When will Hystrix Throw ThreadPoolRejection Error when Synchronous Queue is used?

Hystrix uses SynchronousQueue when MaxQueueSize has been set to -1, and uses LinkedBlockingQueue when MaxQueueSize is set to >=0.

When LinkedBlockingQueue is used and the number of concurrent requests is beyond (threadPool.CoreSize + queueSizeRejectionThreshold), ThreadPoolRejection Exception will be thrown.

When SynchronousQueue is used and the number of concurrent requests is beyond (threadPool.CoreSize), ThreadPoolRejection Exception will be thrown.

Here's a snippet providing it when SynchronousQueue is used:

	import com.netflix.hystrix.HystrixCommand;
	import com.netflix.hystrix.HystrixCommandGroupKey;
	import com.netflix.hystrix.HystrixCommandProperties;
	import com.netflix.hystrix.HystrixThreadPoolProperties;
	import java.util.Observable;
	/**
	* Created by yzhang on 12/6/18.
	*/
	public class TestHystrix {
	public static void main(String[] args) throws InterruptedException {
	HystrixThreadPoolProperties.Setter threadPoolPropertiesSetter = HystrixThreadPoolProperties.Setter()
	.withCoreSize(5)
	.withMaxQueueSize(-1)
	.withQueueSizeRejectionThreshold(5);
	for(int i=0;i<10;i++) {
	final int j = i;
	System.out.println("submitting:" + j);
	HystrixCommand command = new HystrixCommand(
	HystrixCommand.Setter.withGroupKey(HystrixCommandGroupKey.Factory.asKey("test"))
	.andThreadPoolPropertiesDefaults(threadPoolPropertiesSetter)
	.andCommandPropertiesDefaults(HystrixCommandProperties.Setter().withExecutionTimeoutInMilliseconds(10000))) {
	@Override
	protected Object run() throws Exception {
	System.out.println("running: " + j);
	Thread.sleep(Integer.MAX_VALUE);
	return null;
	}
	};
	rx.Observable observable = command.observe();
	if(j > 5) {
	System.out.println("get Object of " + j);
	Object result = observable.toBlocking().single();
	}
	}
	Thread.sleep(3000);
	}
	}

view raw hystrix_synchronous_queue_test.java hosted with ❤ by GitHub

submitting:0
submitting:1
submitting:2
submitting:3
submitting:4
submitting:5
running: 1
running: 2
running: 0
running: 4
running: 3
submitting:6
get Object of 6
Exception in thread "main" com.netflix.hystrix.exception.HystrixRuntimeException: TestHystrix$1 could not be queued for execution and no fallback available.
 at com.netflix.hystrix.AbstractCommand$21.call(AbstractCommand.java:783)
 at com.netflix.hystrix.AbstractCommand$21.call(AbstractCommand.java:768)
 at rx.internal.operators.OperatorOnErrorResumeNextViaFunction$1.onError(OperatorOnErrorResumeNextViaFunction.java:99)
 at rx.internal.operators.OperatorDoOnEach$1.onError(OperatorDoOnEach.java:71)
 at rx.internal.operators.OperatorDoOnEach$1.onError(OperatorDoOnEach.java:71)
...

Note that ThreadPoolRejection starts to happen when all 5 threads in the threadPool have been taken.