分析你的系統

你所認識的分布式系統，哪些是可以進行垂直拆分的？拆分之后系統之間的依賴如何梳理？系統異構之后的穩定性調用如何保證？這些都是可能在分布式場景中面臨的問題。

說個比較常見的問題，大家都知道秒殺系統，秒殺流程在分布式的環境中，就需要依賴：訂單管理，會員管理，支付等，假如在整個系統中會員系統不穩定，導致系統請求擠壓，慢慢的就會導致對其他外部系統的依賴也不可用，所以就要使用業務的隔離，這樣會避免服務的局部不可用導致的全部不可用。

Hystrix 源碼分析

找到HystrixCommand的父類AbstractCommand， 里面有個構造方法，從構造方法可以看出里這里定義了 threadPool對象。代碼如下，關鍵代碼都有做相應的注釋。

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) {

//commandGroup對象，用于組織一類業務相關的對象

this.commandGroup = initGroupKey(group);

// commandKey默認是以類為為名稱的

this.commandKey = initCommandKey(key, getClass());

this.properties = initCommandProperties(this.commandKey, propertiesStrategy, commandPropertiesDefaults);

//這個方法里定義了TheradPool里的關鍵字，默認以傳入的commandGroup 的name做為key的名稱

this.threadPoolKey = initThreadPoolKey(threadPoolKey, this.commandGroup, this.properties.executionIsolationThreadPoolKeyOverride().get());

this.metrics = initMetrics(metrics, this.commandGroup, this.threadPoolKey, this.commandKey, this.properties);

this.circuitBreaker = initCircuitBreaker(this.properties.circuitBreakerEnabled().get(), circuitBreaker, this.commandGroup, this.commandKey, this.properties, this.metrics);

//這里就是線程池對象啦。

this.threadPool = initThreadPool(threadPool, this.threadPoolKey, threadPoolPropertiesDefaults);

//Strategies from plugins

this.eventNotifier = HystrixPlugins.getInstance().getEventNotifier();

this.concurrencyStrategy = HystrixPlugins.getInstance().getConcurrencyStrategy();

HystrixMetricsPublisherFactory.createOrRetrievePublisherForCommand(this.commandKey, this.commandGroup, this.metrics, this.circuitBreaker, this.properties);

this.executionHook = initExecutionHook(executionHook);

this.requestCache = HystrixRequestCache.getInstance(this.commandKey, this.concurrencyStrategy);

this.currentRequestLog = initRequestLog(this.properties.requestLogEnabled().get(), this.concurrencyStrategy);

/* fallback semaphore override if applicable */

this.fallbackSemaphoreOverride = fallbackSemaphore;

/* execution semaphore override if applicable */

this.executionSemaphoreOverride = executionSemaphore;

}

/**

這個方法用于得到HystrixThreadPoolKey 對象， Hystrix內部有大量的Key對象，可以簡單理解這些 Key都是相應對象的唯一標識。從代碼里可以看出，默認情況下Hystrix采用的是commandGroup 的name做為Thread Pool的key值。

**/

private static HystrixThreadPoolKey initThreadPoolKey(HystrixThreadPoolKey threadPoolKey, HystrixCommandGroupKey groupKey, String threadPoolKeyOverride) {

if (threadPoolKeyOverride == null) {

// we don't have a property overriding the value so use either HystrixThreadPoolKey or HystrixCommandGroup

if (threadPoolKey == null) {

/* use HystrixCommandGroup if HystrixThreadPoolKey is null */

return HystrixThreadPoolKey.Factory.asKey(groupKey.name());

} else {

return threadPoolKey;

}

} else {

// we have a property defining the thread-pool so use it instead

return HystrixThreadPoolKey.Factory.asKey(threadPoolKeyOverride);

}

}

/**

在這里將調用具體的構造線程池的方法。

**/

private static HystrixThreadPool initThreadPool(HystrixThreadPool fromConstructor, HystrixThreadPoolKey threadPoolKey, HystrixThreadPoolProperties.Setter threadPoolPropertiesDefaults) {

if (fromConstructor == null) {

// get the default implementation of HystrixThreadPool

return HystrixThreadPool.Factory.getInstance(threadPoolKey, threadPoolPropertiesDefaults);

} else {

return fromConstructor;

}

}