Support event listener of circuit breaker state change event

[huangxfchn]

For example, if sentinel start to switch off, then can call back an interface to notify the application. The same is true when sentinel is out of circuit breaker mode.

[sczyh30]

In fact, Sentinel has provided callback support for StatisticSlot:

• ProcessorSlotEntryCallback: callback when resource entry passed (onPass) or has been blocked (onBlocked)
• ProcessorSlotExitCallback: callback when resource entry successfully exited (onExit)

We can register your own callback via StatisticSlotCallbackRegistry. Maybe here we can just leverage the onBlocked callback to handle the rule trigger alarm. You could get the triggered rule from the BlockException.

You may also refer to #418

[huangxfchn]

Looks a bit like.

I need to catch DegradeException, and determine the count = = 1 representative to downgrade strategy, but onBlocked method should be invoked after the circuit breaker mode, this method was invoked every time during until out of fusing time window. Is that right?

So, what if I knew when to jump out of the circuit breaker mode.

[sczyh30]

You mean notification for circuit breaker CLOSE event? This hasn't been supported yet, but it seems helpful. Maybe we could provide a listener registry for circuit breaker state change events. We could discuss the design here.

[huangxfchn]

Yeah, circuit breaker START and CLOSE event.