Hystrix降级技术解析-Fallback
一、降级 所谓降级，就是指在在Hystrix执行非核心链路功能失败的情况下，我们如何处理，比如我们返回默认值等。如果我们要回退或者降级处理，代码上需要实现HystrixCommand.getFallback()方法或者是HystrixObservableCommand. HystrixObservableCommand()。 publicclassCommandHelloFailureextendsHystrixCommand<String>{ privatefinalStringname; publicCommandHelloFailure(Stringname){ super(HystrixCommandGroupKey.Factory.asKey("ExampleGroup")); this.name=name; } @Override protectedStringrun(){ thrownewRuntimeException("thiscommandalwaysfails"); } @Override protectedStringgetFallback(){ return"HelloFailure"+name+"!"; } } 二、Hystrix的降级回退方式 Hystrix一共有如下几种降级回退模式： 1、Fail Fast 快速失败 @Override protectedStringrun(){ if(throwException){ thrownewRuntimeException("failurefromCommandThatFailsFast"); }else{ return"success"; } } 如果我们实现的是HystrixObservableCommand.java则 重写 resumeWithFallback方法 @Override protectedObservable<String>resumeWithFallback(){ if(throwException){ returnObservable.error(newThrowable("failurefromCommandThatFailsFast")); }else{ returnObservable.just("success"); } } 2、Fail Silent 无声失败 返回null，空Map，空List fail silent.png @Override protectedStringgetFallback(){ returnnull; } @Override protectedList<String>getFallback(){ returnCollections.emptyList(); } @Override protectedObservable<String>resumeWithFallback(){ returnObservable.empty(); } 3、Fallback: Static 返回默认值 回退的时候返回静态嵌入代码中的默认值，这样就不会导致功能以Fail Silent的方式被清楚，也就是用户看不到任何功能了。而是按照一个默认的方式显示。 @Override protectedBooleangetFallback(){ returntrue; } @Override protectedObservable<Boolean>resumeWithFallback(){ returnObservable.just(true); } 4、Fallback: Stubbed 自己组装一个值返回 当我们执行返回的结果是一个包含多个字段的对象时，则会以Stubbed 的方式回退。Stubbed 值我们建议在实例化Command的时候就设置好一个值。以countryCodeFromGeoLookup为例，countryCodeFromGeoLookup的值，是在我们调用的时候就注册进来初始化好的。CommandWithStubbedFallback command = new CommandWithStubbedFallback(1234, "china");主要代码如下： publicclassCommandWithStubbedFallbackextendsHystrixCommand<UserAccount>{ protectedCommandWithStubbedFallback(intcustomerId,StringcountryCodeFromGeoLookup){ super(HystrixCommandGroupKey.Factory.asKey("ExampleGroup")); this.customerId=customerId; this.countryCodeFromGeoLookup=countryCodeFromGeoLookup; } @Override protectedUserAccountgetFallback(){ /** *Returnstubbedfallbackwithsomestaticdefaults,placeholders, *andaninjectedvalue'countryCodeFromGeoLookup'thatwe'lluse *insteadofwhatwewouldhaveretrievedfromtheremoteservice. */ returnnewUserAccount(customerId,"UnknownName", countryCodeFromGeoLookup,true,true,false); } 5、Fallback: Cache via Network 利用远程缓存 通过远程缓存的方式。在失败的情况下再发起一次remote请求，不过这次请求的是一个缓存比如redis。由于是又发起一起远程调用，所以会重新封装一次Command，这个时候要注意，执行fallback的线程一定要跟主线程区分开，也就是重新命名一个ThreadPoolKey。 Cache via Network.png publicclassCommandWithFallbackViaNetworkextendsHystrixCommand<String>{ privatefinalintid; protectedCommandWithFallbackViaNetwork(intid){ super(Setter.withGroupKey(HystrixCommandGroupKey.Factory.asKey("RemoteServiceX")) .andCommandKey(HystrixCommandKey.Factory.asKey("GetValueCommand"))); this.id=id; } @Override protectedStringrun(){ //RemoteServiceXClient.getValue(id); thrownewRuntimeException("forcefailureforexample"); } @Override protectedStringgetFallback(){ returnnewFallbackViaNetwork(id).execute(); } privatestaticclassFallbackViaNetworkextendsHystrixCommand<String>{ privatefinalintid; publicFallbackViaNetwork(intid){ super(Setter.withGroupKey(HystrixCommandGroupKey.Factory.asKey("RemoteServiceX")) .andCommandKey(HystrixCommandKey.Factory.asKey("GetValueFallbackCommand")) //useadifferentthreadpoolforthefallbackcommand //sosaturatingtheRemoteServiceXpoolwon'tprevent //fallbacksfromexecuting .andThreadPoolKey(HystrixThreadPoolKey.Factory.asKey("RemoteServiceXFallback"))); this.id=id; } @Override protectedStringrun(){ MemCacheClient.getValue(id); } @Override protectedStringgetFallback(){ //thefallbackalsofailed //sothisfallback-of-a-fallbackwill //failsilentlyandreturnnull returnnull; } } } 6、Primary + Secondary with Fallback 主次方式回退（主要和次要） 这个有点类似我们日常开发中需要上线一个新功能，但为了防止新功能上线失败可以回退到老的代码，我们会做一个开关比如使用zookeeper做一个配置开关，可以动态切换到老代码功能。那么Hystrix它是使用通过一个配置来在两个command中进行切换。 Primary + Secondary with Fallback.png /** *Sample{@linkHystrixCommand}patternusingasemaphore-isolatedcommand *thatconditionallyinvokesthread-isolatedcommands. */ publicclassCommandFacadeWithPrimarySecondaryextendsHystrixCommand<String>{ privatefinalstaticDynamicBooleanPropertyusePrimary=DynamicPropertyFactory.getInstance().getBooleanProperty("primarySecondary.usePrimary",true); privatefinalintid; publicCommandFacadeWithPrimarySecondary(intid){ super(Setter .withGroupKey(HystrixCommandGroupKey.Factory.asKey("SystemX")) .andCommandKey(HystrixCommandKey.Factory.asKey("PrimarySecondaryCommand")) .andCommandPropertiesDefaults( //wewanttodefaulttosemaphore-isolationsincethiswraps //2otherscommandsthatarealreadythreadisolated //采用信号量的隔离方式 HystrixCommandProperties.Setter() .withExecutionIsolationStrategy(ExecutionIsolationStrategy.SEMAPHORE))); this.id=id; } //通过DynamicPropertyFactory来路由到不同的command @Override protectedStringrun(){ if(usePrimary.get()){ returnnewPrimaryCommand(id).execute(); }else{ returnnewSecondaryCommand(id).execute(); } } @Override protectedStringgetFallback(){ return"static-fallback-"+id; } @Override protectedStringgetCacheKey(){ returnString.valueOf(id); } privatestaticclassPrimaryCommandextendsHystrixCommand<String>{ privatefinalintid; privatePrimaryCommand(intid){ super(Setter .withGroupKey(HystrixCommandGroupKey.Factory.asKey("SystemX")) .andCommandKey(HystrixCommandKey.Factory.asKey("PrimaryCommand")) .andThreadPoolKey(HystrixThreadPoolKey.Factory.asKey("PrimaryCommand")) .andCommandPropertiesDefaults( //wedefaulttoa600mstimeoutforprimary HystrixCommandProperties.Setter().withExecutionTimeoutInMilliseconds(600))); this.id=id; } @Override protectedStringrun(){ //performexpensive'primary'servicecall return"responseFromPrimary-"+id; } } privatestaticclassSecondaryCommandextendsHystrixCommand<String>{ privatefinalintid; privateSecondaryCommand(intid){ super(Setter .withGroupKey(HystrixCommandGroupKey.Factory.asKey("SystemX")) .andCommandKey(HystrixCommandKey.Factory.asKey("SecondaryCommand")) .andThreadPoolKey(HystrixThreadPoolKey.Factory.asKey("SecondaryCommand")) .andCommandPropertiesDefaults( //wedefaulttoa100mstimeoutforsecondary HystrixCommandProperties.Setter().withExecutionTimeoutInMilliseconds(100))); this.id=id; } @Override protectedStringrun(){ //performfast'secondary'servicecall return"responseFromSecondary-"+id; } } publicstaticclassUnitTest{ @Test publicvoidtestPrimary(){ HystrixRequestContextcontext=HystrixRequestContext.initializeContext(); try{ //将属性"primarySecondary.usePrimary"设置为true，则走PrimaryCommand；设置为false，则走SecondaryCommand ConfigurationManager.getConfigInstance().setProperty("primarySecondary.usePrimary",true); assertEquals("responseFromPrimary-20",newCommandFacadeWithPrimarySecondary(20).execute()); }finally{ context.shutdown(); ConfigurationManager.getConfigInstance().clear(); } } @Test publicvoidtestSecondary(){ HystrixRequestContextcontext=HystrixRequestContext.initializeContext(); try{ //将属性"primarySecondary.usePrimary"设置为true，则走PrimaryCommand；设置为false，则走SecondaryCommand ConfigurationManager.getConfigInstance().setProperty("primarySecondary.usePrimary",false); assertEquals("responseFromSecondary-20",newCommandFacadeWithPrimarySecondary(20).execute()); }finally{ context.shutdown(); ConfigurationManager.getConfigInstance().clear(); } } } } 三、总结 降级的处理方式，返回默认值，返回缓存里面的值（包括远程缓存比如redis和本地缓存比如jvmcache）。 但回退的处理方式也有不适合的场景： 1、写操作 2、批处理 3、计算 以上几种情况如果失败，则程序就要将错误返回给调用者。 参考资料：https://github.com/Netflix/Hystrix/wiki
