摘要: 原创出处 http://www.iocoder.cn/Hystrix/circuit-breaker/ 「芋道源码」欢迎转载,保留摘要,谢谢!
关注 微信公众号:【芋道源码】 有福利:
本文主要分享 断路器 HystrixCircuitBreaker 。
HystrixCircuitBreaker 有三种状态 :
CLOSED
:关闭 OPEN
:打开 HALF_OPEN
:半开 其中,断路器处于 OPEN
状态时,链路处于 非健康 状态,命令执行时,直接调用 回退 逻辑,跳过 正常 逻辑。
HystrixCircuitBreaker 状态变迁如下图 :
红线:初始时,断路器处于 CLOSED
状态,链路处于 健康 状态。当满足如下条件,断路器从 CLOSED
变成 OPEN
状态:
HystrixCommandProperties.default_metricsRollingStatisticalWindow = 10000 ms
)内,总请求数超过一定 量 ( 可配, HystrixCommandProperties.circuitBreakerRequestVolumeThreshold = 20
) 。 HystrixCommandProperties.circuitBreakerErrorThresholdPercentage = 50%
) 。 绿线:断路器处于 OPEN
状态,命令执行时,若当前时间超过断路器 开启 时间一定时间( HystrixCommandProperties.circuitBreakerSleepWindowInMilliseconds = 5000 ms
),断路器变成 HALF_OPEN
状态, 尝试 调用 正常 逻辑,根据执行是否成功, 打开或关闭 熔断器【 蓝线 】。
com.netflix.hystrix.HystrixCircuitBreaker
,Hystrix 断路器 接口 。定义接口如下代码 :
public interface HystrixCircuitBreaker{ /** * Every {@link HystrixCommand} requests asks this if it is allowed to proceed or not. It is idempotent and does * not modify any internal state, and takes into account the half-open logic which allows some requests through * after the circuit has been opened * * @return boolean whether a request should be permitted */ boolean allowRequest(); /** * Whether the circuit is currently open (tripped). * * @return boolean state of circuit breaker */ boolean isOpen(); /** * Invoked on successful executions from {@link HystrixCommand} as part of feedback mechanism when in a half-open state. */ void markSuccess(); /** * Invoked on unsuccessful executions from {@link HystrixCommand} as part of feedback mechanism when in a half-open state. */ void markNonSuccess(); /** * Invoked at start of command execution to attempt an execution. This is non-idempotent - it may modify internal * state. */ boolean attemptExecution(); }
#allowRequest()
和 #attemptExecution()
方法,方法目的基本类似,差别在于当断路器满足 尝试关闭 条件时,前者不会将断路器不会修改状态( CLOSE => HALF-OPEN
),而后者会。 HystrixCircuitBreaker 有两个子类实现 :
在 AbstractCommand 创建时,初始化 HystrixCircuitBreaker ,代码如下 :
/* package */abstract class AbstractCommand<R> implements HystrixInvokableInfo<R>, HystrixObservable<R>{ /** * 断路器 */ protected final HystrixCircuitBreaker circuitBreaker; protected AbstractCommand(HystrixCommandGroupKey group, HystrixCommandKey key, HystrixThreadPoolKey threadPoolKey, HystrixCircuitBreaker circuitBreaker, HystrixThreadPool threadPool, HystrixCommandProperties.Setter commandPropertiesDefaults, HystrixThreadPoolProperties.Setter threadPoolPropertiesDefaults, HystrixCommandMetrics metrics, TryableSemaphore fallbackSemaphore, TryableSemaphore executionSemaphore, HystrixPropertiesStrategy propertiesStrategy, HystrixCommandExecutionHook executionHook){ // ... 省略无关代码 // 初始化 断路器 this.circuitBreaker = initCircuitBreaker(this.properties.circuitBreakerEnabled().get(), circuitBreaker, this.commandGroup, this.commandKey, this.properties, this.metrics); // ... 省略无关代码 } private static HystrixCircuitBreaker initCircuitBreaker(boolean enabled, HystrixCircuitBreaker fromConstructor, HystrixCommandGroupKey groupKey, HystrixCommandKey commandKey, HystrixCommandProperties properties, HystrixCommandMetrics metrics){ if (enabled) { if (fromConstructor == null) { // get the default implementation of HystrixCircuitBreaker return HystrixCircuitBreaker.Factory.getInstance(commandKey, groupKey, properties, metrics); } else { return fromConstructor; } } else { return new NoOpCircuitBreaker(); } } }
HystrixCommandProperties.circuitBreakerEnabled = true
时,即断路器功能 开启 ,使用 Factory 获得 HystrixCircuitBreakerImpl 对象。在「3. HystrixCircuitBreaker.Factory」详细解析。 HystrixCommandProperties.circuitBreakerEnabled = false
时,即断路器功能 关闭 ,创建 NoOpCircuitBreaker 对象。另外,NoOpCircuitBreaker 代码简单到脑残,点击 链接 查看实现。 com.netflix.hystrix.HystrixCircuitBreaker.Factory
,HystrixCircuitBreaker 工厂 ,主要用于:
HystrixCircuitBreaker 容器,基于 HystrixCommandKey 维护了 HystrixCircuitBreaker 单例对象 的映射。代码如下 :
private static ConcurrentHashMap<String, HystrixCircuitBreaker> circuitBreakersByCommand = new ConcurrentHashMap<String, HystrixCircuitBreaker>();
整体代码灰常清晰,点击 链接 查看代码。
com.netflix.hystrix.HystrixCircuitBreaker.HystrixCircuitBreakerImpl
, 完整 的断路器实现。
我们来逐个方法看看 HystrixCircuitBreakerImpl 的具体实现。
构造方法,代码如下 :
/* package */class HystrixCircuitBreakerImpl implements HystrixCircuitBreaker{ private final HystrixCommandProperties properties; private final HystrixCommandMetrics metrics; enum Status { CLOSED, OPEN, HALF_OPEN } private final AtomicReference<Status> status = new AtomicReference<Status>(Status.CLOSED); private final AtomicLong circuitOpened = new AtomicLong(-1); private final AtomicReference<Subscription> activeSubscription = new AtomicReference<Subscription>(null); protected HystrixCircuitBreakerImpl(HystrixCommandKey key, HystrixCommandGroupKey commandGroup, final HystrixCommandProperties properties, HystrixCommandMetrics metrics){ this.properties = properties; this.metrics = metrics; //On a timer, this will set the circuit between OPEN/CLOSED as command executions occur Subscription s = subscribeToStream(); activeSubscription.set(s); } }
status
属性,断路器的状态。 circuitOpened
属性,断路器打开,即状态变成 OPEN
的时间。 activeSubscription
属性,基于 Hystrix Metrics 对请求量统计 Observable 的订阅,在「4.2 #subscribeToStream()」详细解析。 #subscribeToStream()
方法,向 Hystrix Metrics 对请求量统计 Observable 的发起订阅。代码如下 :
private Subscription subscribeToStream(){ 1: private Subscription subscribeToStream(){ 2: /* 3: * This stream will recalculate the OPEN/CLOSED status on every onNext from the health stream 4: */ 5: return metrics.getHealthCountsStream() 6: .observe() 7: .subscribe(new Subscriber<HealthCounts>() { 8: @Override 9: public void onCompleted(){ 10: 11: } 12: 13: @Override 14: public void onError(Throwable e){ 15: 16: } 17: 18: @Override 19: public void onNext(HealthCounts hc){ 20: System.out.println("totalRequests" + hc.getTotalRequests()); // 芋艿,用于调试 21: // check if we are past the statisticalWindowVolumeThreshold 22: if (hc.getTotalRequests() < properties.circuitBreakerRequestVolumeThreshold().get()) { 23: // we are not past the minimum volume threshold for the stat window, 24: // so no change to circuit status. 25: // if it was CLOSED, it stays CLOSED 26: // if it was half-open, we need to wait for a successful command execution 27: // if it was open, we need to wait for sleep window to elapse 28: } else { 29: if (hc.getErrorPercentage() < properties.circuitBreakerErrorThresholdPercentage().get()) { 30: //we are not past the minimum error threshold for the stat window, 31: // so no change to circuit status. 32: // if it was CLOSED, it stays CLOSED 33: // if it was half-open, we need to wait for a successful command execution 34: // if it was open, we need to wait for sleep window to elapse 35: } else { 36: // our failure rate is too high, we need to set the state to OPEN 37: if (status.compareAndSet(Status.CLOSED, Status.OPEN)) { 38: circuitOpened.set(System.currentTimeMillis()); 39: } 40: } 41: } 42: } 43: }); 44: }
第 5 至 7 行 :向 Hystrix Metrics 对请求量统计 Observable 的发起订阅。这里的 Observable 基于 RxJava Window 操作符。
FROM 《ReactiveX文档中文翻译》「Window」
定期将来自原始 Observable 的数据分解为一个 Observable 窗口,发射这些窗口,而不是每次发射一项数据
#onNext()
方法将 不断 被调用,每次计算断路器的状态。 第 22 行 :判断 周期 ( 可配, HystrixCommandProperties.default_metricsRollingStatisticalWindow = 10000 ms
)内,总请求数超过一定 量 ( 可配, HystrixCommandProperties.circuitBreakerRequestVolumeThreshold = 20
) 。
00:00
内发起了 N 个请求, 00:11
不计算这 N 个请求。 第 29 行 : 错误 请求占总请求数超过一定 比例 ( 可配, HystrixCommandProperties.circuitBreakerErrorThresholdPercentage = 50%
) 。
第 37 至 39 行 :满足断路器 打开 条件, CAS 修改状态( CLOSED => OPEN
),并设置打开时间( circuitOpened
) 。
【 补充 】第 5 至 7 行 ::smiling_imp: 怕写在上面,大家有压力。Hystrix Metrics 对请求量统计 Observable 使用了 两种 RxJava Window 操作符 :
Observable#window(timespan, unit)
方法, 固定 周期( 可配, HystrixCommandProperties.metricsHealthSnapshotIntervalInMilliseconds = 500 ms
),发射 Observable 窗口。点击 BucketedCounterStream 构造方法 查看调用处的代码。 Observable#window(count, skip)
方法,每发射 一次 ( skip
) Observable 忽略 count
( 可配, HystrixCommandProperties.circuitBreakerRequestVolumeThreshold = 20
) 个数据项。 为什么 ?答案在 第 22 行的代码 ,周期内达到一定请求量是 断路器打开的一个条件 。点击 BucketedRollingCounterStream 构造方法 查看调用处的代码。 目前该方法有 两处 调用 :
CLOSE
)。 如下是 AbstractCommand#applyHystrixSemantics(_cmd)
方法,对 HystrixCircuitBreakerImpl#attemptExecution
方法的调用的代码 :
private Observable<R> applyHystrixSemantics(final AbstractCommand<R> _cmd){ // ... 省略无关代码 /* determine if we're allowed to execute */ if (circuitBreaker.attemptExecution()) { // 执行【正常逻辑】 } else { // 执行【回退逻辑】 } }
HystrixCircuitBreakerImpl#attemptExecution
方法,判断是否可以执行 正常逻辑 。 #attemptExecution
方法,代码如下 :
1: @Override 2: public boolean attemptExecution(){ 3: // 强制 打开 4: if (properties.circuitBreakerForceOpen().get()) { 5: return false; 6: } 7: // 强制 关闭 8: if (properties.circuitBreakerForceClosed().get()) { 9: return true; 10: } 11: // 打开时间为空 12: if (circuitOpened.get() == -1) { 13: return true; 14: } else { 15: // 满足间隔尝试断路器时间 16: if (isAfterSleepWindow()) { 17: //only the first request after sleep window should execute 18: //if the executing command succeeds, the status will transition to CLOSED 19: //if the executing command fails, the status will transition to OPEN 20: //if the executing command gets unsubscribed, the status will transition to OPEN 21: if (status.compareAndSet(Status.OPEN, Status.HALF_OPEN)) { 22: return true; 23: } else { 24: return false; 25: } 26: } else { 27: return false; 28: } 29: } 30: }
HystrixCommandProperties.circuitBreakerForceOpen = true
( 默认值 : false
) 时,即断路器强制 打开 ,返回 false
。当该配置接入配置中心后,可以动态实现 打开 熔断。 为什么会有该配置 ?当 HystrixCircuitBreaker 创建完成后,无法 动态 切换 NoOpCircuitBreaker 和 HystrixCircuitBreakerImpl ,通过该配置以实现类似效果。 HystrixCommandProperties.circuitBreakerForceClose = true
( 默认值 : false
) 时,即断路器强制 关闭 ,返回 true
。当该配置接入配置中心后,可以动态实现 关闭 熔断。 为什么会有该配置 ?当 HystrixCircuitBreaker 创建完成后,无法 动态 切换 NoOpCircuitBreaker 和 HystrixCircuitBreakerImpl ,通过该配置以实现类似效果。 circuitOpened
) 为”空”,返回 true
。 #isAfterSleepWindow()
方法,判断 是否满足尝试调用正常逻辑的间隔时间 。当满足,使用 CAS 方式修改断路器状态( OPEN => HALF_OPEN
),从而保证有且仅有 一个线程 能够尝试调用正常逻辑。 #isAfterSleepWindow()
方法,代码如下 :
private boolean isAfterSleepWindow(){ final long circuitOpenTime = circuitOpened.get(); final long currentTime = System.currentTimeMillis(); final long sleepWindowTime = properties.circuitBreakerSleepWindowInMilliseconds().get(); return currentTime > circuitOpenTime + sleepWindowTime; }
HystrixCommandProperties.circuitBreakerSleepWindowInMilliseconds
( 默认值, 5000 ms
),返回 true
。 当尝试调用正常逻辑 成功 时,调用 #markSuccess()
方法, 关闭 断路器。代码如下 :
1: @Override 2: public void markSuccess(){ 3: if (status.compareAndSet(Status.HALF_OPEN, Status.CLOSED)) { 4: // 清空 Hystrix Metrics 对请求量统计 Observable 的**统计信息** 5: //This thread wins the race to close the circuit - it resets the stream to start it over from 0 6: metrics.resetStream(); 7: // 取消原有订阅 8: Subscription previousSubscription = activeSubscription.get(); 9: if (previousSubscription != null) { 10: previousSubscription.unsubscribe(); 11: } 12: // 发起新的订阅 13: Subscription newSubscription = subscribeToStream(); 14: activeSubscription.set(newSubscription); 15: // 设置断路器打开时间为空 16: circuitOpened.set(-1L); 17: } 18: }
HALF_OPEN => CLOSED
)。 如下两处调用了 #markNonSuccess()
方法 :
markEmits
markOnCompleted
当尝试调用正常逻辑 失败 时,调用 #markNonSuccess()
方法, 重新打开 断路器。代码如下 :
1: @Override 2: public void markNonSuccess(){ 3: if (status.compareAndSet(Status.HALF_OPEN, Status.OPEN)) { 4: //This thread wins the race to re-open the circuit - it resets the start time for the sleep window 5: circuitOpened.set(System.currentTimeMillis()); 6: } 7: }
HALF_OPEN => OPEN
)。 #attemptExecution()
过一段时间,可以再次 尝试 执行正常逻辑。 如下两处调用了 #markNonSuccess()
方法 :
handleFallback
unsubscribeCommandCleanup
#allowRequest()
和 #attemptExecution()
方法,方法目的基本类似,差别在于当断路器满足 尝试关闭 条件时,前者不会将断路器不会修改状态( CLOSE => HALF-OPEN
),而后者会。点击 链接 查看代码实现。
#isOpen()
方法,比较简单,点击 链接 查看代码实现。
呼呼,相对比较干净的一篇文章,满足。
胖友,分享一波朋友圈可好!