Re: [netmod] [Netconf] Alternative YANG library structure for 7895bis

Vladimir Vassilev <vladimir@transpacket.com> Mon, 11 December 2017 15:53 UTC

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To: Robert Wilton <rwilton@cisco.com>, "netmod@ietf.org" <netmod@ietf.org>, Netconf <netconf@ietf.org>
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From: Vladimir Vassilev <vladimir@transpacket.com>
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Date: Mon, 11 Dec 2017 16:53:30 +0100
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Subject: Re: [netmod] [Netconf] Alternative YANG library structure for 7895bis
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On 12/11/2017 12:16 PM, Robert Wilton wrote:

> Hi Vladimir,
>
>
> On 09/12/2017 11:49, Vladimir Vassilev wrote:
>> On 12/08/2017 07:01 PM, Andy Bierman wrote:
>>
>>> Hi,
>>>
>>> A library per datastore sounds too complicated.
>> I am not proposing that.
> I'm slightly lost, because I thought that was exactly what you were 
> proposing! ;-)
I propose a solution that keeps ietf-yang-library a data model of a 
single YANG context specification doing the datastore specific modeling 
in ietf-datastores model instead. The solution can be both flexible 
(independent yang-library data instances per datastore as my example was 
focused on) or constrained ('not-implemented-in' modules leaf-list). 
Here is everything that is needed:

module: ietf-datastores
     +--ro datastores-state
        +--ro datastore* [name]
        +--ro (model)?
           +--:(same-as-operational)
           +--:(constrained-to-operational)
           |  +--ro not-implemented-in*       -> 
/yanglib:module-state/module/name
           +--:(unconstrained)
              +--ro yang-library-datastore?   identityref

YANG:
...
container datastores-state {
     config false;
     list datastore {
       key name;
     }
     choice model {
         case same-as-operational {
         }
         case constrained-to-operational {
           leaf-list not-implemented-in {
             type leafref {
               path "/yanglib:module-state/yanglib:module/yanglib:name";
             }
           }
         }
         case unconstrained {
           leaf yang-library-datastore {
             type identityref {
               base ds:datastore;
           }
         }
       }
     }
   }
...

IMO ietf-yang-library as defined in rfc7895 is modular and reusable. I 
do not see why this has to be compromised.
>>
>> The fundamental point proposed is that the datastore relevant bits 
>> are kept in the ietf-datastores module instead of merging everything 
>> in a new ietf-yang-library entangled monster module. If needed 
>> ietf-datastores can augment ietf-yang-library but ietf-yang-library 
>> should be usable on its own without ietf-datastores. The solution is 
>> coherent and modular and addresses the problem statement.
>
> The issue with this is that datastore augmentations to YANG library 
> would end up changing the meaning of the existing YANG library nodes.  
> E.g. an old client that ignores the datastore augmentations is going 
> to get a nasty surprise when the server does not behave how it 
> expects.  E.g. because the configuration node that it thinks should be 
> there isn't there because it only supported in <operational>.
>
> This was one of the reasons for changing YANG library.
A well written client that finds out unsupported newer versions of 
ietf-yang-library (which is reported in the capabilities) or any of the 
NMDA modules is deployed should not do any damage. How exactly did you 
solve the problem for the bad clients by changing the structure of 
yang-library in a incompatible way is something I do not understand.
>
> In terms of the idea of just re-using YANG library but export a 
> separate copy for each datastore I think that this has its own problems:
> - I don't like the idea of returning meta-data along with 
> configuration for a <get-data> on any of the configuration datastores.
There is no meta data. The datastores contain only the config false; 
/modules-state tree. I think I explained that very clearly.
> - How does a client know whether the YANG library for <running> 
> applies to the whole server (as it does today) vs just applies to the 
> <running> datastore (as it would for an NMDA server)?
All datastore models are "case same-as-operational" for such systems.
> - It requires more handshaking between the client and server to get 
> the schema, since a separate request would be required for each 
> datastore that is supported.
Correct. Flexibility and modularity come at a price. IMO modularity is 
more important in this case. And there are solutions for the problem 
e.g. send all <get-data> requests before waiting for replies. NETCONF 
supports this.
>
> So, for me, I think that the only way that this solution works, would 
> be to define a new <get-server-metadata> RPC, but even then I think 
> that it would make sense to combine the data together into a new YANG 
> library structure.
As I said my focus is on keeping ietf-yang-library modular (single YANG 
model context). <get-server-metadata> or just adding datastore 
identities allowing the client to use <get-data> to achieve the 
retrieval of the data is of little importance to me.

>
> At the end of the day, I don't think that a new YANG library is going 
> to be were the real cost for supporting NMDA comes from.  I think that 
> the real work is supporting <operational> independently from <running> 
> both in the client and servers. But I also think that once servers 
> start implementing this properly that it will simplify automation, 
> because rather than a client having to guess what state a server is 
> in, it can actually querey, or be notified of it, without having to 
> write a lot of model specific code.
+1

Vladimir
>
> Thanks,
> Rob
>
>
>>> I prefer the proposal that was made at the IETF meeting that had
>>> a 'not-implemented-in' leaf-list and a single module list.
>> This constraint is already specified in the text of the revised 
>> datastores draft. Clients conforming to the draft can expect servers 
>> to comply with the MUST requirement even if there is a separate 
>> yang-library data tree for each datastore the constraint of 
>> configuration stores mapping to 'operational' should be enforced 
>> according to the draft. There is no contradiction here.
>>
>> That said I would be also be OK with ietf-datastores augmenting 
>> ietf-yang-library with such a leaf-list ('not-implemented-in' 
>> leaf-list) as a more constrained flavor of the same approach instead 
>> of going for independent copies of yang-library data. For any of that 
>> to happen change in ietf-datastores.yang is needed and change in the 
>> original rfc7895 ietf-yang-library is not needed at all.
>>
>> Vladimir
>>
>>>
>>> Why is it interesting to have a separate module list for regular 
>>> modules and imported modules?
>>> I prefer to keep the conformance leaf and not change the module list.
>>>
>>> NMDA needs to be possible to implement with a single schema tree 
>>> such that a module
>>> is implemented in all datastores, or a subset of all datastores.  
>>> Otherwise it probably won't
>>> get supported in clients.
>>>
>>>
>>> Andy
>>>
>>>
>>>
>>> On Fri, Dec 8, 2017 at 9:21 AM, Kent Watsen <kwatsen@juniper.net 
>>> <mailto:kwatsen@juniper.net>> wrote:
>>>
>>>     CC-ing NETCONF, where the draft is being worked on.
>>>
>>>     Kent
>>>
>>>
>>>     On Fri, 2017-12-08 at 16:34 +0100, Juergen Schoenwaelder wrote:
>>>     > On Fri, Dec 08, 2017 at 04:19:28PM +0100, Vladimir Vassilev 
>>> wrote:
>>>     > >
>>>     > > Yes. The default value for yang-library-datastore leaf is
>>>     ds:operational
>>>     > > (the only possible one for the ds:operational datastore). This
>>>     is backward
>>>     > > compatible. If one needs different model for 'running', etc.
>>>     then a new
>>>     > > datastore identity has to be defined  and set in place of the
>>>     default value.
>>>     > > Then this identity can be used to read the yang-library data 
>>> with
>>>     > > <get-data>.
>>>     > >
>>>     >
>>>     > Sorry, but I have to ask this: How do I obtain the schema for the
>>>     > datastore (lets call it <running-library>) that reports the
>>>     schema for
>>>     > <running>? Is there another <running-library-library> datastore?
>>>     Will
>>>     > the recursion end? Perhaps it does since 
>>> <running-library-library>
>>>     > might have itself listed as the schema defining datastore. I 
>>> guess
>>>     > Lada will like these kind of meta and meta-meta datastores.
>>>
>>>     Not really. Metadata needn't be in datastores.
>>>
>>>     Lada
>>>
>>>     >
>>>     > /js
>>>     >
>>>     --
>>>     Ladislav Lhotka
>>>     Head, CZ.NIC Labs
>>>     PGP Key ID: 0xB8F92B08A9F76C67
>>>
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