Re: [netmod] Questions about how to assign default values with YANG

[Juergen Schoenwaelder <[e-mail]>]

On Wed, Mar 10, 2021 at 05:34:41PM +0000, Italo Busi wrote:
> Juergen,
> 
> I think you have got the problem: "a data model author thought the default is always 0 and later he/she realizes that in some contexts the default should be something different"
> 
> Unless I am missing something, creating a new leaf (e.g., foo-new) would confuse an existing client.

No, it does not confuse the client, the client will ignore it.
 
> For example, within the operational DS, the value of foo will be set to 0 (as per YANG default statement) while the value of foo-new will be set to 10, which represents the actual value in use by the system.

Yes, a new implementation will have to declare that it does not
implement foo.
 
> Now, the existing client, which is not aware of foo-new, when reading the value of foo in the operational DS will incorrectly think that the actual value in use by the system is 0 rather than 10.

A client reading <operational> knows the value in use. But clients do
not have to real operational.

> Am I missing anything?
> 
> Instead, if we can find a magic way to apply the value 10 to foo within the operational DS, the existing client can read the value of foo in the operational DS and correctly understand that the actual value in use by the system is 10 (even if this is not the default value of foo).

In general, you can't assume that clients read operational. I can't
judge the specific circumstances but in traditional NC/RC, a default
statement can be interpreted as "assume the default value is in force
if this lead is not configured" (unless the client uses RFC 6243
report-all). If all clients are in an NMDA world, the issue is much
smaller - it is reduced to "does the client believe the server has a
bug or not". But even then, I continue to believe that a leaf changing
the semantics of another leaf is bad design.

/js

-- 
Juergen Schoenwaelder           Jacobs University Bremen gGmbH
Phone: [phone]         Campus Ring 1 | 28759 Bremen | Germany
Fax:   [phone]         <https://www.jacobs-university.de/>