深刻 Spring Boot : 快速集成 Dubbo + Hystrix

背景

Hystrix 旨在經過控制那些訪問遠程系統、服務和第三方庫的節點，從而對延遲和故障提供更強大的容錯能力。Hystrix具有擁有回退機制和斷路器功能的線程和信號隔離，請求緩存和請求打包，以及監控和配置等功能。

Dubbo是Alibaba開源的，目前國內最流行的java rpc框架。

本文介紹在spring應用裏，怎麼把Dubbo和Hystrix結合起來使用。

• https://github.com/Netflix/Hystrix
• https://github.com/apache/incubator-dubbo

Spring Boot應用

Demo地址： https://github.com/dubbo/dubbo-samples/tree/master/dubbo-samples-spring-boot-hystrix

生成dubbo集成spring boot的應用

對於不熟悉dubbo 集成spring boot應用的同窗，能夠在這裏直接生成dubbo + spring boot的工程： http://start.dubbo.io/

配置spring-cloud-starter-netflix-hystrix

spring boot官方提供了對hystrix的集成，直接在pom.xml里加入依賴：

<dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-netflix-hystrix</artifactId>
            <version>1.4.4.RELEASE</version>
        </dependency>

而後在Application類上增長@EnableHystrix來啓用hystrix starter：

@SpringBootApplication
@EnableHystrix
public class ProviderApplication {

配置Provider端

在Dubbo的Provider上增長@HystrixCommand配置，這樣子調用就會通過Hystrix代理。

@Service(version = "1.0.0")
public class HelloServiceImpl implements HelloService {
    @HystrixCommand(commandProperties = {
                    @HystrixProperty(name = "circuitBreaker.requestVolumeThreshold", value = "10"),
                    @HystrixProperty(name = "execution.isolation.thread.timeoutInMilliseconds", value = "2000") })
    @Override
    public String sayHello(String name) {
        // System.out.println("async provider received: " + name);
        // return "annotation: hello, " + name;
        throw new RuntimeException("Exception to show hystrix enabled.");
    }
}

配置Consumer端

對於Consumer端，則能夠增長一層method調用，並在method上配置@HystrixCommand。當調用出錯時，會走到fallbackMethod = "reliable"的調用裏。

@Reference(version = "1.0.0")
    private HelloService demoService;

    @HystrixCommand(fallbackMethod = "reliable")
    public String doSayHello(String name) {
        return demoService.sayHello(name);
    }
    public String reliable(String name) {
        return "hystrix fallback value";
    }

經過上面的配置，很簡單地就完成了Spring Boot裏Dubbo + Hystrix的集成。

傳統Spring Annotation應用

Demo地址： https://github.com/dubbo/dubbo-samples/tree/master/dubbo-samples-spring-hystrix

傳統spring annotation應用的配置其實也很簡單，和spring boot應用不一樣的是：

1. 顯式配置Spring AOP支持：@EnableAspectJAutoProxy
2. 顯式經過@Configuration配置HystrixCommandAspect Bean。

@Configuration
    @EnableDubbo(scanBasePackages = "com.alibaba.dubbo.samples.annotation.action")
    @PropertySource("classpath:/spring/dubbo-consumer.properties")
    @ComponentScan(value = {"com.alibaba.dubbo.samples.annotation.action"})
    @EnableAspectJAutoProxy
    static public class ConsumerConfiguration {

        @Bean
        public HystrixCommandAspect hystrixCommandAspect() {
            return new HystrixCommandAspect();
        }
    }

Hystrix集成Spring AOP原理

在上面的例子裏能夠看到，Hystrix對Spring的集成是經過Spring AOP來實現的。下面簡單分析下實現。

@Aspect
public class HystrixCommandAspect {
    @Pointcut("@annotation(com.netflix.hystrix.contrib.javanica.annotation.HystrixCommand)")
    public void hystrixCommandAnnotationPointcut() {
    }
    @Pointcut("@annotation(com.netflix.hystrix.contrib.javanica.annotation.HystrixCollapser)")
    public void hystrixCollapserAnnotationPointcut() {
    }

    @Around("hystrixCommandAnnotationPointcut() || hystrixCollapserAnnotationPointcut()")
    public Object methodsAnnotatedWithHystrixCommand(final ProceedingJoinPoint joinPoint) throws Throwable {
        Method method = getMethodFromTarget(joinPoint);
        Validate.notNull(method, "failed to get method from joinPoint: %s", joinPoint);
        if (method.isAnnotationPresent(HystrixCommand.class) && method.isAnnotationPresent(HystrixCollapser.class)) {
            throw new IllegalStateException("method cannot be annotated with HystrixCommand and HystrixCollapser " +
                    "annotations at the same time");
        }
        MetaHolderFactory metaHolderFactory = META_HOLDER_FACTORY_MAP.get(HystrixPointcutType.of(method));
        MetaHolder metaHolder = metaHolderFactory.create(joinPoint);
        HystrixInvokable invokable = HystrixCommandFactory.getInstance().create(metaHolder);
        ExecutionType executionType = metaHolder.isCollapserAnnotationPresent() ?
                metaHolder.getCollapserExecutionType() : metaHolder.getExecutionType();

        Object result;
        try {
            if (!metaHolder.isObservable()) {
                result = CommandExecutor.execute(invokable, executionType, metaHolder);
            } else {
                result = executeObservable(invokable, executionType, metaHolder);
            }
        } catch (HystrixBadRequestException e) {
            throw e.getCause() != null ? e.getCause() : e;
        } catch (HystrixRuntimeException e) {
            throw hystrixRuntimeExceptionToThrowable(metaHolder, e);
        }
        return result;
    }

1. HystrixCommandAspect裏定義了兩個註解的AspectJ Pointcut：@HystrixCommand, @HystrixCollapser。全部帶這兩個註解的spring bean都會通過AOP處理
2. 在@Around AOP處理函數裏，能夠看到Hystrix會建立出HystrixInvokable，再經過CommandExecutor來執行

spring-cloud-starter-netflix-hystrix的代碼分析

1.@EnableHystrix 引入了@EnableCircuitBreaker，@EnableCircuitBreaker引入了EnableCircuitBreakerImportSelector

@EnableCircuitBreaker
public @interface EnableHystrix {
}

@Import(EnableCircuitBreakerImportSelector.class)
public @interface EnableCircuitBreaker {
}

2.EnableCircuitBreakerImportSelector繼承了SpringFactoryImportSelector<EnableCircuitBreaker>，使spring加載META-INF/spring.factories裏的EnableCircuitBreaker聲明的配置

在META-INF/spring.factories裏能夠找到下面的配置，也就是引入了HystrixCircuitBreakerConfiguration。

org.springframework.cloud.client.circuitbreaker.EnableCircuitBreaker=\
org.springframework.cloud.netflix.hystrix.HystrixCircuitBreakerConfiguration

3.在HystrixCircuitBreakerConfiguration裏能夠發現建立了HystrixCommandAspect

@Configuration
public class HystrixCircuitBreakerConfiguration {

    @Bean
    public HystrixCommandAspect hystrixCommandAspect() {
        return new HystrixCommandAspect();
    }

可見spring-cloud-starter-netflix-hystrix實際上也是建立了HystrixCommandAspect來集成Hystrix。

另外spring-cloud-starter-netflix-hystrix裏還有metrics, health, dashboard等集成。

總結

• 對於dubbo provider的@Service是一個spring bean，直接在上面配置@HystrixCommand便可

• 對於dubbo consumer的@Reference，能夠經過加一層簡單的spring method包裝，配置@HystrixCommand便可

• Hystrix自己提供HystrixCommandAspect來集成Spring AOP，配置了@HystrixCommand和@HystrixCollapser的spring method都會被Hystrix處理

連接

• https://github.com/Netflix/Hystrix
• https://github.com/apache/incubator-dubbo
• http://start.dubbo.io/
• https://cloud.spring.io/spring-cloud-netflix/single/spring-cloud-netflix.html#_circuit_breaker_hystrix_clients