概览

Ribbon的Fallback机制

==使用的是@HystrixCommand注解，其实就是一个api ;==

如下：

@HystrixCommand(fallbackMethod = "defaultStores")
这个注解是通用的，不一定是Ribbon才用;

如何使用：

@Component
public class StoreIntegration {

    @HystrixCommand(fallbackMethod = "defaultStores")
    public Object getStores(Map<String, Object> parameters) {
        //do stuff that might fail
    }

    public Object defaultStores(Map<String, Object> parameters) {
        return /* something useful */;
    }
}

如果要让某些线程本地上下文传播到@HystrixCommand，则默认声明不起作用，因为它在线程池中执行该命令（可能造成超时）。 通过配置或直接在注释中切换Hystrix以使用与调用者相同的线程，方法是要求它使用不同的“隔离策略”。

以下示例演示如何在注释中设置线程：

@HystrixCommand(fallbackMethod = "stubMyService",
    commandProperties = {
      @HystrixProperty(name="execution.isolation.strategy", value="SEMAPHORE")
    }
)
...

@HystrixCommand由名为“javanica”的Netflix contrib库提供。 Spring Cloud在连接到Hystrix断路器的代理中自动包装带有该注释的Spring bean。 断路器计算何时打开和关闭电路以及在发生故障时应采取的措施。

See here for more details. See the Hystrix wiki for details on the properties available.

Feign的Fallback机制

==针对一个类,使用的是fallback属性==

如下：

• 第一种fallback形式：@FeignClient(name = "hello", fallback = HystrixClientFallback.class)

  • 代码示例：

  @FeignClient(name = "microservice-provider-user", fallback = HystrixClientFallback.class)
  public interface UserFeignClient {
      @RequestMapping(method = RequestMethod.GET, value = "/user/{id}", consumes = "application/json")
      public User findById(@PathVariable("id") Long id);
  }
      
  
  static class HystrixClientFallback implements UserFeignClient {
      @Override
      public User findById(Long id) {
          User user = new User();
          user.setId(1L);
          return user;
      }
  }
  

• 第二种：fallbackFactory形式@FeignClient(name = "microservice-provider-user", /*fallback = HystrixClientFallback.class, */ fallbackFactory = HystrixClientFallbackFactory.class)

  • 代码示例：

  @FeignClient(name = "microservice-provider-user", /*fallback = HystrixClientFallback.class, */ fallbackFactory = HystrixClientFallbackFactory.class)
  public interface UserFeignClient {
      @RequestMapping(method = RequestMethod.GET, value = "/user/{id}", consumes = "application/json")
      public User findById(@PathVariable("id") Long id);
  }    
      
  
  @Component
  static class HystrixClientFallbackFactory implements FallbackFactory<UserFeignClient> {
      
      private static Logger LOGGER = LoggerFactory.getLogger(HystrixClientFallbackFactory.class);
      
      @Override
      public UserFeignClient create(Throwable cause) {
          //打印日志
          HystrixClientFallbackFactory.LOGGER.info("fallback; reason was: " + cause.getMessage());
          
          return new UserFeignClient() {
              @Override
              public User findById(Long id) {
                  User user = new User();
                  user.setId(-1L);
                  user.setUsername("我是HystrixClientFallbackFactory");
                  
                  return user;
              }
          };
      }
      
  }
  
  

Zuul的Fallback机制

==针对一个微服务==

如下：

Zuul的Fallback回退机制