问题描述
开发Java Spring Cloud应用，需要发送消息到Azure Event Hub中。使用 Spring Cloud Stream Event Hubs Binder 依赖，应用执行一会就会遇见报错：reactor.core.publisher.Sinks$EmissionException: Spec. Rule 1.3 - onSubscribe, onNext, onError and onComplete signaled to a Subscriber MUST be signaled serially.
问题解答
从错误来看，这明显是多线程并发处理时，多个线程同时触发了onSubscribe 或 onNext 或 onError 或 onComplete 事件，而这些事件在与 Subscriber处理时只能一个一个串行处理。
因为SpringCloud的 Controller 并发请求时，会分配多个线程同时调用many.emitNext()，这时如果之前请求线程处理还未结束，新请求的线程会直接这样的报错。
异常产生的代码为：
private final Sinks.EmitFailureHandler emitFailureHandler = (signalType, emitResult) -> emitResult.equals(Sinks.EmitFailureHandler.FAIL_FAST);
Sinks 对 FAIL_FAST 和 FAIL_NON_SERIALIZED的枚举值说明参考：reactor-core/Sinks.java at main · reactor/reactor-core · GitHub
- FAIL_FAST：表示对失败不会进行任何重试，会马上触发异常处理机制，这里就是抛出EmissionException异常。（ A pre-made handler that will not instruct to retry any failure and trigger the failure handling immediately.）
- FAIL_NON_SERIALIZED：表示会持续重试，直至成功。
如果发送到Event Hub的消息允许丢失，可以通过Try Catch捕获异常后记录日志即可。
但是，如果发送的消息不能丢失，必须成功传递到Event Hub中，就可以使用 FAIL_NON_SERIALIZED 模式。
修改为：
private final Sinks.EmitFailureHandler emitFailureHandler = (signalType, emitResult) -> emitResult.equals(Sinks.EmitResult.FAIL_NON_SERIALIZED);
参考资料
ReactorDispatcher: Making sure spec 1.3 is not violated and under race, signals are not lost upon concurrent ClosedChannelException ： https://github.com/Azure/azure-sdk-for-java/issues/27320
FAIL NON SERIALIZED :
/**
* Has successfully emitted the signal
*/
OK,
/**
* Has failed to emit the signal because the sink was previously terminated successfully or with an error
*/
FAIL_TERMINATED,
/**
* Has failed to emit the signal because the sink does not have buffering capacity left
*/
FAIL_OVERFLOW,
/**
* Has failed to emit the signal because the sink was previously interrupted by its consumer
*/
FAIL_CANCELLED,
/**
* Has failed to emit the signal because the access was not serialized
*/
FAIL_NON_SERIALIZED,
/**
* Has failed to emit the signal because the sink has never been subscribed to has no capacity
* to buffer the signal.
*/
FAIL_ZERO_SUBSCRIBER;