Re: [netmod] choice/case in tree diagrams

Martin Bjorklund <mbj@tail-f.com> Tue, 06 March 2018 09:44 UTC

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From: Martin Bjorklund <mbj@tail-f.com>
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Subject: Re: [netmod] choice/case in tree diagrams
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Hi,

After thinking some more about this, realizing that this document is
in AUTH48, and looking at the first sentence in the Abstract:

   This document captures the current syntax used in YANG module tree
   diagrams.

I have reached the conclusion that we probably shouldn't make any
drastic changes.

The current syntax, with flags for choice but not for case, may look a
bit odd, but it does follow RFC 7950 where a choice node can have a
config property, but case cannot.  Also, this syntax has now been used
for several years w/o causing much confusion.

I suggest the following changes to this document:

OLD:

       <flags> is one of:
         rw  for configuration data
         ro  for non-configuration data, output parameters to rpcs
             and actions, and notification parameters
         -w  for input parameters to rpcs and actions
         -u  for uses of a grouping
         -x  for rpcs and actions
         -n  for notifications
         mp  for nodes containing a "mount-point" extension statement

NEW:

       <flags> is one of:
         rw  for configuration data
         ro  for non-configuration data, output parameters to rpcs
             and actions, and notification parameters
         -w  for input parameters to rpcs and actions
         -u  for uses of a grouping
         -x  for rpcs and actions
         -n  for notifications
         mp  for nodes containing a "mount-point" extension statement

         case nodes do not have any <flags>.

Then, since the syntax requires whitespace before <name>:

     <status>--<flags> <name><opts> <type> <if-features>

we need to fix the examples:

OLD:

             +--rw (root-type)
                +--:(vrf-root)

NEW:

             +--rw (root-type)
                +-- :(vrf-root)

(two occurances)



/martin



Vladimir Vassilev <vladimir@transpacket.com> wrote:
> 
> 
> On 03/05/2018 06:40 PM, Per Hedeland wrote:
> > On 2018-03-05 16:06, Ladislav Lhotka wrote:
> >> On Mon, 2018-03-05 at 15:49 +0100, Per Hedeland wrote:
> >>> On 2018-03-05 15:41, Ladislav Lhotka wrote:
> >>>> On Mon, 2018-03-05 at 15:26 +0100, Martin Bjorklund wrote:
> >>>>> Juergen Schoenwaelder <j.schoenwaelder@jacobs-university.de> wrote:
> >>>>>> On Mon, Mar 05, 2018 at 02:54:18PM +0100, Martin Bjorklund wrote:
> >>>>>>>> So it seems the running code got it right. ;-)
> >>>>>>> As the author of that code, I think that was purely by accident...
> >>>>>>>
> >>>>>>> But I'm not convinced it is the correct solution.  We have one example
> >>>>>>> in the other thread where someone was confused by the "rw" flag and
> >>>>>>> thought that it implied that the node would be present in the data
> >>>>>>> tree.
> >>>>>>>
> >>>>>> So what does rw mean?
> >>>>>>
> >>>>>> (i)  The schema node has a rw property.
> >>>>>> (ii) The schema node can be instantiated and the instantiated data
> >>>>>> node
> >>>>>>       has a rw property.
> >>>>>>
> >>>>>> I think it is difficult to have both at the same time. If the tree is
> >>>>>> a representation of schema nodes, then (i) seems to make more
> >>>>>> sense. That said, the explanation in 2.6 is somewhat vague since it
> >>>>>> says 'data' and not 'nodes' (like everywhere else):
> >>>>>>
> >>>>>> OLD:
> >>>>>>
> >>>>>>         <flags> is one of:
> >>>>>>           rw  for configuration data
> >>>>>>           ro  for non-configuration data, output parameters to rpcs
> >>>>>>               and actions, and notification parameters
> >>>>>>
> >>>>>> NEW:
> >>>>>>
> >>>>>>         <flags> is one of:
> >>>>>>           rw  for configuration data nodes
> >>>>>>           ro for non-configuration data nodes, output parameters to
> >>>>>>           rpcs
> >>>>>>               and actions, and notification parameters
> >>>>> I think this is ok.  But that means that we also have to add:
> >>>>>
> >>>>>             --  for a choice or case node
> >>>>>
> >>>>> But in order to be consistent, we should probably have:
> >>>>>
> >>>>>             --  for a choice, case, input or output node
> >>>> But unlike the three other statements, "choice" can have the config
> >>>> substatement, so "rw/ro" makes sense there.
> >>> I don't think so - that config statement does not a define a property
> >>> of
> >>> the choice node (it can obviously neither be read nor written), only a
> >>> default for descendant data nodes, as described in section 7.21.1 of
> >>> RFC
> >>> 7950.
> >> It is not a default - if a choice has "config false", then no
> >> descendant can be
> >> "config true". One of the benefits of having rw/ro in the ascii tree
> >> is to see
> >> where a state data subtree actually starts.
> > It is a default, but yes, it is also a restriction in the specific
> > case
> > of the argument being "false" at a point where the default would
> > otherwise be "true". And in that case it is equivalent to having
> > "config
> > false" on all the descendant data nodes, and they will of course be
> > flagged as "ro" regardless of whether the "config false" comes from
> > the
> > choice or the individual data nodes - and that is where the state
> > *data*
> > suntree(s) actually start(s).
> >
> > So I guess the question then is whether this specific case motivates
> > always having flags on specifically choice nodes, while the other
> > non-data nodes have no flags. Since the 'config' statement is ignored
> > in
> > rpc/action input/output and notification, choice nodes there should
> > then
> > presumably have "-w"/"ro"/"-n". Personally I think the diagram is
> > clearer with flags only on the data nodes.
> When I think about it <flags> do not have any information contents 
> outside of the context of a data tree and its schema. So if we are
> removing clutter we should probably start there by specifying that
> <flags> should be ommited under rpc,notification and action.
> 
> Vladlimir
> >
> > --Per
> >
> >> Lada
> >>
> >>> --Per
> >>>
> >>>> Lada
> >>>>
> >>>>>
> >>>>> This means that the correct tree syntax for choice and case will be:
> >>>>>
> >>>>>       +-- (subnet)?
> >>>>>          +-- :(prefix-length)
> >>>>>          |  +--rw prefix-length?   uint8
> >>>>>          +-- :(netmask)
> >>>>>             +--rw netmask?         yang:dotted-quad
> >>>>>
> >>>>>
> >>>>> /martin
> >>>>>
> >>>>>
> >>>>>> The document (as far as I searched for it) does not clearly say that
> >>>>>> 'node' means 'schema node'. In hindsight, it might have been useful to
> >>>>>> explicitely import terminology from RFC 7950 and to use it carefully
> >>>>>> (RFC 7950 has 'schema node' and 'data node' but here we largely talk
> >>>>>> about 'nodes' - and my assumption is that this means 'schema nodes'.)
> >>>>> _______________________________________________
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