Domain-Driven Design & REST

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

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Background 3

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4 Spring Data Repositories &  Aggregates Spring HATEOAS Hypermedia  for Spring MVC Spring Data REST

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REST ≠   CRUD via HTTP 5

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

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Value objects 9

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

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Stringly typed code 11 public class SomeService { public void createUser(String firstname,  String lastname, String email) { … } }

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

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Value Objects are a  PITA to build in  some languages. 13

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

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Lombok — putting the spice back into Java. 15

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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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AutoValue 17 Project website on GitHub.

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Key opponents: Mapping libraries  that need to  (de)serialize them. 18

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Entities & Repositories 19

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20 Order LineItem Product Invoice Customer Payment Address Email

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20 Order LineItem Product Invoice Customer Payment Address Email

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Entity +  Repository =   Aggregate 21

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Aggregates form nice representation boundaries. 22

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Aggregates become  the key things  to refer to. 23

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Don’t get trapped by datastore thinking. 24

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Try to avoid   bi-directional relationships. 25

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Domain Events 26

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27 Level 0: No events at all

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

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

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

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

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Domain events as  state transitions. 30

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Expose important  events to interested parties via feeds. 31

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

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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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REST 33

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Representation design matters 34

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Aggregates Identiﬁable  Referable  Scope of consistency 35

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Resources Identiﬁable  Referable  Scope of consistency 36

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Hypermedia 37

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Serving data and navigation information  at the same time. 38

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Trading domain knowledge with protocol complexity in clients. 39

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Reducing decisions in clients to whether a  link is present or not. 40

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Prefer explicit   state transitions over poking at your resources using PATCH. 41

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Translate domain concepts into web- appropriate ones. 42

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

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

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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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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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Spring RESTBucks 47

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Web Service Repository - Orders Spring Data Spring Data  REST Payment Spring Data Manual  implementation Manual  implementation

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JacksonCustomizations Externalize tweaks to the general JSON design 49

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Spring Data REST  for the CRUDdy parts. 50

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ResourceProcessor To conditionally sneak links into the default representation. 51