Hystrix实现限流和熔断

Hystrix提供了熔断、隔离、Fallback、cache、监控等功能，能够在一个、或多个依赖同时出现问题时保证系统依然可用。
将服务的接口使用hystrix线程池做隔离，可以实现限流和熔断的效果。配合天舟平台提供的SpringCloudConfig配置中心，可以在不重启服务的情况下，动态调整hystrix限流的参数。

springboot工程使用hystrix的配置步骤:
1.pom.xml:

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

2.开启hystrix及hystrixDashboard:

入口类加注解，@EnableHystrix,@EnableHystrixDashboard
@SpringBootApplication
@EnableHystrix
@EnableHystrixDashboard
public class ContestDemoApplication{
    public static void main(String[] args) {
        SpringApplication.run(ContestDemoApplication.class, args);
    }
}

3.@HystrixCommand注解:

@HystrixCommand 加到服务的接口方法上，可以对接口限流。
下面的代码，给服务的 /hello接口 加了hystrix线程隔离，并且限制并发为5。
当接口熔断或者降级时，会走降级方法，降级方法将异常信息返回，并且返回状态码 503
@RestController
@RequestMapping(value = "/contest/demo")
public class HelloController {
    //对controller层的接口做hystrix线程池隔离，可以起到限流的作用
    @HystrixCommand(
            commandKey = "helloCommand",//缺省为方法名
        threadPoolKey = "helloPool",//缺省为类名
        fallbackMethod = "fallbackMethod",//指定降级方法，在熔断和异常时会走降级方法
        commandProperties = {
        //超时时间
        @HystrixProperty(name = "execution.isolation.thread.timeoutInMilliseconds", value = "1000")
},
        threadPoolProperties = {
                //并发，缺省为10
                @HystrixProperty(name = "coreSize", value = "5")
        }
    )
    @RequestMapping(
            value = "/hello",
            method = RequestMethod.GET
    )
    public String sayHello(HttpServletResponse httpServletResponse){
        return "Hello World!：00000";
    }

    /**
     *  降级方法，状态码返回503
     *  注意，降级方法的返回类型与形参要与原方法相同，可以多一个Throwable参数放到最后，用来获取异常信息
     */
    public String fallbackMethod(HttpServletResponse httpServletResponse,Throwable e){
        httpServletResponse.setStatus(HttpStatus.SERVICE_UNAVAILABLE.value());
        return e.getMessage();
    }
}

hystrix的属性配置:

如果将@HystrixCommand注解加到方法上，不对属性（如CoreSize）做任何配置，那么相当于使用了如下缺省配置。 每个属性的意义可以参考  hystrix学习资料
@HystrixCommand(
            commandProperties = {
                    //execution
                    @HystrixProperty(name = "execution.isolation.strategy", value = "THREAD"),
                    @HystrixProperty(name = "execution.isolation.thread.timeoutInMilliseconds", value = "1000"),
                    @HystrixProperty(name = "execution.timeout.enabled", value = "true"),
                    @HystrixProperty(name = "execution.isolation.thread.interruptOnTimeout", value = "true"),
                    @HystrixProperty(name = "execution.isolation.thread.interruptOnCancel", value = "false"),
                    @HystrixProperty(name = "execution.isolation.semaphore.maxConcurrentRequests", value = "10"),
                    //fallback
                    @HystrixProperty(name = "fallback.isolation.semaphore.maxConcurrentRequests", value = "10"),
                    @HystrixProperty(name = "fallback.enabled", value = "true"),
                    //circuit breaker
                    @HystrixProperty(name = "circuitBreaker.enabled", value = "true"),
                    @HystrixProperty(name = "circuitBreaker.forceClosed", value = "false"),
                    @HystrixProperty(name = "circuitBreaker.forceOpen", value = "false"),
                    @HystrixProperty(name = "circuitBreaker.requestVolumeThreshold", value = "20"),
                    @HystrixProperty(name = "circuitBreaker.sleepWindowInMilliseconds", value = "5000"),
                    @HystrixProperty(name = "circuitBreaker.errorThresholdPercentage", value = "50"),
                    //Metrics
                    @HystrixProperty(name = "metrics.rollingStats.timeInMilliseconds", value = "10000"),
                    @HystrixProperty(name = "metrics.rollingStats.numBuckets", value = "10"),
                    @HystrixProperty(name = "metrics.rollingPercentile.enabled", value = "true"),
                    @HystrixProperty(name = "metrics.rollingPercentile.timeInMilliseconds", value = "60000"),
                    @HystrixProperty(name = "metrics.rollingPercentile.numBuckets", value = "6"),
                    @HystrixProperty(name = "metrics.rollingPercentile.bucketSize", value = "100"),
                    @HystrixProperty(name = "metrics.healthSnapshot.intervalInMilliseconds", value = "500"),
                    //request context
                    @HystrixProperty(name = "requestCache.enabled", value = "true"),
                    @HystrixProperty(name = "requestLog.enabled", value = "true")},
            threadPoolProperties = {
                    @HystrixProperty(name = "coreSize", value = "10"),
                    @HystrixProperty(name = "maximumSize", value = "10"),
                    @HystrixProperty(name = "maxQueueSize", value = "-1"),
                    @HystrixProperty(name = "queueSizeRejectionThreshold", value = "5"),
                    @HystrixProperty(name = "keepAliveTimeMinutes", value = "1"),
                    @HystrixProperty(name = "allowMaximumSizeToDivergeFromCoreSize", value = "false"),
                    @HystrixProperty(name = "metrics.rollingStats.timeInMilliseconds", value = "10000"),
                    @HystrixProperty(name = "metrics.rollingStats.numBuckets", value = "10")
            }
    )
    如果要自定义这些属性，那么需要先了解下hystrix属性配置的优先级。

4 个不同优先级别的配置（优先级由低到高）。
全局配置属性：通过在配置文件中定义全局属性值，在应用启动时或可用Spring Cloud Config和服务本身的动态刷新接口实现的动态刷新配置功能下，可以实现对“全局默认值”的覆盖以及在运行期对“全局默认值”的动态调整。

如下是 threadpool 和 command下的属性全局配置，注意具体的配置都在  default  下
#hystrix全局属性配置
hystrix:
  threadpool:
    default: #对应@HystrixCommand注解中threadPoolKey的属性值，默认为default
      coreSize: 50 #线程池的核心线程数及并发执行的最大线程数
  command:
    default: #对应@HystrixCommand注解中commandKey的属性值，默认为default
      execution:
        isolation:
          thread:
            timeoutInMilliseconds: 1000 #执行超时时间
      fallback:
        isolation:
          semaphore:
            maxConcurrentRequests: 1000 #任意时间点允许的降级方法并发数。当请求达到或超过该设置值其余请求不会调用fallback而是直接被拒绝

实例默认值：通过代码为实例定义的默认值。通过代码的方式为实例设置属性值来覆盖默认的全局配置。

示例如下：
@HystrixCommand(
            commandKey = "helloCommand",//缺省为方法名
        threadPoolKey = "helloPool",//缺省为类名
        fallbackMethod = "fallbackMethod",//指定降级方法，在熔断和异常时会走降级方法
        commandProperties = {
        //超时时间
        @HystrixProperty(name = "execution.isolation.thread.timeoutInMilliseconds", value = "1000")
},
        threadPoolProperties = {
                //并发，缺省为10
                @HystrixProperty(name = "coreSize", value = "5")
        }
    )
    @RequestMapping(
            value = "/hello",
            method = RequestMethod.GET
    )
    public String sayHello(HttpServletResponse httpServletResponse){
        return "Hello World!：00000";
    }

实例配置属性：通过配置文件来为指定的实例进行属性配置，以覆盖前面的三个默认值。它也可用Spring Cloud Config和服务本身的动态刷新接口实现的动态刷新配置功能，实现对具体实例配置的动态调整。

以示例代码中的 /hello 接口为例， commandKey=helloCommand   threadPoolKey=helloPool,那么对其的配置如下
#hystrix参数配置
hystrix:
  threadpool:
    helloPool:  #注解中的threadPoolKey属性值, 如有特殊配置不想使用defualt默认配置可自行添加
      coreSize: 20
  command:
    helloCommand:  #注解中的commandKey属性值,如有特殊配置不想使用默认配置可自行添加
      execution:
        isolation:
          thread:
            timeoutInMilliseconds: 4000 #执行超时时间

Hystrix实现限流和熔断

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