Enable updating EV configs on IS updates

[LZD-PratyushBhatt]

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Currently EV only gets updated only as part of these events CurrentStateChange, LiveInstanceChange, PeriodicRebalance, OnDemandRebalance and ControllerChange.
It doesnt get updated for IdealState changes. When a change is applied to the ideal_state of a resource, it’s expected that the EV reflect the changes. In cases of changes in configs like rebalancer algo, or min_active_replica, the change might not result in an assignment change, and Helix just forgets to update the EV to reflect these .

This change proposes updating the EV when IS changes. Keep EV an honest reflection of the state of the resource, when a resource-change event occurs. if it leads to change in assignment or not .
For updating EV, from IS we only take SimpleFields and putall the simple configs from IS to EV.

Tests

New test class TestExternalViewComputeOnIdealStateChange

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[LZD-PratyushBhatt]

@junkaixue @xyuanlu Can you review this? Thanks!

[xyuanlu]

I am not sure about this "When a change is applied to the ideal_state of a resource, it’s expected that the EV reflect the changes. "
I think EV is aggregating CS. If no CS change, why we want to change EV?

[LZD-PratyushBhatt]

Thanks for the review @xyuanlu
Regarding "I think EV is aggregating CS. If no CS change, why we want to change EV?"
Correct EV is constructed by aggregating CS, but lets say there is a change on IS simpleField that didnt trigger a CurrentStateChange event lets say, then in this case EV simplefields will not be matching with IS simplefields.

[xyuanlu]

I think this is too expensive to add IS listener just for updating configs (map fields) in EV.
EV pipeline is pretty expensive. It computes EV for all resources by aggregating all CS. IS changes pretty frequently during rebalance, if we introduce an EV compute, it will cause a burden for the already overloaded Helix controller.
I recommend the user read the resource config from IS instead of EV.