Spring Under the hood-Inside Feign and client side discovery with consul

Feign using consul as discovery service is easy to setup thanks to Spring Autoconfigurations, but let’s see how this magic is actually done -

Spring cloud open feign comes with FeignLoadBalancerAutoConfiguration . If we see closely it imports three more LoadBalancerAutoConfigurations

  • HttpClientFeignLoadBalancerConfiguration, OkHttpFeignLoadBalancerConfiguration, DefaultFeignLoadBalancerConfiguration

  • If classpath has Apache http client then it uses HttpClientFeignLoadBalancerConfiguration , if path has Ok http client then it uses OkHttpFeignLoadBalancerConfiguration otherwise uses it DefaultFeignLoadBalancerConfiguration which in turn uses Apache Http client.

  • The HttpClientFeignLoadBalancerConfiguration creates a FeignBlockingLoadBalancerClient which uses an instance of LoadBalancerClient.

  • FeignBlockingLoadBalancerClient is a wrapper around ApacheHttpClient vs OkHttpClient.

  • If we look into the class of LoadBalancerClient we see it is an interface and it represents a client side load balancer.

  • spring cloud consul starter brings spring cloud loadbalancer which provides one of the implementations BlockingLoadBalancerClient .

  • BlockingLoadBalancerClient further uses by defaut RoundRobinLoadBalancer .

  • RoundRobinLoadbalancer make use of ServerInstanceListSupplier which provides the list of server.

  • By default we get CachingServiceInstanceListSupplier from LoadBalancerClientConfig. Which in turn uses ConsulDiscoveryClient.

  • ConsulDiscoveryClient is an implementation of DiscoveryClient which reads from service registry .

Thus from the above flow of various configurations Feign gets integrated with Consul and discovers the actual service url to call.