Declarative Clients in Spring

What is a declarative HTTP client vs an imperative one? Why are they useful and why should I care? We’ll talk about options that are available in the Spring Portfolio and what is coming on the horizon.

In this session, we will look at the history of declarative clients in Spring via Spring Cloud OpenFeign. We will also dive into upcoming options that are coming to Spring and the advantages that these new technologies bring to the developer experience.

Transcript

1. Olga Maciaszek-Sharma Rossen Stoyanchev Declarative Clients in Spring Copyright © 2020 VMware, Inc. or its affiliates.
2. About Olga Software Engineer in Spring Cloud team Spring Cloud LoadBalancer, Spring Cloud OpenFeign, Spring Cloud Netflix, Spring Cloud Contract, Spring Cloud Square, RSocket HTTP Bridge
3. About Rossen Spring Framework and Spring for GraphQL teams Spring MVC, WebFlux, WebSocket, RSocket
4. What is a declarative client?
5. Sample applications Customer Service Verification Service HTTP POST /verify Customer Verification Result
6. Direct use of HTTP client
7. Client interface
8. Implementation
9. Implementation with helper methods
10. Client interface usage
11. What if we could just create an annotated interface and have a library/ framework translate it into actual HTTP client requests?
12. Declarative/ interface client Client interface method calls HTTP calls Proxy
13. Feign
14. OpenFeign/Feign
15. Feign Java to HTTP client binder • Core part of a former Spring Cloud Netflix module, passed over to the community • Supports various HTTP clients, including Apache HTTP, Apache HC5, Ok HTTP • Contracts - support for custom annotations and argument resolution • Support for a broad range of encoders/ decoders, including Jackson and GSON • Metrics, including Micrometer support
16. Spring Cloud OpenFeign
17. Spring Cloud OpenFeign Spring MVC and Spring Cloud support for OpenFeign • Spring MVC annotations support • Auto-configurations for Feign-specific beans • Spring Cloud LoadBalancer support • Spring Cloud CircuitBreaker support • Tracing support
18. A declarative/ interface client
19. A declarative/ interface client use
20. Spring Cloud OpenFeign Demo Applications communicating over HTTP Customer Service SC OpenFeign Client SC LoadBalancer SC Sleuth Verification Service POST /verify Customer Verification Result Eureka Service Registry
21. Spring Cloud OpenFeign - popularity Highly-readable interface clients with robust integrations • 797 GitHub stars • 8670549 Maven downloads in 2021 • Nearly flat learning curve for Spring MVC users
22. Spring Cloud OpenFeign - issues • Issues with reusing client interfaces as controllers ◦ @RequestMapping at class level user issues (GH-547, GH-678) ◦ CVE-2021-22044 • Maintenance issues caused by dependence on third party upstream project • Lack of non-blocking support
23. Spring Cloud Square Spring Boot and Spring Cloud support for Retrofit and OkHttp • WebClient support for Retrofit interface clients • Auto-configurations • Load-balancing • Tracing • Only Retrofit annotations • Incubator project
24. RetroSocket Interface client for RSocket • Incubator project
25. Considerations More third-party integrations or an in-house solution? • HTTP/reactive, RSocket/RetroSocket - converging solutions • Opportunity to create a first-class solution in the Spring Framework ◦ No maintenance dependence on third party projects • Previous Spring Cloud support built around external projects ◦ Possibility of providing integration on top of the Spring Framework interface clients
26. HTTP & RSocket Interface Clients in Spring Framework 6.0
27. Interface clients are not new in the Spring Framework Same mechanism, different protocols
28. Why HTTP and RSocket now? Discussions with Spring Cloud around OpenFeign, Retrofit, and RSocket integrations
29. Spring Framework 6.0 already had plans for RSocket interface client
30. Old request for HTTP, deferred to… Spring Cloud OpenFeign
31. Old request for HTTP, deferred to… Spring Cloud OpenFeign
32. Opportunity to revisit Underlying mechanism is not specific to Spring Cloud Cloud features are extensions on top
33. Some issues can only be addressed in Spring Framework programming model vs reusing @RequestMapping
34. Decided to explore for Spring Framework 6.0
35. HTTP interface client programming model Relationship to server @RequestMapping?
36. Two goals Define a single request (no wildcards of any kind!) Avoid outright server-specific
37. @RequestMapping attributes For mapping of 1..N requests Any HTTP method Any URL path
38. @RequestMapping method parameters ServletRequest HttpMethod @RequestHeader @RequestParam @RequestBody @RequestAttribute @ModelAttribute …
39. @RequestMapping return values ResponseEntity HttpHeaders Body void Model View SseEmitter …
40. New method annotation to declare rather than map requests @RequestMapping @HttpExchange
41. @HttpExchange attributes url method contentType accept
42. @HttpExchange method parameters Subset of those for @RequestMapping i.e. reusing method parameter annotations
43. @HttpExchange method parameters URI HttpMethod @RequestHeader @PathVariable @RequestBody @RequestParam @CookieValue
44. @HttpExchange return values Subset of those for @RequestMapping i.e. also using return value types
46. Blocking
47. Reactive
48. status, headers, body
49. headers
50. execute only
52. Spring Boot can declare this as a bean
53. RSocket interface client planned for Spring Framework 6.0 M5
54. Resources • Demo code • Spring Cloud OpenFeign • Spring Framework documentation for HTTP interface client • Spring Cloud Square • RetroSocket
55. Thank you Twitter: @olga_maciaszek, @rstoya05 © 2020 Spring. A VMware-backed project.