Re: [netmod] Proposal to enhance the YANG tree output

[Lou Berger <[e-mail]>]

Martin,

Speaking as a contributor:


On 9/15/2017 7:40 AM, Martin Bjorklund wrote:
> Robert Wilton <[e-mail]> wrote:
>> On 15/09/2017 11:21, Ladislav Lhotka wrote:
>>> Andy Bierman píše v Čt 14. 09. 2017 v 08:43 -0700:
>>>> Hi,
>>>>
>>>>
>>>> Actually I liked the early pyang output that was concise and easy to
>>>> remember.
>>>> The current format gets very cluttered and there are too many little
>>>> symbols
>>>> to remember them all.
>>> I agree.
> Me too.  The current draft adds three new magic symbols: "mp" "@" and
> "/".
>
> "mp" is for a mount point, and it can be generated directly from the
> YANG modules.
>
> Directly under a "mp", "/" and "@" are used to indicate that a node is mounted
> or available through a parent reference, respectively.
>
> I actually question the usability of "/" and "@".  

I agree that / and @ are something new, and enabled by schema mount. 
There have been repeated comments in the RTG WG that there needs to be
some way for vendors to convey what they have implemented with Schema
mount and this is one way to help convey (a) what is expected of server
implementors and (b) what client implementors should expect.   Hence the
current draft text:

  In describing the intended use of a module containing a mount point,
   it is helpful to show how the mount point would look with mounted
   modules.

The whole point of trees is to facilitate understanding for those who
are less familiar with a model than the authors, and IMO that's the
paramount perspective in this discussion.

> Since a parent
> reference can be very specific, e.g. one specific interface, it isn't
> really accurate to show:
>
>                   +--mp vrf-root
>                      +--rw rt:routing/
>                      |  ...
>                      +--ro if:interfaces@
This is just a conditional and we have precedent on how to handle
conditional representation.   

>
> And the trailing "/" on rt:routing doesn't add any information we
> don't already know.  Since vrf-root is a mount point, it follows that
> its children are mounted.

The issue is a bit more complex when considering some real use cases,
particularity when parent references and augmentations are used.  For
example consider the following *without* the use / or @:

module: ietf-network-instance
  +--rw network-instances
     +--rw network-instance* [name]
        | ...
        +--rw (root-type)
           +--:(vrf-root)
              +--mp vrf-root
                 +--ro rt:routing-state
                 |  +--ro router-id?                 yang:dotted-quad
                 |  +--ro control-plane-protocols
                 |     +--ro control-plane-protocol* [type name]
                 |        +--ro ospf:ospf
                 |           +--ro instance* [af]
                 +--rw rt:routing
                 |  +--rw router-id?                 yang:dotted-quad
                 |  +--rw control-plane-protocols
                 |     +--rw control-plane-protocol* [type name]
                 |     +--rw ospf:ospf
                 |        +--rw instance* [af]
                 |           +--rw areas
                 |              +--rw area* [area-id]
                 |                 +--rw interfaces
                 |                    +--rw interface* [name]
                 |                       +--rw name if:interface-ref
                 |                       +--rw cost?   uint16
                 +--ro if:interfaces
                 |  ...
                 +--ro if:interfaces-state
                 |  ...


It's certainly not too hard to spot the top level mounts, but it is
impossible to distinguish the parent references from the actual mounts. 
Further more, some mounts are hard to spot.  For example, notice ospf. 
Did you notice that it's a mount? Is it a mount or parent reference? 
With the / and @ both cases are transparent:

module: ietf-network-instance
  +--rw network-instances
     +--rw network-instance* [name]
        | ...
        +--rw (root-type)
           +--:(vrf-root)
              +--mp vrf-root
                 +--ro rt:routing-state/
                 |  +--ro router-id?                 yang:dotted-quad
                 |  +--ro control-plane-protocols
                 |     +--ro control-plane-protocol* [type name]
                 |        +--ro ospf:ospf/
                 |           +--ro instance* [af]
                 +--rw rt:routing/
                 |  +--rw router-id?                 yang:dotted-quad
                 |  +--rw control-plane-protocols
                 |     +--rw control-plane-protocol* [type name]
                 |     +--rw ospf:ospf/
                 |        +--rw instance* [af]
                 |           +--rw areas
                 |              +--rw area* [area-id]
                 |                 +--rw interfaces
                 |                    +--rw interface* [name]
                 |                       +--rw name if:interface-ref
                 |                       +--rw cost?   uint16
                 +--ro if:interfaces@
                 |  ...
                 +--ro if:interfaces-state@
                 |  ...

> Also, what is mounted under a mount point is not defined in the
> schema, so a tool cannot generate this from the YANG modules.

I think this is a limitation in the current schema-mount definition that
perhaps will be revisited to support design time mounts, but
nonetheless  still has value to model any reader and implementor.
...

Lou