[OPSAWG] Question on how to design a Yang model to reflect auto-asignment of a give leaf

[Oscar González de Dios <[e-mail]>]

Dear OPSAWG and Netmod colleagues,

                During last IETF Opsawg meeting we raised a question (and there was some discussion during the meeting) that we have found yet no good answer and we would like to discuss it with operations and Yang experts.

                The use case is the following:  We have a yang module which holds certain optional leafs. The behaviors that we would like to have (and distinguish between them) are:


a)       The user does not provide the value and such value is auto-assigned by the system (a  device (if it is a device module) or a controller (if it is a network/service module)).

b)      The user does not provide a value and wants that such value IS NOT set by the system (as assigning a value has implications). That is, intentionally it is aimed at being left "empty" and should not be expanded. So, either the value is set or should remain empty

What is the best way to model this behavior? I see that some yang modules have added an "auto-assignment" leaf to express if auto-assignment is desired or not. (hence, auto-assignment false, and leaf not set, would  do not assign).

Which is the "default" rule for a leaf that is not set? It is that the system is free to create it (via template or any means of auto-assignment) or should leave it as is, that is, empty?

In NMDA, the system is allowed to expand a given configuration. This fact, in my personal view,  implies that by "default" any system could implement the "auto-assignment" behavior being compliant with Neconf/Restconf/NMDA rules (but I am not sure if the interpretation is correct).

Best Regards,

                Óscar

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