Partial Success Fallback - HystrixObservableCommand

[benjchristensen]

A HystrixObservableCommand can stream multiple values. This makes fallback a little more nuanced.

If it fails immediately, then nothing has been emitted via onNext and the getFallback() is easy to reason about.

If some values are emitted via onNext() but then it fails with onError (for any reason, including timeout) then it is partially successful.

The fallback() will then be applied with Observable.onErrorResumeNext which means the partially successful onNext already emitted are still there, then the fallback applies.

The question is ... what does this mean for a user? for the developer of the fallback?

It is far more nuanced than a scalar response (using HystrixCommand or HystrixAsyncCommand).

I think we should at least rename getFallback to resumeWithFallback or something that communicates it's not an either-or, but additive (by resuming).

An implementation of a command should maintain state internally if needed so the fallback can know about partial success. I don't want to make that part of the HystrixObservableCommand itself though, as some will possibly want to cache the results to know what has been sent, but that's expensive to do all the time when not needed.

I'd appreciate further thoughts on how this should behave and be implemented.

[benjchristensen]

/cc @KoltonAndrus @mattrjacobs

[benjchristensen]

Possible signature:

protected Observable<R> onFailureResumeWithFallback()

[mattrjacobs]

This has already been implemented and unit-tested as resumeWithFallback()