Spring Cloud - fusing mechanism - breaker Spring Cloud Start Tutorial (VII): fuse mechanism - Breaker Breaker (Curcuit Breaker) mode

Spring Cloud Getting Started tutorial (VII): fuse mechanism - Circuit Breakers

Circuit breaker model is not clear, you can see another blog post: Breaker (Curcuit Breaker) mode , directly below describes how to use Spring Cloud breaker.

SpringCloud Netflix realized the name of the circuit breaker at the library called Hystrix micro-service architecture, usually have multiple levels of service calls is below incognito architecture, browser API access backstage pass a schematic diagram of the micro-services..:

A micro-services timeout failure could lead to a chain reaction waterfall, the figure below, Hystrix by independent feedback circuit breakers implemented to prevent this from happening.

Figure B service fails for some reason, becomes unavailable, all calls to the service B timeout. B when the call fails to reach a particular threshold value (within 5 seconds occurs 20 times the default value Hystrix failure is defined above), a link will be in the open state, then all calls to all service B will not It is executed, replaced by a circuit breaker provided a link open message Fallback representation. Hystrix provides the appropriate mechanism that allows developers to define the Fallbak news.

open link blocked waterfall error, can be submerged or let the wrong time to repair service. This fallback may call another Hystrix protection, static data, or legally null values. Fallbacks can form a chain structure, so that the bottom of the first call Fallback other business services to return static data.

Below, we entered, adding circuit breakers in the previous two HELLO WORLD cluster service, which prevent hang after a Hello world, leading to system failure occurs chain timeout.

Add hystrix library support breaker 1. maven project (described in the previous section Ribbon or Feign engineering) of the pom.xml

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

2. Use a circuit breaker in the Ribbon application

1) Adding @EnableCircuitBreaker comment on Spring Boot startup class

1 @SpringBootApplication
2 @EnableDiscoveryClient
3 @EnableCircuitBreaker
4 public class ServiceRibbonApplication {
5 
6     public static void main(String[] args) {
7         SpringApplication.run(ServiceRibbonApplication.class, args);
8     }
。。。 。。。

2). Tagging access services with @HystrixCommand comment

 1 @Service
 2 public class HelloService {
 3     @Autowired RestTemplate restTemplate;
 4 
 5     @HystrixCommand(fallbackMethod = "serviceFailure")
 6     public String getHelloContent() {
 7         return restTemplate.getForObject("http://SERVICE-HELLOWORLD/",String.class);
 8     }
 9 
10     public String serviceFailure() {
11         return "hello world service is not available !";
12     }
13 }

@HystrixCommand annotations define a circuit breaker, which encapsulates getHelloContant () method, when SERVICE-HELLOWORLD access failures reaches its threshold, will not make the call SERVICE-HELLOWORLD, instead return a method defined by fallbackMethod serviceFailure ( ). @HystrixCommand fallbackMethod method defined annotations, you need to pay special attention to two points:

第一， fallbackMethod的返回值和参数类型需要和被@HystrixCommand注解的方法完全一致。否则会在运行时抛出异常。比如本例中，serviceFailure（）的返回值和getHelloContant()方法的返回值都是String。

第二，当底层服务失败后，fallbackMethod替换的不是整个被@HystrixCommand注解的方法（本例中的getHelloContant), 替换的只是通过restTemplate去访问的具体服务。可以从中的system输出看到，即使失败，控制台输出里面依然会有“call SERVICE-HELLOWORLD”。

启动eureka服务，只启动两个Helloworld服务，然后中断其中一个（模拟其中一个微服务挂起），访问http://localhost:8901/然后刷新, 由于有负载均衡可以看到以下两个页面交替出现。可以看到第二个被挂起的服务，被定义在Ribbon应该里面的错误处理方法替换了。

4. 在Feign应用中使用断路器

1）. Feign内部已经支持了断路器，所以不需要想Ribbon方式一样，在Spring Boot启动类上加额外注解

2）. 用@FeignClient注解添加fallback类，该类必须实现@FeignClient修饰的接口。

1 @FeignClient(name = "SERVICE-HELLOWORLD", fallback = HelloWorldServiceFailure.class)
2 public interface HelloWorldService {
3     @RequestMapping(value = "/", method = RequestMethod.GET)
4     public String sayHello();
5 }

3). 创建HelloWorldServiceFailure类，必须实现被@FeignClient修饰的HelloWorldService接口。注意添加@Component或者@Service注解，在Spring容器中生成一个Bean

1 @Component
2 public class HelloWorldServiceFailure implements HelloWorldService {
3     @Override
4     public String sayHello() {
5         System.out.println("hello world service is not available !");
6         return "hello world service is not available !";
7     }
8 }

4）. Spring Cloud之前的Brixton版本中，Feign是缺省是自动激活了断路器的，但最近的Dalston版本已经将缺省配置修改为禁止。

原因参见： https://github.com/spring-cloud/spring-cloud-netflix/issues/1277，这一点要注意。所以要在Feign中使用断路器，必须在application.yml中添加如下配置：

feign:
   hystrix:
     enabled: true

5）. 启动Feign应用，访问http://localhost：8902/hello，可以一看到和Ribbon一样的效果。

对断路器模式不太清楚的话，可以参看另一篇博文：断路器（Curcuit Breaker）模式，下面直接介绍Spring Cloud的断路器如何使用。

SpringCloud Netflix实现了断路器库的名字叫Hystrix. 在微服务架构下，通常会有多个层次的服务调用. 下面是微服架构下, 浏览器端通过API访问后台微服务的一个示意图：

一个微服务的超时失败可能导致瀑布式连锁反映，下图中，Hystrix通过自主反馈实现的断路器，防止了这种情况发生。

图中的服务B因为某些原因失败，变得不可用，所有对服务B的调用都会超时。当对B的调用失败达到一个特定的阀值(5秒之内发生20次失败是Hystrix定义的缺省值), 链路就会被处于open状态，之后所有所有对服务B的调用都不会被执行，取而代之的是由断路器提供的一个表示链路open的Fallback消息. Hystrix提供了相应机制，可以让开发者定义这个Fallbak消息.

open的链路阻断了瀑布式错误，可以让被淹没或者错误的服务有时间进行修复。这个fallback可以是另外一个Hystrix保护的调用, 静态数据，或者合法的空值. Fallbacks可以组成链式结构，所以，最底层调用其它业务服务的第一个Fallback返回静态数据.

下面，进入正题，在之前的两HELLO WORLD服务集群中加入断路器，防止其中一个Hello world挂掉后，导致系统发生连锁超时失败。

1. 在maven工程（前面章节中介绍的Ribbon或者Feign工程）的pom.xml中添加hystrix库支持断路器

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

2. 在Ribbon应用中使用断路器

1). 在Spring Boot启动类上添加@EnableCircuitBreaker注解

1 @SpringBootApplication
2 @EnableDiscoveryClient
3 @EnableCircuitBreaker
4 public class ServiceRibbonApplication {
5 
6     public static void main(String[] args) {
7         SpringApplication.run(ServiceRibbonApplication.class, args);
8     }
。。。 。。。

2). 用@HystrixCommand注解标注访问服务的方法

 1 @Service
 2 public class HelloService {
 3     @Autowired RestTemplate restTemplate;
 4 
 5     @HystrixCommand(fallbackMethod = "serviceFailure")
 6     public String getHelloContent() {
 7         return restTemplate.getForObject("http://SERVICE-HELLOWORLD/",String.class);
 8     }
 9 
10     public String serviceFailure() {
11         return "hello world service is not available !";
12     }
13 }

@HystrixCommand注解定义了一个断路器，它封装了getHelloContant()方法，当它访问的SERVICE-HELLOWORLD失败达到阀值后，将不会再调用SERVICE-HELLOWORLD，取而代之的是返回由fallbackMethod定义的方法serviceFailure（）。@HystrixCommand注解定义的fallbackMethod方法，需要特别注意的有两点：

4. 在Feign应用中使用断路器

1）. Feign内部已经支持了断路器，所以不需要想Ribbon方式一样，在Spring Boot启动类上加额外注解

2）. 用@FeignClient注解添加fallback类，该类必须实现@FeignClient修饰的接口。

1 @FeignClient(name = "SERVICE-HELLOWORLD", fallback = HelloWorldServiceFailure.class)
2 public interface HelloWorldService {
3     @RequestMapping(value = "/", method = RequestMethod.GET)
4     public String sayHello();
5 }

3). 创建HelloWorldServiceFailure类，必须实现被@FeignClient修饰的HelloWorldService接口。注意添加@Component或者@Service注解，在Spring容器中生成一个Bean

1 @Component
2 public class HelloWorldServiceFailure implements HelloWorldService {
3     @Override
4     public String sayHello() {
5         System.out.println("hello world service is not available !");
6         return "hello world service is not available !";
7     }
8 }

4). Brixton in versions prior to Spring Cloud, Feign default is automatically activated a circuit breaker, but recent Dalston version has been amended to prohibit the default configuration.

See reasons: https://github.com/spring-cloud/spring-cloud-netflix/issues/1277 , we should pay attention to. So to use a circuit breaker in Feign, you must add the following configuration in application.yml in:

feign:
   hystrix:
     enabled: true

5) Start Feign application, visit http: // localhost: 8902 / hello, and you can see a Ribbon same effect.

Spring Cloud - fusing mechanism - breaker Spring Cloud Start Tutorial (VII): fuse mechanism - Breaker Breaker (Curcuit Breaker) mode

Spring Cloud Getting Started tutorial (VII): fuse mechanism - Circuit Breakers

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