Hystrix 源码解析 —— 断路器 HystrixCircuitBreaker
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!
本文主要基于 Hystrix 1.5.X 版本
1. 概述
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:
- 周期( 可配,
HystrixCommandProperties.default_metricsRollingStatisticalWindow = 10000 ms
)内,总请求数超过一定量( 可配,HystrixCommandProperties.circuitBreakerRequestVolumeThreshold = 20
) 。- 错误请求占总请求数超过一定比例( 可配,
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】。
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2. HystrixCircuitBreaker
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 :
- NoOpCircuitBreaker :空的断路器实现,用于不开启断路器功能的情况。
- HystrixCircuitBreakerImpl :完整的断路器实现。
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 HystrixCircuitBreakerreturn HystrixCircuitBreaker.Factory.getInstance(commandKey, groupKey, properties, metrics);} else {return fromConstructor;}} else {return new NoOpCircuitBreaker();}}}
- 当
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。- 当
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。3. HystrixCircuitBreaker.Factory
com.netflix.hystrix.HystrixCircuitBreaker.Factory
,HystrixCircuitBreaker 工厂,主要用于:
- 创建 HystrixCircuitBreaker 对象,目前只创建 HystrixCircuitBreakerImpl 。
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。
4. HystrixCircuitBreakerImpl
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。
4.1 构造方法
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 occurSubscription s = subscribeToStream();activeSubscription.set(s);}}
- Status 枚举类,断路器的三种状态。
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。4.2 #subscribeToStream()
#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 stream4: */5: return metrics.getHealthCountsStream()6: .observe()7: .subscribe(new Subscriber<HealthCounts>() {8:9: public void onCompleted() {10:11: }12:13:14: public void onError(Throwable e) {15:16: }17:18:19: 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 statisticalWindowVolumeThreshold22: 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 CLOSED26: // if it was half-open, we need to wait for a successful command execution27: // if it was open, we need to wait for sleep window to elapse28: } 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 CLOSED33: // if it was half-open, we need to wait for a successful command execution34: // if it was open, we need to wait for sleep window to elapse35: } else {36: // our failure rate is too high, we need to set the state to OPEN37: 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
- 简单来说,固定间隔,
#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
) 。
- 这里要注意下,请求次数统计的是周期内,超过周期的不计算在内。例如说,
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 :�� 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 :
- 「4.1 构造方法」,在创建 HystrixCircuitBreakerImpl 时,向 Hystrix Metrics 对请求量统计 Observable 的发起订阅。固定间隔,计算断路器是否要关闭(
CLOSE
)。- 「4.4 #markSuccess()」,清空 Hystrix Metrics 对请求量统计 Observable 的统计信息,取消原有订阅,并发起新的订阅。
4.3 #attemptExecution()
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】}}
- 使用
HystrixCircuitBreakerImpl#attemptExecution
方法,判断是否可以执行正常逻辑。
#attemptExecution
方法,代码如下 :
1:2: 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 execute18: //if the executing command succeeds, the status will transition to CLOSED19: //if the executing command fails, the status will transition to OPEN20: //if the executing command gets unsubscribed, the status will transition to OPEN21: 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: }
- 第 4 至 6 行 :当
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。- 第 8 至 10 行 :当
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。- 第 12 至 13 行 :断路器打开时间(
circuitOpened
) 为”空”,返回true
。- 第 16 至 28 行 :调用
#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;}
- 在当前时间超过断路器打开时间
HystrixCommandProperties.circuitBreakerSleepWindowInMilliseconds
( 默认值,5000 ms
),返回true
。4.4 #markSuccess()
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 :
1:2: 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 06: 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: }
- 第 3 行 :使用 CAS 方式,修改断路器状态(
HALF_OPEN => CLOSED
)。- 第 6 行 :清空 Hystrix Metrics 对请求量统计 Observable 的统计信息。
- 第 8 至 14 行 :取消原有订阅,发起新的订阅。
- 第 16 行 :设置断路器打开时间为”空” 。
如下两处调用了
#markNonSuccess()
方法 :4.5 #markNonSuccess()
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 :
1:2: 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 window5: circuitOpened.set(System.currentTimeMillis());6: }7: }
- 第 3 行 :使用 CAS 方式,修改断路器状态(
HALF_OPEN => OPEN
)。- 第 5 行 :设置设置断路器打开时间为当前时间。这样,
#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 :4.6 #allowRequest()
#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。4.7 #isOpen()
#isOpen()
方法,比较简单,点击 链接 查看代码实现。