Fallback相当于是降级操作。所谓降级,就是指在Hystrix执行非核心链路功能失败的情况下,该如何处理,比如返回默认值或者从缓存中取值。

触发降级的情况
1、hystrix调用各种接口,或者访问外部依赖(如mysql、redis等等)时,执行方法中抛出了异常。
2、对每个外部依赖,无论是服务接口,中间件,资源隔离,对外部依赖只能用一定量的资源去访问,线程池/信号量,如果资源池已满,则后续的请求将会被 reject,即进行限流。
3、访问外部依赖的时候,访问时间过长,可能就会导致超时,报一个TimeoutException异常,即Timeout机制。
上述三种情况,都是常见的异常情况,对外部依赖的东西访问的时候出现了异常,发送异常事件到断路器中去进行统计。
4、如果断路器发现异常事件的占比达到了一定的比例,直接开启断路器。
上述四种情况,都会去调用fallback降级机制。
注意:
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。
如果要实现回退或者降级处理,代码上需要实现HystrixCommand.getFallback()方法或者是HystrixObservableCommand. HystrixObservableCommand()。
例如:
public class CommandHelloFailure extends HystrixCommand<String> { private final String name; public CommandHelloFailure(String name) { super(HystrixCommandGroupKey.Factory.asKey("ExampleGroup")); this.name = name; } @Override protected String run() { throw new RuntimeException("this command always fails"); } @Override protected String getFallback() { return "Hello Failure " + name + "!"; } } |
Hystrix的降级回退方式
1、Fail Fast 快速失败
就是不给fallback降级逻辑,运行过程中如果有报错,直接会把这个报错抛出来,给tomcat调用线程。
HystrixCommand的实现方式:
@Override protected String run() { if (throwException) { throw new RuntimeException("failure from CommandThatFailsFast"); } else { return "success"; } } |
HystrixObservableCommand的实现方式:
@Override protected Observable<String> resumeWithFallback() { if (throwException) { return Observable.error(new Throwable("failure from CommandThatFailsFast")); } else { return Observable.just("success"); } } |
2、Fail Silent 无声失败
给一个fallback降级逻辑。如果是HystrixCommand,则返回null、空Map、空List 等;如果是HystrixObservableCommand,则返回 Observable.empty()。

@Override protected String getFallback() { return null; } @Override protected List<String> getFallback() { return Collections.emptyList(); } @Override protected Observable<String> resumeWithFallback() { return Observable.empty(); } |
3、Static Fallback 默认值降级
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的方式被清除,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 protected Boolean getFallback() { return true; } @Override protected Observable<Boolean> resumeWithFallback() { return Observable.just( true ); } |
4、Stubbed Fallback 残缺降级
用请求中的部分数据拼装成结果,然后再填充一些默认值返回。比如说,当发起了一个请求,然后请求中可能本身就附带了一些信息,如果主请求失败了,走到降级逻辑,在降级逻辑里面,可以将这个请求中的数据,以及部分本地缓存有的数据拼装在一起,再给数据填充一些简单的默认值,然后尽可能将自己有的数据返回到请求方。
public class CommandWithStubbedFallback extends HystrixCommand<UserAccount> { protected CommandWithStubbedFallback(int customerId, String countryCodeFromGeoLookup) { super(HystrixCommandGroupKey.Factory.asKey("ExampleGroup")); this.customerId = customerId; this.countryCodeFromGeoLookup = countryCodeFromGeoLookup; } @Override protected UserAccount getFallback() { /** * Return stubbed fallback with some static defaults, placeholders, * and an injected value 'countryCodeFromGeoLookup' that we'll use * instead of what we would have retrieved from the remote service. */ return new UserAccount(customerId, "Unknown Name", countryCodeFromGeoLookup, true, true, false); } } |
5、多层嵌套Fallback
多层嵌套Fallback是指,在第一次请求失败的情况下,再发起第二次 remote 请求,这次请求的是一个缓存比如Redis;在第二次请求又失败的情况下,再发起最后一次请求,这次请求是去本地ehcache缓存中去获取数据。由于重新发起了远程调用,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,以防止因主线程池已满而无法运行降级的command,也就是重新命名一个 ThreadPoolKey。

public class CommandWithFallbackViaNetwork extends HystrixCommand<String> { private final int id; protected CommandWithFallbackViaNetwork(int id) { super(Setter.withGroupKey(HystrixCommandGroupKey.Factory.asKey("RemoteServiceX")) .andCommandKey(HystrixCommandKey.Factory.asKey("GetValueCommand"))); this.id = id; } @Override protected String run() { // RemoteServiceXClient.getValue(id); throw new RuntimeException("force failure for example"); } @Override protected String getFallback() { return new FallbackViaNetwork(id).execute(); } private static class FallbackViaNetwork extends HystrixCommand<String> { private final int id; public FallbackViaNetwork(int id) { super(Setter.withGroupKey(HystrixCommandGroupKey.Factory.asKey("RemoteServiceX")) .andCommandKey(HystrixCommandKey.Factory.asKey("GetValueFallbackCommand")) // use a different threadpool for the fallback command // so saturating the RemoteServiceX pool won't prevent // fallbacks from executing .andThreadPoolKey(HystrixThreadPoolKey.Factory.asKey("RemoteServiceXFallback"))); this.id = id; } @Override protected String run() { MemCacheClient.getValue(id); } @Override protected String getFallback() { // the fallback also failed // so this fallback-of-a-fallback will // fail silently and return null return null; } } } |
6、Facade Command 手动降级
一般来说,程序都是去执行一个主流程的command,如果发现主流程中出现了问题,需要可以通过设置一个标志为来让程序去执行备用command,又或者,在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。此时可以通过做一个开关比如使用zookeeper做一个配置开关,可以动态切换到备用command或者老代码功能。那么Hystrix它是使用通过一个配置来在两个command中进行切换。

/** * Sample {@link HystrixCommand} pattern using a semaphore-isolated command * that conditionally invokes thread-isolated commands. */ public class CommandFacadeWithPrimarySecondary extends HystrixCommand<String> { private final static DynamicBooleanProperty usePrimary = DynamicPropertyFactory.getInstance().getBooleanProperty("primarySecondary.usePrimary", true); private final int id; public CommandFacadeWithPrimarySecondary(int id) { super(Setter .withGroupKey(HystrixCommandGroupKey.Factory.asKey("SystemX")) .andCommandKey(HystrixCommandKey.Factory.asKey("PrimarySecondaryCommand")) .andCommandPropertiesDefaults( // we want to default to semaphore-isolation since this wraps // 2 others commands that are already thread isolated // 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 protected String run() { if (usePrimary.get()) { return new PrimaryCommand(id).execute(); } else { return new SecondaryCommand(id).execute(); } } @Override protected String getFallback() { return "static-fallback-" + id; } @Override protected String getCacheKey() { return String.valueOf(id); } private static class PrimaryCommand extends HystrixCommand<String> { private final int id; private PrimaryCommand(int id) { super(Setter .withGroupKey(HystrixCommandGroupKey.Factory.asKey("SystemX")) .andCommandKey(HystrixCommandKey.Factory.asKey("PrimaryCommand")) .andThreadPoolKey(HystrixThreadPoolKey.Factory.asKey("PrimaryCommand")) .andCommandPropertiesDefaults( // we default to a 600ms timeout for primary HystrixCommandProperties.Setter().withExecutionTimeoutInMilliseconds(600))); this.id = id; } @Override protected String run() { // perform expensive 'primary' service call return "responseFromPrimary-" + id; } } private static class SecondaryCommand extends HystrixCommand<String> { private final int id; private SecondaryCommand(int id) { super(Setter .withGroupKey(HystrixCommandGroupKey.Factory.asKey("SystemX")) .andCommandKey(HystrixCommandKey.Factory.asKey("SecondaryCommand")) .andThreadPoolKey(HystrixThreadPoolKey.Factory.asKey("SecondaryCommand")) .andCommandPropertiesDefaults( // we default to a 100ms timeout for secondary HystrixCommandProperties.Setter().withExecutionTimeoutInMilliseconds(100))); this.id = id; } @Override protected String run() { // perform fast 'secondary' service call return "responseFromSecondary-" + id; } } public static class UnitTest { @Test public void testPrimary() { HystrixRequestContext context = 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", new CommandFacadeWithPrimarySecondary(20).execute()); } finally { context.shutdown(); ConfigurationManager.getConfigInstance().clear(); } } @Test public void testSecondary() { HystrixRequestContext context = 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", new CommandFacadeWithPrimarySecondary(20).execute()); } finally { context.shutdown(); ConfigurationManager.getConfigInstance().clear(); } } } } |