Check current instance status before update.

[OlgaMaciaszek]

This is related to spring-cloud/spring-cloud-netflix#4094 as an attempted fix.

[OlgaMaciaszek]

@howardyuan could you please take a look?

[OlgaMaciaszek]

Adding pic with comments from offline discussion:

[OlgaMaciaszek]

Hi @howardyuan, regarding your comments before, I still use the pre-existing method under the hood from here, so setIsDirty will be called as well if we do change the status. I have added an exception when we verify the previous status first, as in this scenario we assume that it is the status that should be there in a correct workflow and if it's not set then possibly there was a race condition. null is returned in the pre-existing implementation if the status we want to change to is also the one already set - so nothing needs to be done. Seems like a different scenario to me. Let me know what you think. CC @spencergibb

[howardyuan]

I'm a bit concerned that we throw an exception here instead of like before which is simply returning null like before? Also, we don't setIsDirty() here anymore? not sure why we introduce some discrepancy here?

[howardyuan]

Hi @OlgaMaciaszek My concern was that even if we have a race condition, should we throw exceptions or we just skip this update and let eventual consistency catches up? I'm not sure if we will catch those exception at the top, but I think it's too much a disruption if we crash the app whenever we detect a race condition updating the instance status? lmk what you think? I'll take a further look at the whole thing but this is my quick thinking.