我的一個項目並發200+，觀察服務器的日志發現出現了大量的熔斷超時偶爾會閃現出RejectedExecutionException: Rejected command because thread-pool queueSize is at rejection threshold。
在高並發的前提下出現熔斷超時：
1.先確定是否是自己接口的問題，接口平均響應時長是多少？
舉個例子，倘若平均響應時長為200ms,單線程處理的話5次/秒，tomcat最大並發線程數按照100個來算的話，那就是100*5次/秒=500次/秒
那也就是正常情況下，可以承受500次/秒的並發請求。
但是現在200次就不停的刷超時熔斷！
2.在接口上使用@HystrixCommand注解少配置了參數
之前是按照下面的方式配置的
@HystrixCommand(fallbackMethod = "your_interface", commandProperties = {
@HystrixProperty(name = "execution.isolation.thread.timeoutInMilliseconds", value = "5000") })
注意這里有個坑：
- hystrix.command.default.fallback.isolation.semaphore.maxConcurrentRequests 如果並發數達到該設置值，請求會被拒絕和拋出異常並且fallback不會被調用。默認10
- hystrix.threadpool.default.coreSize 並發執行的最大線程數，默認10
- hystrix.threadpool.default.maxQueueSize BlockingQueue的最大隊列數，
- hystrix.threadpool.default.queueSizeRejectionThreshold 即使maxQueueSize沒有達到，達到queueSizeRejectionThreshold該值后，請求也會被拒絕。
原來是因為這里如果不顯示設置coreSize的化，那么就會按照默認的10來進行處理。這就很好理解為什么大量熔斷超時了，10-thread*5次/秒/單線程=50次/秒<200次並發量。
然后調整參數，最后解決了這個問題：
@HystrixCommand(fallbackMethod = "otaCheckUpdateFallback", commandProperties = {
@HystrixProperty(name = "execution.isolation.thread.timeoutInMilliseconds", value = "5000"),
}, threadPoolProperties = { @HystrixProperty(name = "coreSize", value = "100"),
@HystrixProperty(name = "maxQueueSize", value = "20"),
@HystrixProperty(name = "queueSizeRejectionThreshold", value = "20")})
同樣將參數queueSizeRejectionThreshold調大之后，即便請求頻率超過預期，那么當達到queueSizeRejectionThreshold之后會拋出下面的異常。
2018-04-11 20:33:16.294 ERROR [http-nio-9098-exec-30][DirectJDKLog.java:181] - Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is com.netflix.hystrix.exception.HystrixRuntimeException: otaCheckUpdate fallback execution rejected.] with root cause
com.netflix.hystrix.exception.HystrixRuntimeException: your_interface fallback execution rejected.
at com.netflix.hystrix.AbstractCommand.handleFallbackRejectionByEmittingError(AbstractCommand.java:1043) ~[hystrix-core-1.5.10.jar!/:1.5.10]
at com.netflix.hystrix.AbstractCommand.getFallbackOrThrowException(AbstractCommand.java:875) ~[hystrix-core-1.5.10.jar!/:1.5.10]
at com.netflix.hystrix.AbstractCommand.handleThreadPoolRejectionViaFallback(AbstractCommand.java:993) ~[hystrix-core-1.5.10.jar!/:1.5.10]
at com.netflix.hystrix.AbstractCommand.access$400(AbstractCommand.java:60) ~[hystrix-core-1.5.10.jar!/:1.5.10]
at com.netflix.hystrix.AbstractCommand$12.call(AbstractCommand.java:607) ~[hystrix-core-1.5.10.jar!/:1.5.10]
at com.netflix.hystrix.AbstractCommand$12.call(AbstractCommand.java:601) ~[hystrix-core-1.5.10.jar!/:1.5.10]
at rx.internal.operators.OperatorOnErrorResumeNextViaFunction$4.onError(OperatorOnErrorResumeNextViaFunction.java:140) ~[rxjava-1.1.10.jar!/:1.1.10]
at rx.internal.operators.OnSubscribeDoOnEach$DoOnEachSubscriber.onError(OnSubscribeDoOnEach.java:87) ~[rxjava-1.1.10.jar!/:1.1.10]
at rx.internal.operators.OnSubscribeDoOnEach$DoOnEachSubscriber.onError(OnSubscribeDoOnEach.java:87) ~[rxjava-1.1.10.jar!/:1.1.10]
at com.netflix.hystrix.AbstractCommand$HystrixObservableTimeoutOperator$3.onError(AbstractCommand.java:1194) ~[hystrix-core-1.5.10.jar!/:1.5.10]
at rx.internal.operators.OnSubscribeLift.call(OnSubscribeLift.java:54) ~[rxjava-1.1.10.jar!/:1.1.10]
at rx.internal.operators.OnSubscribeLift.call(OnSubscribeLift.java:30) ~[rxjava-1.1.10.jar!/:1.1.10]
at rx.Observable.unsafeSubscribe(Observable.java:10211) ~[rxjava-1.1.10.jar!/:1.1.10]
at rx.internal.operators.OnSubscribeDoOnEach.call(OnSubscribeDoOnEach.java:41) ~[rxjava-1.1.10.jar!/:1.1.10]
at rx.internal.operators.OnSubscribeDoOnEach.call(OnSubscribeDoOnEach.java:30) ~[rxjava-1.1.10.jar!/:1.1.10]
at rx.Observable.unsafeSubscribe(Observable.java:10211) ~[rxjava-1.1.10.jar!/:1.1.10]
at rx.internal.operators.OnSubscribeDoOnEach.call(OnSubscribeDoOnEach.java:41) ~[rxjava-1.1.10.jar!/:1.1.10]
at rx.internal.operators.OnSubscribeDoOnEach.call(OnSubscribeDoOnEach.java:30) ~[rxjava-1.1.10.jar!/:1.1.10]
at rx.internal.operators.OnSubscribeLift.call(OnSubscribeLift.java:48) ~[rxjava-1.1.10.jar!/:1.1.10]
at rx.internal.operators.OnSubscribeLift.call(OnSubscribeLift.java:30) ~[rxjava-1.1.10.jar!/:1.1.10]
at rx.Observable.unsafeSubscribe(Observable.java:10211) ~[rxjava-1.1.10.jar!/:1.1.10]
at rx.internal.operators.OnSubscribeDoOnEach.call(OnSubscribeDoOnEach.java:41) ~[rxjava-1.1.10.jar!/:1.1.10]
at rx.internal.operators.OnSubscribeDoOnEach.call(OnSubscribeDoOnEach.java:30) ~[rxjava-1.1.10.jar!/:1.1.10]
at rx.Observable.unsafeSubscribe(Observable.java:10211) ~[rxjava-1.1.10.jar!/:1.1.10]
at rx.internal.operators.OnSubscribeDoOnEach.call(OnSubscribeDoOnEach.java:41) ~[rxjava-1.1.10.jar!/:1.1.10]
at rx.internal.operators.OnSubscribeDoOnEach.call(OnSubscribeDoOnEach.java:30) ~[rxjava-1.1.10.jar!/:1.1.10]
at rx.Observable.unsafeSubscribe(Observable.java:10211) ~[rxjava-1.1.10.jar!/:1.1.10]
at rx.internal.operators.OnSubscribeDoOnEach.call(OnSubscribeDoOnEach.java:41) ~[rxjava-1.1.10.jar!/:1.1.10]
at rx.internal.operators.OnSubscribeDoOnEach.call(OnSubscribeDoOnEach.java:30) ~[rxjava-1.1.10.jar!/:1.1.10]
at rx.internal.operators.OnSubscribeLift.call(OnSubscribeLift.java:48) ~[rxjava-1.1.10.jar!/:1.1.10]
at rx.internal.operators.OnSubscribeLift.call(OnSubscribeLift.java:30) ~[rxjava-1.1.10.jar!/:1.1.10]
at rx.Observable.unsafeSubscribe(Observable.java:10211) ~[rxjava-1.1.10.jar!/:1.1.10]
at rx.internal.operators.OnSubscribeDefer.call(OnSubscribeDefer.java:51) ~[rxjava-1.1.10.jar!/:1.1.10]
at rx.internal.operators.OnSubscribeDefer.call(OnSubscribeDefer.java:35) ~[rxjava-1.1.10.jar!/:1.1.10]
at rx.Observable.unsafeSubscribe(Observable.java:10211) ~[rxjava-1.1.10.jar!/:1.1.10]
at rx.internal.operators.OnSubscribeMap.call(OnSubscribeMap.java:48) ~[rxjava-1.1.10.jar!/:1.1.10]
at rx.internal.operators.OnSubscribeMap.call(OnSubscribeMap.java:33) ~[rxjava-1.1.10.jar!/:1.1.10]
at rx.Observable.unsafeSubscribe(Observable.java:10211) ~[rxjava-1.1.10.jar!/:1.1.10]
at rx.internal.operators.OnSubscribeDoOnEach.call(OnSubscribeDoOnEach.java:41) ~[rxjava-1.1.10.jar!/:1.1.10]
at rx.internal.operators.OnSubscribeDoOnEach.call(OnSubscribeDoOnEach.java:30) ~[rxjava-1.1.10.jar!/:1.1.10]