ability to deal with server side defaults/initializers

[surajkota]

Is your feature request related to a problem?

Ability to specify fields initialized by service post create. Following are the patterns observed:

1. Generated fields. Example:
   1. roleARN field in ScalableTarget resource is generated by service. See Handle server-side defaults applicationautoscaling-controller#12 (comment) (service - Applicationautoscaling)
   2. staticHyperparameter field in Hyperparameter tuning resource. Slice has some extra fields returned from the service which should be ignored for comparison. See customSetDefaults in HPJob (service - SageMaker)
2. Default value for fields
   1. Boolean fields returned as False.
      • Scalable target struct in ScalableTarget resource - SuspendedState.<*> (all fields) returned as False by default. See setDefaults in autoscaling (service - Applicationautoscaling)
   2. String fields return default values. e.g. Model resource - PrimaryContainer.Mode retuned as SingleMode by default (service - SageMaker)

Describe the solution you'd like
1.a, 2.a, 2.b - if DesiredResource.Spec.field = nil set these values from latestResource
1.b - a hook to write custom algorithm for comparison and remove service generated fields, ignore setting values from latestResource to desired

Note: In 2.a,

• whole desired.spec.SuspendedState == nil and service still returns false values for all fields in the structs (DynamicScalingInSuspended, DynamicScalingInSuspended, ScheduledScalingSuspended)
• desired.spec.SuspendedState != nil but desired.spec.SuspendedState.DynamicScalingInSuspended == nil and service still returns False for desired.spec.SuspendedState.DynamicScalingInSuspended

Describe alternatives you've considered
1.b - a hook to write custom logic to insert only server generated <key, value> pairs into the slice only if diff is False
1.b - a hook to write custom algorithm for comparison and insert service generated fields, set values from latestResource into desired only for comparison

[echen-98]

1.b - a hook to write custom algorithm for comparison and remove service generated fields, ignore setting values from latestResource to desired

@surajkota do you mean a hook that would give service teams the ability to control whether or not the diff for a certain field should be added to the delta? i.e. essentially overwriting any one of these blocks: https://github.com/aws-controllers-k8s/elasticache-controller/blob/main/pkg/resource/user/delta.go#L35-L41

I definitely agree if so, but please let me know if I misinterpreted the suggestion.

1.a, 2.a, 2.b - if DesiredResource.Spec.field = nil set these values from latestResource

Also, could you elaborate on this suggestion? Do you mean directly populating desired.Spec with the server-returned default values?

[echen-98]

In general, the generated code for adding a diff to the delta makes the assumption that a nil difference either direction is sufficient grounds for invoking an Update.

However, this is a case in which desired.Spec.field = nil and latest.Spec.field = non-nil, but we don't want to invoke an Update (because by not specifying the field, the user has signaled that they are okay with the default configuration for that field). Even if we did, we would be populating field with the equivalent of nil in the Update payload, which doesn't really make sense.

So some possible solutions would be:

1. Changing ackcompare.HasNilDifference to only check for the nil difference in one direction, rather than both, on every Spec field. In other words, only add the diff if desired.Spec.field = non-nil and latest.Spec.field = nil but not the other way around.
2. Adding support in the generator config to enable the above behavior, but on a per-field basis

@vijtrip2 do you have any thoughts on this, and has work already been done on this that I've missed or am not aware of?

[surajkota]

essentially overwriting any one of these blocks:

yes

Do you mean directly populating desired.Spec with the server-returned default values?

yes