Microservice Resiliency

Feign Clients

https://github.com/jeffgbutler/microservice-workshop-kotlin/

https://jeffgbutler.github.io/microservice-workshop-kotlin/

What is Feign?

• Feign is a declarative REST client
• Originated at Netflix, now in a separate orgainzation (Open Feign)
• It is a method of calling a remote web service by defining the request and response only
• Feign handles all the marshalling/error handling/etc. for calling a remote service
• Spring Cloud Feign integrates Feign with the Spring web service annotations
• Fully integrated with Eureka and Hystrix
• Meant to be a sinpler alternative for Spring REST Template

Adding Feign to Spring Boot

• Available in Spring Initializr as a standard dependency
• Globally enabled with the @EnableFeignClients annotation on a configuration class

Example Feign Client

          
            @FeignClient(name = "movie-award-service", url = "http://localhost:8083")
            interface MovieAwardService {
            
                @GetMapping("/award/search")
                fun findAwardsForMovie(@RequestParam("movieId") movieId: Int): List
            }
          
        

This will call a web service with a URL like http://localhost:8080/award/search?movieId=3

Returned JSON will be automatically unmarshalled to the class you define

Handling 404

          
            @FeignClient(name = "movie-service", decode404 = true, url = "http://localhost:8081")
            interface MovieService {
            
                @GetMapping("/movie/{id}")
                fun findById(@PathVariable("id") id: Int): Movie?
            }
          
        

With decode404=true the code will return null if there is a 404 (NOT_FOUND) error

Application Integration

• Interfaces annotated with @FeignClient are automatically discovered and available in the Spring container
• If you wish to share service and model definitions between a client and server:
  • Create a common interface with just the mappings and path or request valiables
  • In the server, extend the common interface and add @RestController
  • In the client, extend the common interface and add @FeignClient

Exercise

Create Feign clients in the aggregate web service