Re: [netmod] OpsState Direction Impact on Recommended IETF YANG Model Structure

[Robert Wilton <[e-mail]>]

Please see RW: inline


On 12/07/2016 20:15, Andy Bierman wrote:
>
>
> On Tue, Jul 12, 2016 at 12:07 PM, Juergen Schoenwaelder 
> <[e-mail] 
> <mailto:[e-mail]>> wrote:
>
>     On Tue, Jul 12, 2016 at 11:36:03AM -0700, Andy Bierman wrote:
>     > Yes there is value in modeling conventions in general.
>     > I am trying to understand the value of this specific convention.
>     >
>     > If I have an RPC that asks for applied state, then it doesn't
>     really matter
>     > how the config and state is organized.  There is only overlap in
>     the objects
>     > if the data model is designed that way.
>     >
>     > Whether or not an edit is applied yet is a property of the
>     implementation,
>     > not the data model.
>
>     The current /foo and /foo-state approach requires some amount of
>     duplication. If the trees contain some nested lists of lists, you have
>     to at least replicate all the key leafs in the schema definition. For
>     a small model, this is not a bit deal; for more complex models, this
>     becomes complex and there are possible sources of errors. If it is
>     possible to simply data models, this may be a long-term win.
>
>
>
> It only requires duplication if you insist on modeling opstate objects.
> IMO an RPC + notification approach is far superior.
RW:

Are you thinking of a single global notification of convergence?

Although such a scheme may be sufficient if the configuration is 
changing infrequently, it is not sufficient for a system in which 
configuration is changing frequently (e.g. say to provision very short 
lived tunnels across a data center network).

If the client is interested when a particular config change has 
converged then a global converged flag:
  (i) may be delayed a long time (if at all) after the config change has 
completed (because other subsequent config operations are in progress)
  (ii) may be hard to relate back to the original config change

Hence, I think that there are only 2 technical solutions to this problem:
(a) reporting/notifying the state of the applied configuration nodes (as 
per the requirements stated in draft-ietf-netmod-opstate-reqs, and 
covered by the all the various solution drafts)
(b) report the success/failure of the particular config operation.

My opinion, is that as network configuration becomes more dynamic, and 
programmatically controlled then (a) is the easier solution and more 
robust to failure over (b).  But both approaches can be supported by 
devices.


> In this approach I only need to identify the intended config object
> and retrieve (or be notified) of the applied-state transition.
RW:

This is only part of the solution, since metadata can only be used when 
nodes exist in the YANG data tree.  So, this method alone cannot 
indicate configuration that is applied by the device and is in the 
process of being deleted.

That is primarily why draft-wilton-netconf-opstate-metadata-00 offers 
metadata both on the "persistent" datastore and "operational state 
datastore".



>
>     The reason why we have the /foo and /foo-state convention is, I
>     believe, the design of the NETCONF <get/> operation, which assumes
>     state and config can be represented together in a single tree. And we
>     have carried this further into YANG (e.g., the way contexts are
>     constructed for xpath expressions). In hindsight, I am not sure the
>     consequences of the <get/> design were that well thought through - but
>     I am not complaining, at that time we did not have YANG nor did we
>     have experience with bigger data models written in YANG. Is it worth
>     fixing it? This is a tough question and I understand that people have
>     different opinions and also different views on where we are on the
>     lifecycle of this technology. The challenge, as always, is to evolve a
>     technology along with its usage and upcoming new requirements. If the
>     evolution is too fast, you risk fragmentation since people will then
>     run many different versions. If evolution is too slow or stops
>     entirely, you pave the way for complete replacements of the
>     technology.
>
>     I think it is worth to step back for a moment and to think about where
>     we like to be in 5 or 10 years from now when we discuss architectural
>     questions.
>
>
>
> We have read-only foo-state because we have only been standardizing
> monitoring data for the last 30 years.  I completely agree that if the 
> state depends
> on the config in order for an instance to exist, then co-locate the 
> config and
> state and share the node naming.
>
>
> But modeling admin-state and oper-state with the same object is not a 
> requirement.
RW:

I agree, but when the oper-state exactly of a node reflects the applied 
admin-state then I see no benefit in having a separate duplicate "config 
false" node.

Rob


> Being able to determine if the intended config is applied yet is a 
> requirement.
> This can be done with protocol operations so it is 100% data model 
> neutral.
>
>
>
>     /js
>
>
> Andy
>
>
>     --
>     Juergen Schoenwaelder           Jacobs University Bremen gGmbH
>     Phone: [phone]         Campus Ring 1 | 28759 Bremen | Germany
>     Fax:   [phone]         <http://www.jacobs-university.de/>
>
>
>
>
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