Hypermedia Cannot be the Engine

1. HYPERMEDIA  cannot be the engine HYPERMEDIA  cannot be the engine Ruben Verborgh  Ghent University – iMinds

2. “REST APIS MUST BE  HYPERTEXT-DRIVEN.” Roy T. Fielding

3. HYPERMEDIA  cannot be the engine Hypermedia Heaven & Hell Self-descriptive Salvation Angels of Affordance

4. REST “REDUCES COUPLING”. WS-*/ESB RPC over HTTP RESTful HTTP Deg Loo Tigh Des Discovery Identification Binding Platform Interaction Interface Orientation Interface Orientation Model Granularity State Evolution Generated Code Generated Code Conversation Discovery Identification Binding Platform Interaction Model Granularity State Evolution Conversation (a) RESTful HTTP (b) RPC over HTTP Figure 3: Measuring the degree of coupling implied by diﬀerent W RESTful HTTP RPC over HTTP REST RPC C. Pautasso & E. Wilde, “Why is the Web Loosely Coupled? A Multi-Faceted Metric for Service Design”

5. REST HAS  A UNIFORM INTERFACE. resources representations self-descriptiveness hypermedia

6. RESOURCES identify resources, not actions resources are cacheable

7. REPRESENTATIONS share identifiers across systems different needs,  different representations

8. SELF-DESCRIPTIVENESS no external documentation clients interpret representations

9. HYPERMEDIA no hard-coded URLs retrieve a resource, follow links

10. SO FAR  FROM HEAVEN so close  to hell.

11. DOES SELF-DESCRIPTIVENESS REDUCE COUPLING? YES. only the message itself  is needed for interpretation “in-band”

12. DOES SELF-DESCRIPTIVENESS REDUCE COUPLING? NO. REST APIs rely on media types. “in-band”??

13. SO YOU UNDERSTAND JSON? application/alto-costmap+json application/alto-costmapfilter+json application/alto-directory+json application/alto-endpointprop+json application/alto-endpointpropparams+json application/alto-endpointcost+json application/alto-endpointcostparams+json application/alto-error+json application/alto-networkmapfilter+json application/alto-networkmap+json application/jrd+json application/json application/json-patch+json application/ld+json application/reputon+json application/vcard+json application/vnd.api+json application/vnd.bekitzur-stech+json application/vnd.collection.doc+json application/vnd.collection+json application/vnd.collection.next+json application/vnd.document+json application/vnd.hal+json application/vnd.heroku+json application/vnd.oftn.l10n+json application/vnd.siren+json application/vnd.xacml+json

14. DOES SELF-DESCRIPTIVENESS REDUCE COUPLING? Clients can interpret the message  using only in-band information… …if they understand  the media type… …which is out-of-band.

15. DOES HYPERMEDIA REDUCE COUPLING? In each representation, the server includes links towards other resources.

16. DOES HYPERMEDIA REDUCE COUPLING? YES. Clients don’t hard-code  a server’s URLs. Clients don’t hard-code  the path to information.

17. DOES HYPERMEDIA REDUCE COUPLING? In each representation, the server includes links towards other resources. NO. How can the server know what links its clients need?

18. DOES HYPERMEDIA REDUCE COUPLING? The server sends you an image. Scale it.  Crop it. But you want to do something else. Black & white.  Make profile pic.

19. HYPERMEDIA  cannot be the engine media type coupling affordance coupling

21. ENOUGH WITH THE  MEDIA TYPES ALREADY.

22. CHOOSE A SELF-DESCRIPTIVE  MEDIA TYPE. JSON is just JSON. “image” likely has  a hundred meanings.

23. CHOOSE A SELF-DESCRIPTIVE  MEDIA TYPE. JSON-LD is self-descriptive. “image” is actually a URL. Clients can look it up.

24. CHOOSE A SELF-DESCRIPTIVE  MEDIA TYPE. The client still has to  understand this media type. But it’s not task-  or application-specific.

25. ADD SELF-DESCRIPTIVE  HYPERMEDIA CONTROLS. <form> <label>Person:</label> <input name=“person” /> </form> What does this do?

26. ADD SELF-DESCRIPTIVE  HYPERMEDIA CONTROLS. Hydra inside JSON-LD response relate controls  to data properties

27. DESCRIBE THE  API BEFOREHAND. in a hypermedia-based way describe the links (imageA) —B&W—> (imageB)

28. DESCRIBE THE  API BEFOREHAND. in a resource-based way describe the functionality (imageA) ——> (imageB&W)

30. HYPERMEDIA RESPONSES SHOULD AFFORD GOING TO NEXT STEPS. Server: Here’s an image. Client: Thanks. I want to scale it. Server: I thought you would, so I included a link.

31. HYPERMEDIA RESPONSES SHOULD AFFORD GOING TO NEXT STEPS. Client: Now use it as my  Facebook profile picture. Server: Err…  why would you want to do that?

32. IT’S HARD TO IMAGINE ALL WAYS  IN WHICH CLIENTS USE YOUR API. IMPOSSIBLE SELF-DESCRIPTIVENESS  GIVES THEM A CHOICE.

33. COMBINE SELF-DESCRIPTIVENESS  OF AN API’S RESPONSES… …WITH SELF-DESCRIPTIVENESS  OF AN API’S FUNCTIONALITY “This is a photo  of Ellen.” “This API takes an image,  returns a black & white version.”

34. …TO COMPLETE THE INTERACTION  THAT HYPERMEDIA COULDN’T.

35. HYPERMEDIA can be the engine of a single API. HYPERMEDIA cannot the engine of cross-API interactions.

36. BUILDING API DESCRIPTIONS is thinking about Web APIs on a Web scale.

Hypermedia Cannot be the Engine

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