How to log the reason that hystrix fallback method invoked

Question

I am creating a REST client using Fiegn. I have got my calls working, but I want to log the exception whitch tigger the fallback method invoked.
Code like this：

public interface FooService {
    Foo queryFoo(Integer fooId);
}

public interface FooServiceFallback implements FooService {
    @Override
    Foo queryFoo(Integer fooId) {
        return new Foo();
    }
}

@Configuration
public class FooServiceConfiguration {
    @Bean
    public FooService() {
        return HystrixFeign.builder().[...].target(FooService.class, "http://xxx", FooServiceFallback.class);
    }
}

The fallback method can be invoked when exception occurs, but noting is logged.

How to log the exception which trigger the fallback method invoked？
Like connectionTimeoutException.

Answer 1

Fallback method can take an extra argument of type Throwable, which will indicate the cause.

For example if your method is like this

@HystrixCommand(fallbackMethod = "fallbackMethod")
public String mainMethod(String s) {
 .....
}

Your fallback method can be like this

public String fallbackMethod(String s) {
     ......
}

or

public String fallbackMethod(String s, Throwable throwable) {
     //log the cause using throwable instance
     ......
}

In your case use the second one.

EDIT:

If you are using HystrixFeign, this is how you do it. You should make use of FallbackFactory

@Component
public class FooServiceFallbackFactory implements FallbackFactory<FooService> {

    @Override
    public FooService create(Throwable throwable) {
        return new FooServiceFallback(throwable);
    }

}

Your fallback class would look like

@Component
public class FooServiceFallback implements FooService {

   private final Throwable cause;

   public FooServiceFallback(Throwable cause) {
      this.cause = cause;
   }

   @Override
   Foo queryFoo(Integer fooId) {
       //You have access to cause now, which will have the real exception thrown
   }

}

Also you need to change a bit of your configuration class

@Configuration
public class FooServiceConfiguration {
    @Bean
    public FooService() {
        return HystrixFeign.builder().[...].target(FooService.class, "http://xxx", FooServiceFallbackFactory.class);
    }
}