Spring Feign Client

Often a service needs to call Http endpoints , Feign comes from the OpenFeign project makes it easier to call http endpoints in a declarative fashion.
Spring has openfeign integraton through its Spring Cloud OpenFeign integration.

How To include feign clients

The actual project for feign is OpenFeign https://github.com/OpenFeign/feign . Spring comes with it's own starter . Add below dependency to the project

    <dependency>      <groupId>org.springframework.cloud</groupId>      <artifactId>spring-cloud-starter-openfeign</artifactId>    </dependency>

For an example let's create a service named feign-serviceA which will call feign-serviceB . feign-serviceB exposes a GET endpoint /hello which returns a simple hello response body.

In order to call the endpoint using a feign client from feign-serviceA we need to do following -

• Create an interface annotated with @FeignClient like below

@FeignClient(url = "<host:port for serviceB>", name = "serviceB")public interface ServiceBClient{    @GetMapping("/hello")    public ResponseEntity<String> sayHello();}

• Add @EnableFeignClients annotation to main class .

and we are all set we can simply autowire ServiceBClient and call method sayHello like any normal method call .

@EnableFeignClients takes a list of clients if we have many feign clients then it is better to mention the clients as argument otherwise spring will do a classpath scan to find out feign clients.

Feign Configuration

Under the hood feign comes with some components which are used to make a call to remote endpoints and encode/decode request response .

• Client - To make HTTP call feign requires http client. By default openfeign comes with a Default Client. We can override it with ApacheHttpClient, OkHttpClient or ApacheHC5FeignClient . These feign clients are wrapper around a delegate client. For example ApacheHttpClient wraps a httpcomponents httpclient and converts the response to feign response.

• Decoder - Something needs to convert the feign Response to the actual type of the feign method's return type. Decoders are that instruments. By default spring provides an OptionalDecoder which delegates to ResponseEntityDecoder which further delegates to SpringDecoder. We can override it by defining a bean of Decoder .

• Encoder - We call feign methods by passing objects to it something needs to convert it to http request body. Encoder does that job. Again By default spring provides SpringEncoder.

Along side above components there are also support for caching, metrics provided by spring feign starter .

We can create a configuration class and override the defaults for the above components.
If we want to override default for single components @Feign accepts configuration arguments which we can use to define custom override for default values.

Retry

Feign has baked in support for retry mechanism. However by default it uses Retry.NEVER_RETRY . For example we can create a custom retryer which will retry any status code > 400. Below is the code for our CustomRetryer .

public class CustomRetryer extends Retryer.Default{    public CustomRetryer(long period, long maxPeriod, int maxAttempts){        super(period, maxPeriod, maxAttempts);    }    @Override    public void continueOrPropagate(RetryableException e){        log.info("Going to retry for ", e);        super.continueOrPropagate(e);    }    @Override    public Retryer clone(){        return new CustomRetryer(5,SECONDS.toMillis(1), 5);    }}

One important fact is feign Retry works either on IOException or RetryableException thrown from some errorDecoder . Below is how a custom decoder looks like -

    @Bean    public ErrorDecoder errorDecoder(){        return (methodKey, response) -> {            byte[] body = {};            try {                if (response.body() != null) {                    body = Util.toByteArray(response.body().asInputStream());                }            } catch (IOException ignored) { // NOPMD            }            FeignException exception = new FeignException.BadRequest(response.reason(), response.request(), body, response.headers());            if (response.status() >= 400) {                return new RetryableException(                        response.status(),                        response.reason(),                        response.request().httpMethod(),                        exception,                        Date.from(Instant.now().plus(15, ChronoUnit.MILLIS)),                        response.request());            }            return exception;        };    }

Though documentation says feign retries on IOException internally when an IOException occurs it wraps it in a RetryableException.

Support for resiliency

One form of resiliency through retries we saw in last section. Spring has CircuitBreaker support for feign . It achieves it through a separate Feign builder FeignCircuitBreaker.Builder . The actual implementation of circuitbreaker comes from resilience4j library.

Interceptor

Sometimes we want to modify the request by adding some extra information. For example we may add some header for each request. We can achieve this by using RequestInterceptor. For experiment I added below interceptor which populates a header userid.

    @Bean    public RequestInterceptor userIdRequestInterceptor(){        return (template) -> {            template.header("userid", "somerandomtext");        };    }

feign-serviceB reads this header and returns back as header. If we do a curl we get below response

< HTTP/1.1 200< connection: keep-alive< date: Sat, 20 Aug 2022 15:27:47 GMT< keep-alive: timeout=60< userid: somerandomtext< Content-Type: text/plain;charset=UTF-8< Content-Length: 7<* transfer closed with 7 bytes remaining to read

We see userid is added to the response.
A very useful application for interceptor is when feign has to send oauth2 access token . Out of the spring provides a OAuth2FeignRequestInterceptor which adds access token for each request.

Client side loadbalancing support

From spring boot 2.4.0 feign has integration with spring-cloud-loadbalancer which can fetch client url info from various service discovery providers and make that info available to feign .

Usage of feign simplifies various aspect of making http request. In a typical production environment we may need to override several components like clients, decoder, errorDecoder etc . Also within Spring ecosystem feign is nicely integrated with resiliency, loadbalancing , metrics etc which makes it automatic choice when we are working in a microservices architecture .