Reactive: add Sentinel Reactor adapter module

[sczyh30]

Describe what this PR does / why we need it

Add Sentinel Reactor adapter module and implement reactive transformer.

Does this pull request fix one issue?

Resolves #544

Describe how you did it

API Design:

someService.doSomething()
   .transform(new SentinelReactorTransformer<>(resourceName))
   .subscribe();

Internal implementation:

Here I've implemented a SentinelReactorTransformer that will augment the original stream with the encapsulated Sentinel operator during the assembly stage. There are two kinds of operators:

• MonoSentinelOperator implements MonoOperator
• FluxSentinelOperator implements FluxOperator

Both will wrap the actual subscriber with the SentinelReactorSubscriber during subscribe. SentinelReactorSubscriber is derived from the Reactor BaseSubscriber (due to the fact that the original BaseSubscriber declares all event callbacks as final methods, I have to copy the original code and modify it with so that the methods are inheritable).

Entry logic of Sentinel is wrapped in each hook function (hookOnSubscribe, hookOnNext, hookOnError, hookOnComplete). The stage:

• The hookOnSubscribe will be called during the subscribe stage. Here Sentinel will execute SphU.asyncEntry(xxx) and propagate the event. If the request is blocked, Sentinel will cancel the upstream and propagate the corresponding BlockException with onError callback.

• The hookOnNext will be called when receiving data. If the upstream has been canceled, Sentinel will just try to exit the entry.

  • For unary publishers (Mono), the onNext should be called at most once, followed by the terminal signal. There may be such kind of event stream: onSubscribe() -> onNext() -> cancel() -> onComplete(). In that case, the onComplete event would be dropped. So for Mono we need to tryCompleteEntry in both hookOnNext and hookOnComplete.
  • For Flux, we just call onNext to propagate the event and data.

• In hookOnComplete we just tryCompleteEntry and call actual.onComplete() to propagate the event.

• In hookOnError, if the error is biz-exception (not a BlockException), we'll record the exception within the entry. Then we'll tryCompleteEntry and propogate the error.

• How to carry Context of Sentinel to build the invocation chain: via Reactor Context (override the actualContext)

• The response time: from onSubscribe to terminal state

Describe how to verify it

Run the test cases.

Special notes for reviews

The TCK did not work well so here I removed tests with TCK. Please check whether the SentinelReactorSubscriber obeys the Reactive Streams specification.

[CarpenterLee]

LGTM