Re: [netmod] Schema-mount question: Augmentation to the Mounted Module

["Xufeng Liu" <[e-mail]>]

Hi Martin,

> -----Original Message-----
> From: Martin Bjorklund [mailto:[e-mail]]
> Sent: Wednesday, June 14, 2017 1:07 PM
> To: [e-mail]
> Cc: [e-mail]; [e-mail]; draft-ietf-netmod-schema-
> [e-mail]; [e-mail]
> Subject: Re: [netmod] Schema-mount question: Augmentation to the Mounted
> Module
> 
> Ladislav Lhotka <[e-mail]> wrote:
> >
> > > On 14 Jun 2017, at 13:43, Lou Berger <[e-mail]> wrote:
> > >
> > > Hi,
> > >
> > > (speaking as contributor...)
> > >
> > >
> > > On 6/14/2017 7:17 AM, Ladislav Lhotka wrote:
> > >> Hi Xufeng,
> > >>
> > >> please see my answers inline.
> > >>
> > >> Xufeng Liu <[e-mail]> writes:
> > >>
> > >>> Hi Lada,
> > >>>
> > >>>
> > >>>
> > >>> We have got two questions on how to specify the module entries in
> > >>> a
> > >>> schema:
> > >>>
> > >>>
> > >>>
> > >>> 1. Are augmentations of parent modules inherited when augmented
> > >>> module is listed in schema-mounts schema?
> > >>>
> > >>> For example, ietf-ospf module augments ietf-routing. When we
> > >>> include ietf-routing to the schema entry, is ietf-ospf automatically
> included?
> > >> No, you also have to include "ietf-ospf" in the "module" list
> > >> inside the corresponding "schema" entry, exactly as you do in the
> > >> top level YANG library, otherwise ietf-ospf won't be mounted.
> > >
> > > I agree.  The draft should have text that makes this explicit.
> >
> > Why? It should be sufficiently clear that modules that are not listed
> > in "schema" are not present in the mounted schema. An augment is just
> > a special mechanism of contributing schema nodes.
> 
> Yes I agree.  But let's see if we can clarify the text.  Xufeng, what in
the current
> text led you to believe that a module in the parent schema would be
> automatically present in the mounted schema?
> 
[Xufeng] Thanks for looking at this. The confusion is because of the lack of
text, I would say. The term "mount" has an analogy to the Unix file system
"mount", where what we only specify the parent directory and child file
system (the connecting relationship at the connection point). Also, similar
is the command for the Unix soft/hard links, where we don't need to check if
there are other links under the child. 

> > >>> 2.	When we have ietf-yang-library mounted under a parent (LNE),
does
> > >>> ietf-yang-library have to contain exactly the same list of Yang
> > >>> modules as the list contained in the "schema" entry under
> > >>> "schema-mount"?
> > >> I am not sure I understand but do you mean an LNE mounted schema
> > >> defined via the "use-schema" case that also includes
> > >> ietf-yang-library? This is a corner case we probably haven't
> > >> thought about but it IMO doesn't make any sense to do so because
> > >> the applicable YANG library that counts is inside the "schema"
> > >> entry. Martin, should we address this anomaly?
> > >
> > > I think this is a very real scenario for LNE.  Consider a 'host'
> > > system
> > > that allows read only views of the LNE and wants the benefit of
> > > "use-schema".  In this case, library under the mount point is still
> > > needed for client access within the mounted LNE.
> >
> > In this case it would IMO be much better if the server inside the LNE
> > provide YANG library data separately for its clients. The client of
> > the host system needn't see it because it is just redundant.
> >
> > >
> > >
> > > It seems to me that in this case the mounted library module data
> > > must exactly match what is listed in the corresponding "schema"
> > > entry under "schema-mount" in order to ensure deterministic client
> views/behavior.
> > > Again, I think this should be made explicit in the draft.
> >
> > Another option is to ban ietf-yang-library in schemas mounted via
> > "use-schema".
> 
> This won't help.  The question is still the same - if you use
"use-schema", and
> one of the mounted modules is "ietf-yang-library", does the contents of
the
> yang library in both places have to be the same?
> 
> Anyway, I think the answer is that from the current specification, it
follows that
> the contents *will* be the same.
> 
> 
> /martin
> 
> 
> 
> > I still think it is wrong to tout "inline" and "use-schema" as the
> > same "schema mount" concept. If we clearly separated the two concepts,
> > "inline" would become an absolute no-brainer requiring just a single
> > YANG extension statement, and "use-schema" would also be easier to
> > explain with no confusing exceptions and qualifications. It's just
> > simple divide-and-conquer in terms of the spec, with no limitations
> > compared to the current options.
> >
> > Lada
> >
> > >
> > >> BTW, I think that normally LNE schema is supposed to be mounted
> > >> using the "inline" case, and then of course ietf-yang-library is
> > >> required but there is no "schema" entry under "schema-mounts" to
> > >> worry about.
> > > Both inline and non-inline LNE usage is expected in real systems...
> > >
> > > Lou
> > >> Lada
> > >>
> > >>> For example, ietf-ospf module augments ietf-routing. When we mount
> > >>> ietf-routing ietf-yang-library to LNE, should we list ietf-ospf in
> > >>> the mount module list? And also in ietf-yang-library?
> > >>>
> > >>>
> > >>>
> > >>> It would be great if these can be clarified.
> > >>>
> > >>>
> > >>>
> > >>> Thanks,
> > >>>
> > >>> - Xufeng
> >
> > --
> > Ladislav Lhotka, CZ.NIC Labs
> > PGP Key ID: 0xB8F92B08A9F76C67
> >
> >
> >
> >
> >