
    47
    

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53. Web
    Service
    Repository
    -
    Orders
    Spring Data
    Spring Data

    REST
    Payment
    Spring Data
    Manual

    implementation
    Manual

    implementation
    

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54. JacksonCustomizations
    Externalize tweaks to the
    general JSON design
    49
    

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55. Spring Data REST

    for the CRUDdy parts.
    50
    

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56. ResourceProcessor
    To conditionally sneak
    links into the default
    representation.
    51
    

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57. Code
    52
    Spring RESTBucks - https://github.com/olivergierke/spring-restbucks
    

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58. Questions?
    53
    

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