As you may have guessed, the circuit breaker is a very important pattern when it comes to handling service failures. It is worth spending some more time understanding how to implement it. Let's take a hypothetical problem; say we have a movie recommendation service. Users will make a call to our service, which in turn gets the recommendation for the current user from a sophisticated machine learning implementation. In case the external recommendation service goes down, we still want our users to be able to get recommendations.

We will try implementing the following solution:

To get started, we will create a couple of Spring Boot applications, as will do in the rest of the book. Here is the recommendation engine service:

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.client.RestTemplate;

/***
* This class is responsible for applying machine learning algorithm for
* the current user and find out personalized algorithm.
*
*/
@RestController
public class RecommendationController {

  private final RestTemplate restTemplate;

  public RecommendationController(RestTemplate restTemplate) {
    this.restTemplate = restTemplate;
  }

  /***
  * This method calculates and return movies recommendation
  * @return Movies
  */
  @GetMapping("/movies")
  public String getRecommendedMovies() {
  // Applies number cruncher algo and come up with movies for logged in used
  // For sake of thie example we are returning fixed set of movies
    return("Movies Recommended for you. 
 1. Jumanji: Welcome to the Jungle. 
2. Inception 
3. The Dark  knight.");
  }

}

But we are more interested in the calling method, where we will implement our circuit breaker. We will include a Netflix Hystrix implementation to use the circuit breaker:

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

And here is the controller API and service class that implements the circuit breaker:

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.web.client.RestTemplateBuilder;
import org.springframework.cloud.client.circuitbreaker.EnableCircuitBreaker;
import org.springframework.context.annotation.Bean;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.client.RestTemplate;

@EnableCircuitBreaker
@RestController
@SpringBootApplication
public class ServiceoneApplication {

  @Autowired
  private RecommendationService recommendationService;

  public static void main(String[] args) {
    SpringApplication.run(ServiceoneApplication.class, args);
  }

  @Bean
  public RestTemplate rest(RestTemplateBuilder builder) {
    return builder.build();
  }

  @RequestMapping("/movies")
  public String getRecommendedMovies() {
    return recommendationService.recommend();
  }
}

The following is the for the recommendation service:

import com.netflix.hystrix.contrib.javanica.annotation.HystrixCommand;
import org.springframework.stereotype.Service;
import org.springframework.web.client.RestTemplate;

import java.net.URI;

@Service
public class RecommendationService {

  private final RestTemplate restTemplate;

  public RecommendationService(RestTemplate rest) {
    this.restTemplate = rest;
  }

  @HystrixCommand(fallbackMethod = "reliable")
  public String recommend() {
    URI uri = URI.create("http://localhost:8081/movies");
    return this.restTemplate.getForObject(uri, String.class);
  }

  public String reliable() {
    return "Top 3 Movies for the month. 
 1. Batman Begins
 2. Interstellar
 3. Justice League. ";
  }

}