Subject: Re: [netmod] Motivations for Structuring Models

From: Randy Presuhn <[email-masked]>
Date: Sat, 29 Aug 2015 15:08:52 -0400 (EDT)

Hi -

It is with no little amusement that I watch this thread struggling
with questions that were solved fairly neatly a quarter century ago
in GDMO/CMIP-land.  I'm *not* suggesting we go back there, but would
like to offer an observation about modeling that might help.

The organization of instance data in SNMP is a direct mirror of
the "object" definitions.  Simple at first, but quickly becoming
baroque as various minds of "multiplexing" are added to compensate
for post hoc deficiencies in the index structures.

Life is such that once a resource has been modeled, it will be
used/re-used/embedded in systems in ways in which its designers
couldn't be expected to imagine.  A consequence of this is that
if instance naming is completely locked down when the management
interface for a resource is first defined (as it is in SNMP) then
all sorts of peculiar hacks will be needed to deal with, for example,
virtual routers.  Unfortunately, an SNMP/SMI-like mindset is so
pervasive that folks seem to overlook that there are other ways
to deal with this situation.

What GDMO did was to use a separate "NAME BINDING" construct to
specify contexts in which instances might show up, allowing
instances to be put in places that weren't even imagined when
the original class definition was written.  Name bindings could
be standardized, or be vendor or even product-specific, allowing
the simplicity or complexity of a given system's instance tree
to reflect the actual simplicity or complexity of that system,
rather than requiring all systems to be structured for the
worst case.

Yes, separating the specification of instance naming in large part
from class definition does have implications for how one does access
control, and how clients figure out how to ask a server to create
something, but it's not a huge deal - it's just not like VACM, and a
whole slew of hacky solutions and "wierd plumbing adapters" (to borrow
from Jeff Case) just go away.  Strangely, it makes the job of the
initial modeler and of the eventual user much easier.

Randy

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From: Ladislav Lhotka <[email-masked]>
Date: Mon, 31 Aug 2015 17:04:42 +0200

> rather than requiring all systems to be structured for the

How could this be expressed in YANG terms? (I tried to figure it out
myself but I unfortunately couldn't make any sense of sec. 8.6 in CCITT
Recommendation X.722).

Thanks, Lada

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Ladislav Lhotka, CZ.NIC Labs
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