`

使用Hystrix守护应用(2)

阅读更多
接上篇(http://ningandjiao.iteye.com/blog/2171185),
执行方式
HystrixCommand提供了3种执行方式:

同步执行:即一旦开始执行该命令,当前线程就得阻塞着直到该命令返回结果,然后才能继续执行下面的逻辑。当调用命令的execute()方法即为同步执行, 示例:
  
@Test
    public void synchronousExecute() throws Exception {
        ThreadEchoCommand command = new ThreadEchoCommand("xianlinbox");
        String result = command.execute();
        assertThat(result,equalTo("Echo: xianlinbox"));
    }

异步执行:命令开始执行会返回一个Future<T>的对象,不阻塞后面的逻辑,开发者自己根据需要去获取结果。当调用HystrixCommand的queue()方法即为异步执行
   
@Test
    public void asynchronousExecute() throws Exception {
        ThreadEchoCommand command = new ThreadEchoCommand("xianlinbox");
        Future<String> result = command.queue();
        while (!result.isDone()){
            System.out.println("Do other things ...");
        }
        assertThat(result.get(),equalTo("Echo: xianlinbox"));
    }

响应式执行:命令开始执行会返回一个Observable<T> 对象,开发者可以给给Obeservable对象注册上Observer或者Action1对象,响应式地处理命令执行过程中的不同阶段。当调用HystrixCommand的observe()方法,或使用Observable的工厂方法(just(),from())即为响应式执行,这个功能的实现是基于Netflix的另一个开源项目RxJava(https://github.com/Netflix/RxJava)来的,更细节的用法可以参考:https://github.com/Netflix/Hystrix/wiki/How-To-Use#wiki-Reactive-Execution。 示例:
   
@Test
    public void reactiveExecute1() throws Exception {
        ThreadEchoCommand command1 = new ThreadEchoCommand("xianlinbox");
        Observable<String> result = command1.observe();
        result.subscribe(new Action1<String>() {
            @Override
            public void call(String s) {
                logger.info("Command called. Result is:{}", s);
            }1
        });
        Thread.sleep(1000);
    }

    @Test
    public void reactiveExecute2() throws Exception {
        ThreadEchoCommand command = new ThreadEchoCommand("xianlinbox");
        Observable<String> result = command.observe();
        result.subscribe(new Observer<String>() {
            @Override
            public void onCompleted() {
                logger.info("Command Completed");
            }

            @Override
            public void onError(Throwable e) {
                logger.error("Command failled", e);
            }

            @Override
            public void onNext(String args) {
                logger.info("Command finished,result is {}", args);
            }
        });
        Thread.sleep(1000);
    }

隔离方式(Thread Pool和Semaphores)
Hystrix支持2种隔离方式:
ThreadPool:即根据配置把不同的命令分配到不同的线程池中,这是比较常用的隔离策略,该策略的优点是隔离性好,并且可以配置断路,某个依赖被设置断路之后,系统不会再尝试新起线程运行它,而是直接提示失败,或返回fallback值;缺点是新起线程执行命令,在执行的时候必然涉及到上下文的切换,这会造成一定的性能消耗,但是Netflix做过实验,这种消耗对比其带来的价值是完全可以接受的,具体的数据参见Hystrix Wiki(https://github.com/Netflix/Hystrix/wiki/How-it-Works#wiki-Isolation)。 本文前面的例子都是使用的TheadPool隔离策略。

Semaphores:信号量,顾名思义就是使用一个信号量来做隔离,开发者可以限制系统对某一个依赖的最高并发数。这个基本上就是一个限流的策略。每次调用依赖时都会检查一下是否到达信号量的限制值,如达到,则拒绝。该隔离策略的优点不新起线程执行命令,减少上下文切换,缺点是无法配置断路,每次都一定会去尝试获取信号量。示例:

public class SemaphoreEchoCommand extends HystrixCommand<String> {
    private Logger logger = LoggerFactory.getLogger(ThreadEchoCommand.class);
    private String input;

    protected SemaphoreEchoCommand(String input) {
        super(Setter.withGroupKey(HystrixCommandGroupKey.Factory.asKey("Semaphore Echo"))
                .andCommandKey(HystrixCommandKey.Factory.asKey("Echo"))
                .andCommandPropertiesDefaults(HystrixCommandProperties.Setter()
                        .withExecutionIsolationStrategy(HystrixCommandProperties.ExecutionIsolationStrategy.SEMAPHORE)
                        .withExecutionIsolationSemaphoreMaxConcurrentRequests(2)));
        this.input = input;
    }

    @Override
    protected String run() throws Exception {
        logger.info("Run command with input: {}", input);
        Thread.currentThread().sleep(100);
        return "Echo: " + input;
    }
}
    @Test
    public void semaphoresCommandExecute() throws Exception {
        SemaphoreEchoCommand command = new SemaphoreEchoCommand("xianlinbox");
        assertThat(command.execute(), equalTo("Echo: xianlinbox"));
    }

    @Test
    public void semaphoresCommandMultiExecute() throws Exception {
        for (int i = 0; i < 5; i++) {
            final SemaphoreEchoCommand command = new SemaphoreEchoCommand("xianlinbox-" + i);
            Thread thread = new Thread(new Runnable() {
                @Override
                public void run() {
                    command.queue();
                }
            });
            thread.start();
        }
        Thread.sleep(1000);
    }

第一个测试的运行日志如下:
23:10:34.996 [main] INFO  c.n.c.DynamicPropertyFactory - DynamicPropertyFactory is initialized with configuration sources: com.netflix.config.ConcurrentCompositeConfiguration@2224df87
23:10:35.045 [main] INFO  d.ThreadEchoCommand - Run command with input: xianlinbox
从运行日志可以看到,HystrixCommand一样是在主线程中执行。

第二个测试的运行日志如下:
14:56:22.285 [Thread-5] INFO  d.ThreadEchoCommand - Run command with input: xianlinbox-4
14:56:22.285 [Thread-1] INFO  d.ThreadEchoCommand - Run command with input: xianlinbox-0
Exception in thread "Thread-2" Exception in thread "Thread-4" com.netflix.hystrix.exception.HystrixRuntimeException: Echo could not acquire a semaphore for execution and no fallback available.
示例中,设置的信号量最大值为2, 因此可以看到有2个线程可以成功运行命令,第三个则会得到一个无法获取信号量的HystrixRuntimeException。
优雅降级
在调用第三方服务时,总是无可避免会出现一些错误(fail,timeout等),再加上上面提到的线程池大小,信号量的限制等等,在执行HystrixComamnd的过程中,总难免会抛出一些异常。而Hystrix为执行过程中的异常情况提供了优雅的降级方案,只需要在自己的HystrixCommand中实现getFallback()方法,当异常出现时,就会自动调用getFallback()方法的值. 示例:为第一小节中的AddressHystrixCommand和ContactHystrixCommand添加getFallback()方法, 当有异常发生的时候,直接返回null:
   
@Override
    protected Contact getFallback() {
        logger.info("Met error, using fallback value: {}", customerId);
        return null;
    }

然后,停掉Stub的Contact和Address服务, 再次调用GetCustomer服务(http://localhost:8080/HystrixDemo/customers/1),得到结果如下:
{"id":"1","name":"xianlinbox","contact":null,"address":null}
运行日志:
15:22:08.847 [hystrix-Contact-1] INFO  c.x.h.d.ContactHystrixCommand - Get contact for customer 1
15:22:09.098 [hystrix-Contact-1] INFO  c.x.h.d.ContactHystrixCommand - Met error, using fallback value: 1
15:22:09.101 [hystrix-Address-1] INFO  c.x.h.d.AddressHystrixCommand - Get address for customer 1
15:22:09.103 [hystrix-Address-1] INFO  c.x.h.d.AddressHystrixCommand - Met error, using fallback value: 1
请求作用域特性
作用域设置:要想要使用请求作用域特性,首先必须把HystrixCommand置于HystrixRequestContext的生命周期管理中,其典型用法是在Web应用中增加一个ServletFilter,把每个用户Request用HystrixRequestContext包起来。示例:
public class HystrixRequestContextServletFilter implements Filter {
	...
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) 
     throws IOException, ServletException {
     	//启动HystrixRequestContext	
        HystrixRequestContext context = HystrixRequestContext.initializeContext();
        try {
            chain.doFilter(request, response);
        } finally {
        	//关闭HystrixRequestContext
            context.shutdown();
        }
    }
}
然后把该SevletFilter配置到web.xml中:
   
<filter>
        <display-name>HystrixRequestContextServletFilter</display-name>
        <filter-name>HystrixRequestContextServletFilter</filter-name>
        <filter-class>com.xianlinbox.hystrix.filter.HystrixRequestContextServletFilter</filter-class>
    </filter>

    <filter-mapping>
        <filter-name>HystrixRequestContextServletFilter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>

设置了请求作用域之后,接下来看看,我们从中可以得到哪些好处:

请求缓存(Request Cache):即当用户调用HystrixCommand时,HystrixCommand直接从缓存中取而不需要调用外部服务。HystrixCommand从缓存中取需要3个条件:
1. 该HystrixCommand被包裹一个HystrixRequestContext中
2. 该HystrixCommand实现了getCacheKey()方法
3. 在HystrixRequestContext中已有相同Cache Key值的缓存
示例:
public void requestCache() throws Exception {
        HystrixRequestContext context = HystrixRequestContext.initializeContext();
        try {
            ThreadEchoCommand command1 = new ThreadEchoCommand("xianlinbox");
            ThreadEchoCommand command2 = new ThreadEchoCommand("xianlinbox");

            assertThat(command1.execute(),equalTo("Echo: xianlinbox"));
            assertThat(command1.isResponseFromCache(),equalTo(false));
            assertThat(command2.execute(),equalTo("Echo: xianlinbox"));
            assertThat(command2.isResponseFromCache(),equalTo(true));
        } finally {
            context.shutdown();
        }

        context = HystrixRequestContext.initializeContext();
        try {
            ThreadEchoCommand command3 = new ThreadEchoCommand("xianlinbox");
            assertThat(command3.execute(),equalTo("Echo: xianlinbox"));
            assertThat(command3.isResponseFromCache(),equalTo(false));
        } finally {
            context.shutdown();
        }
    }
从上面的例子看以得到,一旦重新初始化了RequestContext,Cache也都全部失效了。另外,从Cache中获取值不会去执行HystrixCommand的run()方法。

除了重新初始化RequestContext,Hystrix还提供了另外一种方式来刷新Cache,该方式需要使用HystrixRequestCache的clear()方法,示例:在ThreadEchoCommand中实现一个静态方法flushCache( ),该方法会调用HystrixRequestCache的clear方法清理Cache
  
 public static void flushCache(String cacheKey) {
        HystrixRequestCache.getInstance(HystrixCommandKey.Factory.asKey("Echo"),
                HystrixConcurrencyStrategyDefault.getInstance()).clear(cacheKey);
    }
    
    @Test
    public void flushCacheTest() throws Exception {
        HystrixRequestContext context = HystrixRequestContext.initializeContext();
        try {
            ThreadEchoCommand command1 = new ThreadEchoCommand("xianlinbox");
            ThreadEchoCommand command2 = new ThreadEchoCommand("xianlinbox");

            assertThat(command1.execute(), equalTo("Echo: xianlinbox"));
            assertThat(command1.isResponseFromCache(), equalTo(false));
            assertThat(command2.execute(), equalTo("Echo: xianlinbox"));
            assertThat(command2.isResponseFromCache(), equalTo(true));

            ThreadEchoCommand.flushCache("xianlinbox");
            ThreadEchoCommand command3 = new ThreadEchoCommand("xianlinbox");
            assertThat(command3.execute(), equalTo("Echo: xianlinbox"));
            assertThat(command3.isResponseFromCache(), equalTo(false));
        } finally {
            context.shutdown();
        }
    }

通过这个机制,开发者可以实现Get-Set-Get的Cache验证机制,防止因为Cache导致的不一致状况。

批量执行请求(Request Collapsing):即用户可以把多个命令封装到一个HystrixCommand中执行以提升效率,这多个命令会在一个线程中依次执行(注:经笔者测试,在JDK6下线程数固定,但是在JDK7下的运行线程数不固定)。要使用该特性需要把依赖调用封装到一个HystrixCollapser<BatchReturnType,ResponseType,RequestArgumentType>中, 该抽象类的主要作用有3个:
1. 把所有的依赖调用封装到一个CollapseRequest的集合中
2. 以第一步得到的CollapseRequest集合为参数创建一个HystrixCommand
3. 把第二步得到的结果集一一对应的设置到对应的CollapseRequest中

为了支持上面的功能,该抽象类提供了3个泛型参数:
BatchReturnType:即BatchCommand的返回值,通常为ResponseType的集合。
ResponseType:依赖调用的返回值。
RequestArgumentType:依赖调用的参数,如果有多个参数,需封装为一个对象或使用集合。
示例:
public class CollapseEchoHystrixCommand extends HystrixCollapser<List<String>, String, String> {
    private Logger logger = LoggerFactory.getLogger(CollapseEchoHystrixCommand.class);
    private String input;

    public CollapseEchoHystrixCommand(String input) {
        super(HystrixCollapser.Setter
                .withCollapserKey(HystrixCollapserKey.Factory.asKey("Echo Collapse")));
        this.input = input;
    }

    @Override
    public String getRequestArgument() {
        return input;
    }

    @Override
    protected HystrixCommand<List<String>> createCommand(Collection<CollapsedRequest<String, String>> collapsedRequests) {
        return new BatchCommand(collapsedRequests);
    }

    @Override
    protected void mapResponseToRequests(List<String> batchResponse, Collection<CollapsedRequest<String, String>> collapsedRequests) {
        logger.info("Mapping response to Request");
        int count = 0;
        for (CollapsedRequest<String, String> request : collapsedRequests) {
            request.setResponse(batchResponse.get(count++));
        }

    }

    private class BatchCommand extends HystrixCommand<List<String>> {
        private Collection<CollapsedRequest<String, String>> requests;

        public BatchCommand(Collection<CollapsedRequest<String, String>> requests) {
            super(HystrixCommandGroupKey.Factory.asKey("Batch"));
            this.requests = requests;
        }

        @Override
        protected List<String> run() throws Exception {
            logger.info("Run batch command");
            List<String> responses = new ArrayList<String>();
            for (CollapsedRequest<String, String> request : requests) {
                logger.info("Run request: {}", request.getArgument());
                responses.add("Echo: " + request.getArgument());
            }
            return responses;
        }
    }
}

    @Test
    public void collapseCommandTest() throws Exception {
        HystrixRequestContext context = HystrixRequestContext.initializeContext();

        try {
            Future<String> result1 = new CollapseEchoHystrixCommand("xianlinbox-1").queue();
            Future<String> result2 = new CollapseEchoHystrixCommand("xianlinbox-2").queue();
            Future<String> result3 = new CollapseEchoHystrixCommand("xianlinbox-3").queue();

            assertThat(result1.get(),equalTo("Echo: xianlinbox-1"));
            assertThat(result2.get(),equalTo("Echo: xianlinbox-2"));
            assertThat(result3.get(),equalTo("Echo: xianlinbox-3"));

            assertEquals(1, HystrixRequestLog.getCurrentRequest().getExecutedCommands().size());
        } finally {
            context.shutdown();
        }
    }

运行日志:
03:10:58.584 [main] INFO  d.CollapseEchoHystrixCommand - Get argument
03:10:58.597 [main] INFO  d.CollapseEchoHystrixCommand - Get argument
03:10:58.597 [main] INFO  d.CollapseEchoHystrixCommand - Get argument
03:10:58.598 [HystrixTimer-1] INFO  d.CollapseEchoHystrixCommand - Create batch command
03:10:58.637 [hystrix-Batch-1] INFO  d.CollapseEchoHystrixCommand - Run batch command
03:10:58.637 [hystrix-Batch-1] INFO  d.CollapseEchoHystrixCommand - Run request: xianlinbox-1
03:10:58.639 [hystrix-Batch-1] INFO  d.CollapseEchoHystrixCommand - Run request: xianlinbox-2
03:10:58.639 [hystrix-Batch-1] INFO  d.CollapseEchoHystrixCommand - Run request: xianlinbox-3
03:10:58.644 [RxComputationThreadPool-1] INFO  d.CollapseEchoHystrixCommand - Mapping response to Request

从运行日志可以看到,整个Collapser的运行过程:
1. 获取调用参数,封装到CollapseRequest中
2. 以封装后的List<CollapseRequest>为参数创建Batch HystrixComand
3. Batch HystrixCommand运行所有的请求,把所有的返回放到List<Response>中
4. 把Response设置到对应的CollapseRequest中,返回给调用者。
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