iss1403: Support @HystrixCommand for rx.Single and rx.Completable sim…

[dmgcodevil]

Requested by #1403

[mattrjacobs]

Can you add tests showing the case where fallbacks get triggered? Specifically, I think that a mismatch between execution type and fallback type should be disallowed. For example, an execution returning a Completable and a fallback returning an Observable is nonsensical.

[dmgcodevil]

@mattrjacobs thanks for reviewing. I added more test cases for successful scenarios, reviled some issues and resolved them. Regarding a 'failure' scenarios, there is only one special that requires a workaround is Completable, Completable isn't parametrized class but it's still a RX type which is a container in some sense. I added support for RX type because from javanica perspective the following code is correct:

public static class Service {
        @HystrixCommand(fallbackMethod = "fallback")
        public Observable<User> command() { throw new IllegalStateException(); }
        private User fallback() { return null; }
    }

after a little thinking I thought that it makes sense to support this behavior for Completable, so the following code is legit as well:

public static class Service {
        @HystrixCommand(fallbackMethod = "fallback")
        public Completable command() { throw new IllegalStateException(); }
        private User fallback() { return null; }
    }

I will update docs shortly

[dmgcodevil]

@mattrjacobs I guess you can merge this one

[dmgcodevil]

@mattrjacobs

• review
• merge

[mattrjacobs]

Sorry for the delay on this @dmgcodevil . I'm still trying to figure out how much value this adds if hystrix-core doesn't contain the equivalent functionality.

My understanding of this feature is that it will take a user-provided Single/Completable and convert that into a HystrixObservableCommand.

I'm writing a unit test to check that submitting a Completable even works in vanilla Hystrix

[mattrjacobs]

Confirmed this in #1574

[mattrjacobs]

So my thought is that this will help users on hystrix-javanica and I don't see a reason not to merge.