Declarative Clients in Spring

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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

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About Rossen Spring Framework and Spring for GraphQL teams Spring MVC, WebFlux, WebSocket, RSocket

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What is a declarative client?

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Sample applications Customer Service Veriﬁcation Service HTTP POST /verify Customer Veriﬁcation Result

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Direct use of HTTP client

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Client interface

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Implementation

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Implementation with helper methods

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Client interface usage

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What if we could just create an annotated interface and have a library/ framework translate it into actual HTTP client requests?

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Declarative/ interface client Client interface method calls HTTP calls Proxy

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Feign

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OpenFeign/Feign

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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

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Spring Cloud OpenFeign

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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

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A declarative/ interface client

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A declarative/ interface client use

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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

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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

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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

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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

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RetroSocket Interface client for RSocket ● Incubator project

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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

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HTTP & RSocket Interface Clients in Spring Framework 6.0

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Interface clients are not new in the Spring Framework Same mechanism, diﬀerent protocols

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Why HTTP and RSocket now? Discussions with Spring Cloud around OpenFeign, Retroﬁt, and RSocket integrations

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Spring Framework 6.0 already had plans for RSocket interface client

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Old request for HTTP, deferred to… Spring Cloud OpenFeign

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Old request for HTTP, deferred to… Spring Cloud OpenFeign

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Opportunity to revisit Underlying mechanism is not speciﬁc to Spring Cloud Cloud features are extensions on top

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Some issues can only be addressed in Spring Framework programming model vs reusing @RequestMapping

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Decided to explore for Spring Framework 6.0

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HTTP interface client programming model Relationship to server @RequestMapping?

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Two goals Deﬁne a single request (no wildcards of any kind!) Avoid outright server-speciﬁc

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@RequestMapping attributes For mapping of 1..N requests Any HTTP method Any URL path

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@RequestMapping method parameters ServletRequest HttpMethod @RequestHeader @RequestParam @RequestBody @RequestAttribute @ModelAttribute …

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@RequestMapping return values ResponseEntity HttpHeaders Body void Model View SseEmitter …

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New method annotation to declare rather than map requests @RequestMapping @HttpExchange

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@HttpExchange attributes url method contentType accept

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@HttpExchange method parameters Subset of those for @RequestMapping i.e. reusing method parameter annotations

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@HttpExchange method parameters URI HttpMethod @RequestHeader @PathVariable @RequestBody @RequestParam @CookieValue

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@HttpExchange return values Subset of those for @RequestMapping i.e. also using return value types

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Blocking

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Reactive

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status, headers, body

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headers

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execute only

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Spring Boot can declare this as a bean

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RSocket interface client planned for Spring Framework 6.0 M5

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Resources ● Demo code ● Spring Cloud OpenFeign ● Spring Framework documentation for HTTP interface client ● Spring Cloud Square ● RetroSocket