Re: [yang-doctors] Style question from individuals working on IEEE YANG

[Schönwälder, Jürgen <[e-mail]>]

On Wed, Jan 15, 2020 at 12:15:19PM +0000, Scott Mansfield wrote:
> 
> The YANG people from IEEE 802.1 discovered a symmetric modelling
> issue related to YANG lists, and kindly request for feedback about
> the common practices in YANG modules beyond those from IEEE 802.1.
> 
> Consider the following YANG code example:
> 
> 01: leaf my-list-size {
> 02:   type uint32;
> 03:   config false;
> 04:   description "The number of elements in 'my-list'";
> 06: }
> 
> 07: leaf-list my-list {
> 08:   type my-list-element-type;
> 09: }
> 
> The state value of leaf my-list-size (L01) needs to be consistent with the
> number of elements in leaf-list my-list (L07). Therefore, IEEE 802.1 
> identified the following options:
> 
> A)           Delete the size leaf (i.e., my-list-size).
> B)            Formulate consistency requirements in IEEE standards documents
> C)            Formulate consistency requirements in YANG description nodes
> (e.g., L04).
> D)           Formulate consistency requirements in YANG must statement(s)
> E)            ???

Looking at this from an NMDA perspective, the my-list-size leaf can
only report correctly the size of my-list in the <operational>
datastore, i.e., the applied config. The my-list that exists in lets
say the <running> datastore may have more or fewer entries.

For configuration datastores, I would rather not have such size leafs.
For the operation state datastore, they are easy to do and I would
define the requirements where the size leaf is defined, i.e., the YANG
description statement. Note that the operational state datastore
should return the true state of the device, i.e., YANG must statements
would not help you much since there is no configuration change
operation that can be rejected due to a must violation.

/js

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