Spring Cloud Hystrix_springcloud hystrix 是观察者模式嘛-优快云博客

Spring Cloud Hystrix

Spring Cloud Hystrix

Hystrix是什么

Hystrix 能使你的系统在出现依赖服务失效的时候，通过隔离系统所依赖的服务，防止服务级联失败，同时提供失败回退机制，更优雅地应对失效，并使你的系统能更快地从异常中恢复。

Hystrix能做什么

在通过第三方客户端访问（通常是通过网络）依赖服务出现高延迟或者失败时，为系统提供保护和控制
在分布式系统中防止级联失败
快速失败（Fail fast）同时能快速恢复
提供失败回退（Fallback）和优雅的服务降级机制
提供近实时的监控、报警和运维控制手段

Hystrix设计原则

防止单个依赖耗尽容器（例如 Tomcat）内所有用户线程
降低系统负载，对无法及时处理的请求快速失败（fail fast）而不是排队
提供失败回退，以在必要时让失效对用户透明化
使用隔离机制（例如『舱壁』/『泳道』模式，熔断器模式等）降低依赖服务对整个系统的影响
针对系统服务的度量、监控和报警，提供优化以满足近实时性的要求
在 Hystrix 绝大部分需要动态调整配置并快速部署到所有应用方面，提供优化以满足快速恢复的要求
能保护应用不受依赖服务的整个执行过程中失败的影响，而不仅仅是网络请求

Hystrix 实现原理

1.舱壁隔离模式

顾名思义，该模式像舱壁一样对资源或失败单元进行隔离，如果一个船舱破了进水，只损失一个船舱，其它船舱可以不受影响。线程隔离(Thread Isolation)就是舱壁隔离模式的一个例子，假定一个应用程序A调用了Svc1/Svc2/Svc3三个服务，且部署A的容器一共有120个工作线程，采用线程隔离机制，可以给对Svc1/Svc2/Svc3的调用各分配40个线程，当Svc2慢了，给Svc2分配的40个线程因慢而阻塞并最终耗尽，线程隔离可以保证给Svc1/Svc3分配的80个线程可以不受影响，如果没有这种隔离机制，当Svc2慢的时候，120个工作线程会很快全部被对Svc2的调用吃光，整个应用程序会全部慢下来。

隔离策略

应用在复杂的分布式结构中，可能会依赖许多其他的服务，并且这些服务都不可避免地有失效的可能。如果一个应用没有与依赖服务的失效隔离开来，那么它将有可能因为依赖服务的失效而失效。
Hystrix将货仓模式运用到了服务调用者上。为每一个依赖服务维护一个线程池（或者信号量），当线程池占满，该依赖服务将会立即拒绝服务而不是排队等待。
每个依赖服务都被隔离开来，Hystrix 会严格控制其对资源的占用，并在任何失效发生时，执行失败回退逻辑。

2.观察者模式

Hystrix通过观察者模式对服务进行状态监听。
每个任务都包含有一个对应的Metrics，所有Metrics都由一个ConcurrentHashMap来进行维护，Key是CommandKey.name()

    private static final ConcurrentHashMap<String, HystrixCommandMetrics> metrics = new ConcurrentHashMap<String, HystrixCommandMetrics>();

在任务的不同阶段会往Metrics中写入不同的信息，Metrics会对统计到的历史信息进行统计汇总，供熔断器以及Dashboard监控时使用。

3.AOP配置命令切入

注解切入的AOP

@Aspect
public class HystrixCommandAspect {

    private static final Map<HystrixPointcutType, MetaHolderFactory> META_HOLDER_FACTORY_MAP;

    static {
        META_HOLDER_FACTORY_MAP = ImmutableMap.<HystrixPointcutType, MetaHolderFactory>builder()
                .put(HystrixPointcutType.COMMAND, new CommandMetaHolderFactory())
                .put(HystrixPointcutType.COLLAPSER, new CollapserMetaHolderFactory())
                .build();
    }

    @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;
    }

Hystrix 使用

/**
 * Copyright 2012 Netflix, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.netflix.hystrix.contrib.javanica.annotation;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;


/**
 * This annotation used to specify some methods which should be processes as hystrix commands.
 */
@Target({ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Inherited
@Documented
public @interface HystrixCommand {

    /**
     * The command group key is used for grouping together commands such as for reporting,
     * alerting, dashboards or team/library ownership.
     * <p/>
     * default => the runtime class name of annotated method
     *
     * @return group key
     */
    String groupKey() default "";

    /**
     * Hystrix command key.
     * <p/>
     * default => the name of annotated method. for example:
     * <code>
     *     ...
     *     @HystrixCommand
     *     public User getUserById(...)
     *     ...
     *     the command name will be: 'getUserById'
     * </code>
     *
     * @return command key
     */
    String commandKey() default "";

    /**
     * The thread-pool key is used to represent a
     * HystrixThreadPool for monitoring, metrics publishing, caching and other such uses.
     *
     * @return thread pool key
     */
    String threadPoolKey() default "";

    /**
     * Specifies a method to process fallback logic.
     * A fallback method should be defined in the same class where is HystrixCommand.
     * Also a fallback method should have same signature to a method which was invoked as hystrix command.
     * for example:
     * <code>
     *      @HystrixCommand(fallbackMethod = "getByIdFallback")
     *      public String getById(String id) {...}
     *
     *      private String getByIdFallback(String id) {...}
     * </code>
     * Also a fallback method can be annotated with {@link HystrixCommand}
     * <p/>
     * default => see {@link com.netflix.hystrix.contrib.javanica.command.GenericCommand#getFallback()}
     *
     * @return method name
     */
    String fallbackMethod() default "";

    /**
     * Specifies command properties.
     *
     * @return command properties
     */
    HystrixProperty[] commandProperties() default {};

    /**
     * Specifies thread pool properties.
     *
     * @return thread pool properties
     */
    HystrixProperty[] threadPoolProperties() default {};

    /**
     * Defines exceptions which should be ignored.
     * Optionally these can be wrapped in HystrixRuntimeException if raiseHystrixExceptions contains RUNTIME_EXCEPTION.
     *
     * @return exceptions to ignore
     */
    Class<? extends Throwable>[] ignoreExceptions() default {};

    /**
     * Specifies the mode that should be used to execute hystrix observable command.
     * For more information see {@link ObservableExecutionMode}.
     *
     * @return observable execution mode
     */
    ObservableExecutionMode observableExecutionMode() default ObservableExecutionMode.EAGER;

    /**
     * When includes RUNTIME_EXCEPTION, any exceptions that are not ignored are wrapped in HystrixRuntimeException.
     *
     * @return exceptions to wrap
     */
    HystrixException[] raiseHystrixExceptions() default {};

    /**
     * Specifies default fallback method for the command. If both {@link #fallbackMethod} and {@link #defaultFallback}
     * methods are specified then specific one is used.
     * note: default fallback method cannot have parameters, return type should be compatible with command return type.
     *
     * @return the name of default fallback method
     */
    String defaultFallback() default "";
}

@Service
public class HelloService
{
    @Autowired
    RestTemplate restTemplate;

    /**
     * 设置命令名称，分组，线程池划分，用于船舱模式 线程池隔离对依赖服务的调用
     * @param name
     * @return
     */
    @HystrixCommand(commandKey = "getHello", groupKey = "helloService", threadPoolKey = "getHelloThread", fallbackMethod = "defaultFallBack")
    public String helloService(String name)
    {
        return restTemplate.getForEntity("http://eureka-client/name?name={0}", String.class, name).getBody();
    }

    public String defaultFallBack(String name)
    {
        return "get by eureka client error!";
    }
}