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This guide walks you through the process of applying circuit breakers to potentially-failing method calls using the Netflix Hystrix fault tolerance library.
You’ll build a microservice application that uses the Circuit Breaker pattern to gracefully degrade functionality when a method call fails. Use of the Circuit Breaker pattern can allow a microservice to continue operating when a related service fails, preventing the failure from cascading and giving the failing service time to recover.
bookstore/build.gradle
link:https://raw.githubusercontent.com/spring-guides/gs-circuit-breaker/master/initial/bookstore/build.gradle[role=include]reading/build.gradle
link:https://raw.githubusercontent.com/spring-guides/gs-circuit-breaker/master/initial/reading/build.gradle[role=include]bookstore/pom.xml
link:https://raw.githubusercontent.com/spring-guides/gs-circuit-breaker/master/initial/bookstore/pom.xml[role=include]reading/pom.xml
link:https://raw.githubusercontent.com/spring-guides/gs-circuit-breaker/master/initial/reading/pom.xml[role=include]The Bookstore service will have a single endpoint. It will be accessible at /recommended, and will (for simplicity) return a String recommended reading list.
Edit our main class, in BookstoreApplication.java. It should look like this:
bookstore/src/main/java/hello/BookstoreApplication.java
link:complete/bookstore/src/main/java/hello/BookstoreApplication.java[role=include]The @RestController annotation marks BookstoreApplication as a controller class, like @Controller does, and also ensures that @RequestMapping methods in this class will behave as though annotated with @ResponseBody. That is, the return values of @RequestMapping methods in this class will be automatically converted appropriately from their original types and will be written directly to the response body.
We’re going to run this application locally alongside a client service application, so in src/main/resources/application.properties, set server.port so that the Bookstore service won’t conflict with the client when we get that running.
bookstore/src/main/resources/application.properties
link:complete/bookstore/src/main/resources/application.properties[role=include]The Reading application will be our front-end (as it were) to the Bookstore application. We’ll be able to view our reading list there at /to-read, and that reading list will be retrieved from the Bookstore service application.
reading/src/main/java/hello/ReadingApplication.java
package hello;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.client.RestTemplate;
import java.net.URI;
@RestController
@SpringBootApplication
public class ReadingApplication {
@RequestMapping("/to-read")
public String readingList() {
RestTemplate restTemplate = new RestTemplate();
URI uri = URI.create("http://localhost:8090/recommended");
return restTemplate.getForObject(uri, String.class);
}
public static void main(String[] args) {
SpringApplication.run(ReadingApplication.class, args);
}
}To get the list from Bookstore, we’re using Spring’s RestTemplate template class. RestTemplate makes an HTTP GET request to the Bookstore service’s URL as we provide it and then returns the result as a String. (For more information on using Spring to consume a RESTful service, see the Consuming a RESTful Web Service guide.)
Add the server.port property to src/main/resources/application.properties:
reading/src/main/resources/application.properties
link:complete/reading/src/main/resources/application.properties[role=include]We now can access, in a browser, the /to-read endpoint on our Reading application, and see our reading list. Yet since we rely on the Bookstore application, if anything happens to it, or if Reading is simply unable to access Bookstore, we’ll have no list and our users will get a nasty HTTP 500 error message.
Netflix’s Hystrix library provides an implementation of the Circuit Breaker pattern: when we apply a circuit breaker to a method, Hystrix watches for failing calls to that method, and if failures build up to a threshold, Hystrix opens the circuit so that subsequent calls automatically fail. While the circuit is open, Hystrix redirects calls to the method, and they’re passed on to our specified fallback method.
Spring Cloud Netflix Hystrix looks for any method annotated with the @HystrixCommand annotation, and wraps that method in a proxy connected to a circuit breaker so that Hystrix can monitor it. This currently only works in a class marked with @Component or @Service, so in the Reading application, under src/main/java/hello, add a new class: BookService.
Move the RestTemplate method to BookService. The complete class should look like this:
reading/src/main/java/hello/BookService.java
link:complete/reading/src/main/java/hello/BookService.java[role=include]We’ve applied @HystrixCommand to our original readingList() method. We also have a new method here: reliable(). The @HystrixCommand annotation has reliable as its fallbackMethod, so if, for some reason, Hystrix opens the circuit on readingList(), we’ll have an excellent (if short) placeholder reading list ready for our users.
In our main class, ReadingApplication, let’s inject the BookService and switch to calling it for our reading list:
reading/src/main/java/hello/ReadingApplication.java
link:complete/reading/src/main/java/hello/ReadingApplication.java[role=include]Now, to retrieve the list from the Bookstore service, we call bookService.readingList(). You’ll also notice that we’ve added one last annotation, @EnableCircuitBreaker; that’s necessary to tell Spring Cloud that the Reading application uses circuit breakers and to enable their monitoring, opening, and closing (behavior supplied, in our case, by Hystrix).
Run both the Bookstore service and the Reading service, and then open a browser to the Reading service, at localhost:8080/to-read. You should see the complete recommended reading list:
Spring in Action (Manning), Cloud Native Java (O'Reilly), Learning Spring Boot (Packt)
Now shut down the Bookstore application. Our list source is gone, but thanks to Hystrix and Spring Cloud Netflix, we have a reliable abbreviated list to stand in the gap; you should see:
Cloud Native Java (O'Reilly)
Congratulations! You’ve just developed a Spring application that uses the Circuit Breaker pattern to protect against cascading failures and to provide fallback behavior for potentially failing calls.
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