Execution Hook differences for HystrixCommand/HystrixObservableCommand

[mattrjacobs]

Ran into this while working on implementing #600
Currently, HystrixCommandExecutionHook has the following methods (among others):

• onRunStart(HystrixInvokable<T> instance)
• onRunSuccess(HystrixInvokable<T> instance, T response)
• onRunError(HystrixInvokable<T> instance, Exception ex)
• onStart(HystrixInvokable<T> instance)
• onComplete(HystrixInvokable<T> instance, T response)
• onError(HystrixInvokable<T> instance, FailureType failureType, Exception ex)

This set of methods matches well to HystrixCommand, but not HystrixObservableCommand.

• HystrixCommand has a run() method, HystrixObservableCommand has construct(), so the naming feels strange
• onRunSuccess conflates an emission (it contains the response) and a terminal signal. HystrixObservableCommand may emit many values before the overall command is marked as a 'success' (an onCompleted)
• onComplete also suffers this conflation

\cc @benjchristensen for thoughts on design

[mattrjacobs]

Here are my thoughts on how to handle this mismatch:

1. Deprecate onRunStart, onRunSuccess, onRunError because the name is misleading. Keep their behavior the same
2. Deprecate onFallbackSuccess, onComplete because they include a value. This is only appropriate for the single-valued case
3. This is the new set of un-deprecated hooks (new are bolded):
   • onStart(HystrixInvokable<T> commandInstance)
   • onEmit(HystrixInvokable<T> commandInstance, T value)
   • onError(HystrixInvokable<T> commandInstance, FailureType failureType, Exception e)
   • onSuccess(HystrixInvokable<T> commandInstance)
   • onThreadStart(HystrixInvokable<T> commandInstance)
   • onThreadComplete(HystrixInvokable<T> commandInstance)
   • onExecutionStart(HystrixInvokable<T> commandInstance)
   • onExecutionEmit(HystrixInvokable<T> commandInstance, T value)
   • onExecutionError(HystrixInvokable<T> commandInstance, Exception e)
   • onExecutionSuccess(HystrixInvokable<T> commandInstance)
   • onFallbackStart(HystrixInvokable<T> commandInstance)
   • onFallbackEmit(HystrixInvokable<T> commandInstance, T value)
   • onFallbackError(HystrixInvokable<T> commandInstance, Exception e)
   • onFallbackSuccess(HystrixInvokable<T> commandInstance)
   • onCacheHit(HystrixInvokable<T> commandInstance)

[benjchristensen]

I think that's good.

[mattrjacobs]

Completed in #683