[netmod] Proposal for minimalist full NMDA support in schema mount

[Lou Berger <[e-mail]>]

Hi,

(I have a bunch of different roles WRT this work.  This mail is being 
sent as an individual - as chair, I fully support the previous chair 
statements on this draft.)

Chris and I have come up with a proposal on how to provide full NMDA as 
part the existing schema-mount module.  Our motivation was to enable 
full NMDA support with *minimal* change to the model and disruption to 
the LC'ed work.  The key NMDA limitation, with -08, that we are aiming 
to address is the ability to support different mounted schema in 
different datastores for non-inline mount points. (See more detailed 
description below if interested full nuances of limitations of -08)

What we came up with was  to simply add a (leaf)list to identify in 
which datastores a
schema-mount schema is valid/present. This is somewhat similar to YL-bis 
schema/module-set. Specifically we're proposing (see below for full tree 
below):

            +--ro schema* [name]
               +--ro name           string
ADD          +--ro datastore*    ds:datastore-ref {revised-datastores}

This approach has the advantages of supporting different mounted schema 
in different DSes, working with both NMDA and non-NMDA implementations, 
supporting all of the extensively discussed features of schema mount 
(including recursive mounts), and having minor/scoped impact on all 
dependent work.  The main downside is that it isn't the most 
optimal/compact solution possible if we were to base this work on 
YL-bis/pre09 draft.  Of course -08 isn't necessarily optimal from all 
perspectives, but it is what was agreed to as sufficient by those who 
contribute to the WG discussion.

In short, we see this as a solution to  addresses the raised last call 
issue with the minimal impact on -08 and dependent work -- which is what 
is appropriate given where we are in the process.

So our/my question really is:

     Is this a solution that you/all can live with?

Note: optimization, design preference and perfect alignment with use or 
YL-bis are not part of our question as we both don't think that is the 
right question given where we are in the WG process.

Lou (with ideas developed with Chris, and chair hat off)

======
Details -- for those who want
======
As background, my understanding/view is that the -08 version of the both 
NMDA and non-NMDA supporting implementations, but there are limitations 
in its NMDA applicability. Used with Yang Library, [rfc7895], only 
non-NMDA implementations can be supported.  When used with the revised 
Yang Library defined in [I.D.ietf-netconf-rfc7895bis-],  NMDA 
implementations  can be supported with certain limitations. 
Specifically, this document requires use of the now deprecated 
module-list grouping, and the same schema represented in schema list of 
the Schema Mount module MUST be used in all datastores.  Inline type 
mount points, which don't use the schema list,  can support different 
schema in different data stores not by instantiating the 
[I.D.ietf-netconf-rfc7895bis-] version of YANG library under the inline 
mount point.

     module: ietf-yang-schema-mount
         +--ro schema-mounts
            +--ro namespace* [prefix]
            |  +--ro prefix    yang:yang-identifier
            |  +--ro uri?      inet:uri
            +--ro mount-point* [module name]
            |  +--ro module        yang:yang-identifier
            |  +--ro name          yang:yang-identifier
            |  +--ro config?       boolean
            |  +--ro (schema-ref)?
            |     +--:(inline)
            |     |  +--ro inline?       empty
            |     +--:(use-schema)
            |        +--ro use-schema* [name]
            |           +--ro name
            |           |       -> /schema-mounts/schema/name
            |           +--ro parent-reference*   yang:xpath1.0
            +--ro schema* [name]
               +--ro name           string
ADD          +--ro datastore*	ds:datastore-ref {revised-datastores}
               +--ro module* [name revision]
               |  +--ro name                yang:yang-identifier
               |  +--ro revision            union
               |  +--ro schema?             inet:uri
               |  +--ro namespace           inet:uri
               |  +--ro feature*            yang:yang-identifier
               |  +--ro deviation* [name revision]
               |  |  +--ro name        yang:yang-identifier
               |  |  +--ro revision    union
               |  +--ro conformance-type    enumeration
               |  +--ro submodule* [name revision]
               |     +--ro name        yang:yang-identifier
               |     +--ro revision    union
               |     +--ro schema?     inet:uri
               +--ro mount-point* [module name]
                  +--ro module        yang:yang-identifier
                  +--ro name          yang:yang-identifier
                  +--ro config?       boolean
                  +--ro (schema-ref)?
                     +--:(inline)
                     |  +--ro inline?       empty
                     +--:(use-schema)
                        +--ro use-schema* [name]
                           +--ro name
                           |       -> /schema-mounts/schema/name
                           +--ro parent-reference*   yang:xpath1.0

We would expect that the revised-datastores feature would be used
(perhaps required) for any implementation that supports ietf-datastores
and yl-bis.