一、概念

1、什么是服务降级

有了服务的熔断，随之就会有服务的降级，所谓服务降级，就是当某个服务熔断之后，服务端提供的服务将不再被调用，此时由客户端自己准备一个本地的fallback回调，返回一个默认值来代表服务端的返回; 这种做法，虽然不能得到正确的返回结果，但至少保证了服务的可用，比直接抛出错误或者服务不可用要好很多。

二、如何进行服务降级

(1) 服务端

1、POM

  1.   <dependencies>
  2.         <!--hystrix-->
  3.         <dependency>
  4.             <groupId>org.springframework.cloud</groupId>
  5.             <artifactId>spring-cloud-starter-netflix-hystrix</artifactId>
  6.         </dependency>
  7.         <!--eureka client-->
  8.         <dependency>
  9.             <groupId>org.springframework.cloud</groupId>
  10.             <artifactId>spring-cloud-starter-netflix-eureka-client</artifactId>
  11.         </dependency>
  12.         <!--web-->
  13.         <dependency>
  14.             <groupId>org.springframework.boot</groupId>
  15.             <artifactId>spring-boot-starter-web</artifactId>
  16.         </dependency>
  17.         <dependency>
  18.             <groupId>org.springframework.boot</groupId>
  19.             <artifactId>spring-boot-starter-actuator</artifactId>
  20.         </dependency>
  21.         <dependency>
  22.             <groupId>com.tfjy.springcloud</groupId>
  23.             <artifactId>cloud-api-commons</artifactId>
  24.             <version>1.0-SNAPSHOT</version>
  25.         </dependency>
  26.         <dependency>
  27.             <groupId>org.springframework.boot</groupId>
  28.             <artifactId>spring-boot-devtools</artifactId>
  29.             <scope>runtime</scope>
  30.             <optional>true</optional>
  31.         </dependency>
  32.         <dependency>
  33.             <groupId>org.projectlombok</groupId>
  34.             <artifactId>lombok</artifactId>
  35.             <optional>true</optional>
  36.         </dependency>
  37.         <dependency>
  38.             <groupId>org.springframework.boot</groupId>
  39.             <artifactId>spring-boot-starter-test</artifactId>
  40.             <scope>test</scope>
  41.         </dependency>
  42.     </dependencies>

2、YML

  1. server:
  2.   port: 8001
  4. spring:
  5.   application:
  6.     name: cloud-provider-hystrix-payment
  8. eureka:
  9.   client:
  10.     register-with-eureka: true
  11.     fetch-registry: true
  12.     service-url:
  13.       #defaultZone: http://eureka7001.com:7001/eureka,http://eureka7002.com:7002/eureka
  14.       defaultZone: http://localhost:7001/eureka/

3、主启动

需要在主启动上加上
image.png

4、业务类

controller

  1.     @GetMapping("/payment/hystrix/timeout/{id}")
  2.     public String paymentInfo_TimeOut(@PathVariable("id") Integer id){
  3.         String result = paymentService.paymentInfo_TimeOut(id);
  4.         log.info("*****result"+result);
  5.         return result;
  6.     }

service

  1.   @HystrixCommand(fallbackMethod = "paymentInfo_TimeOutHandler",commandProperties = {
  2.             @HystrixProperty(name="execution.isolation.thread.timeoutInMilliseconds",value="5000")
  3.     })
  4.     public String paymentInfo_TimeOut(Integer id){
  5.        try{
  6.            TimeUnit.SECONDS.sleep(5);
  7.         }catch (InterruptedException e){
  8.            e.printStackTrace();
  9.         }
  10.         return "线程池:"+Thread.currentThread().getName()+"paymentInfo_TimeOut,id"+id+"O(∩_∩)O哈哈~"+"耗时5秒钟";
  11.     }
  13.     public String paymentInfo_TimeOutHandler(Integer id){
  14.         return "线程池:"+Thread.currentThread().getName()+"paymentInfo_TimeOutHandler,id"+id+"o(╥﹏╥)o";
  15.     }

在代码上加上HystrixCommand注解，然后由个属性fallbackMethod，value值可以填写方法的名字。
当发生超时的时候，就会走下面的方法。

（2）客户端

服务降级 - 图2

1、POM

  1. <dependencies>
  2.     <!--openfeign-->
  3.     <dependency>
  4.         <groupId>org.springframework.cloud</groupId>
  5.         <artifactId>spring-cloud-starter-openfeign</artifactId>
  6.     </dependency>
  7.     <!--hystrix-->
  8.     <dependency>
  9.         <groupId>org.springframework.cloud</groupId>
  10.         <artifactId>spring-cloud-starter-netflix-hystrix</artifactId>
  11.     </dependency>
  12.     <!--eureka client-->
  13.     <dependency>
  14.         <groupId>org.springframework.cloud</groupId>
  15.         <artifactId>spring-cloud-starter-netflix-eureka-client</artifactId>
  16.     </dependency>
  17.     <!-- 引入自己定义的api通用包，可以使用Payment支付Entity -->
  18.     <dependency>
  19.         <groupId>com.tfjy.springcloud</groupId>
  20.         <artifactId>cloud-api-commons</artifactId>
  21.         <version>1.0-SNAPSHOT</version>
  22.     </dependency>
  23.     <!--web-->
  24.     <dependency>
  25.         <groupId>org.springframework.boot</groupId>
  26.         <artifactId>spring-boot-starter-web</artifactId>
  27.     </dependency>
  28.     <dependency>
  29.         <groupId>org.springframework.boot</groupId>
  30.         <artifactId>spring-boot-starter-actuator</artifactId>
  31.     </dependency>
  32.     <!--一般基础通用配置-->
  33.     <dependency>
  34.         <groupId>org.springframework.boot</groupId>
  35.         <artifactId>spring-boot-devtools</artifactId>
  36.         <scope>runtime</scope>
  37.         <optional>true</optional>
  38.     </dependency>
  39.     <dependency>
  40.         <groupId>org.projectlombok</groupId>
  41.         <artifactId>lombok</artifactId>
  42.         <optional>true</optional>
  43.     </dependency>
  44.     <dependency>
  45.         <groupId>org.springframework.boot</groupId>
  46.         <artifactId>spring-boot-starter-test</artifactId>
  47.         <scope>test</scope>
  48.     </dependency>
  49. </dependencies>

2、YML

  1. server:
  2.   port: 80
  4. eureka:
  5.   client:
  6.     register-with-eureka: false
  7.     service-url:
  8.       defaultZone: http://localhost:7001/eureka/
  10. feign:
  11.   hystrix:
  12.     enabled: true

3、主启动

image.png

4、业务

  1.     @GetMapping("/consumer/payment/hystrix/timeout/{id}")
  2.     @HystrixCommand(fallbackMethod = "paymentTimeOutFallbackMethod",commandProperties = {
  3.             @HystrixProperty(name="execution.isolation.thread.timeoutInMilliseconds",value="1500")
  4.     })
  5. //    @HystrixCommand
  6.     public String paymentInfo_TimeOut(@PathVariable("id") Integer id)
  7.     {
  8.         int age = 10/0;
  9.         String result = paymentHystrixService.paymentInfo_TimeOut(id);
  10.         return result;
  11.     }
  12.     public String paymentTimeOutFallbackMethod(@PathVariable("id") Integer id)
  13.     {
  14.         return "我是消费者80,对方支付系统繁忙请10秒钟后再试或者自己运行出错请检查自己,o(╥﹏╥)o";
  15.     }
  1. @Component
  2. @FeignClient(value = "CLOUD-PROVIDER-HYSTRIX-PAYMENT",fallback = PaymentFallbackService.class)
  3. public interface PaymentHystrixService {
  5.     @GetMapping("/payment/hystrix/ok/{id}")
  6.     public   String paymentInfo_ok(@PathVariable("id") Integer id);
  8.     @GetMapping("/payment/hystrix/timeout/{id}")
  9.     public   String paymentInfo_TimeOut(@PathVariable("id") Integer id);
  10. }

这个参数要一样

这个只能等待1.5s但是在8001要5s
image.png

三、全局配置

  1. @RestController
  2. @Slf4j
  3. @DefaultProperties(defaultFallback = "payment_Global_FallbackMethod")
  4. public class OrderHystirxController {
  5.    @GetMapping("/consumer/payment/hystrix/timeout/{id}")
  6. //    @HystrixCommand(fallbackMethod = "paymentTimeOutFallbackMethod",commandProperties = {
  7. //            @HystrixProperty(name="execution.isolation.thread.timeoutInMilliseconds",value="1500")
  8. //    })
  9.     @HystrixCommand
  10.     public String paymentInfo_TimeOut(@PathVariable("id") Integer id)
  11.     {
  12.         int age = 10/0;
  13.         String result = paymentHystrixService.paymentInfo_TimeOut(id);
  14.         return result;
  15.     }
  16.     public String paymentTimeOutFallbackMethod(@PathVariable("id") Integer id)
  17.     {
  18.         return "我是消费者80,对方支付系统繁忙请10秒钟后再试或者自己运行出错请检查自己,o(╥﹏╥)o";
  19.     }
  21.     // 下面是全局fallback方法
  22.     public String payment_Global_FallbackMethod()
  23.     {
  24.         return "Global异常处理信息，请稍后再试，/(ㄒoㄒ)/~~";
  25.     }
  26. }

@DefaultProperties(defaultFallback = “payment_Global_FallbackMethod”)

1:1 每个方法配置一个服务降级方法，技术上可以，实际上傻X
1:N 除了个别重要核心业务有专属，其它普通的可以通过@DefaultProperties(defaultFallback = “”) 统一跳转到统一处理结果页面

通用和独享的各自分开，避免了代码膨胀，合理减少了代码量
image.png

三、哪些情况会发生服务降级

(1) 程序运行异常 (当我们把service方法中改成int i =10/0)
(2) 超时
(3) 服务熔断触发服务降级
(4) 线程池/信号量也会导致服务降级