Re: [netmod] yang-data-ext issues

Robert Wilton <rwilton@cisco.com> Fri, 27 April 2018 11:19 UTC

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To: Ladislav Lhotka <lhotka@nic.cz>, Martin Bjorklund <mbj@tail-f.com>
Cc: netmod@ietf.org
References: <87muxq1pk9.fsf@nic.cz> <CABCOCHSwB8m6Uk82MZHnjnVi=0ofRohqEPT0Q06NM=+3n5gtFA@mail.gmail.com> <065a363753a67ee12f8bde6224009207b5fe7ee7.camel@nic.cz> <20180427.120325.419501937185262392.mbj@tail-f.com> <11da9315-40d9-60cd-d32f-b0ac4a5640c4@cisco.com> <45f5a97d205a9251b382687f54865c62250787cb.camel@nic.cz>
From: Robert Wilton <rwilton@cisco.com>
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Date: Fri, 27 Apr 2018 12:19:41 +0100
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Subject: Re: [netmod] yang-data-ext issues
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On 27/04/2018 12:03, Ladislav Lhotka wrote:
> On Fri, 2018-04-27 at 11:23 +0100, Robert Wilton wrote:
>> On 27/04/2018 11:03, Martin Bjorklund wrote:
>>> Hi,
>>>
>>> Ladislav Lhotka <lhotka@nic.cz>; wrote:
>>>> On Thu, 2018-04-26 at 17:52 -0700, Andy Bierman wrote:
>>>>> On Wed, Apr 25, 2018 at 10:53 PM, Ladislav Lhotka <lhotka@nic.cz>; wrote:
>>>>>> Ladislav Lhotka <lhotka@nic.cz>; writes:
>>>>>>
>>>>>>> On Wed, 2018-04-25 at 08:04 -0700, Andy Bierman wrote:
>>>>>>>> On Wed, Apr 25, 2018 at 7:05 AM, Ladislav Lhotka <lhotka@nic.cz>;
>>>>>>>> wrote:
>>>>>>>>> On Wed, 2018-04-25 at 15:55 +0200, Juergen Schoenwaelder wrote:
>>>>>>>>>> On Tue, Apr 24, 2018 at 04:36:01PM +0200, Martin Bjorklund
>>>>>>>>>> wrote:
>>>>>>>>>>> Ladislav Lhotka <lhotka@nic.cz>; wrote:
>>>>>>>>>>>> Martin Bjorklund <mbj@tail-f.com>; writes:
>>>>>>>>>>>>
>>>>>>>>>>>>> Hi,
>>>>>>>>>>>>>
>>>>>>>>>>>>> I am not sure what this statement tells us re. the issue
>>>>>>>>>>>>> in
>>>>>> this
>>>>>>>>> email
>>>>>>>>>>>>> thread.
>>>>>>>>>>>> It tells us that, in my view, the approach taken in this
>>>>>>>>>>>> document
>>>>>> is a
>>>>>>>>>>>> bad idea.
>>>>>>>>>>> Do you mean that the WG shoud drop this document?  And
>>>>>>>>>>> people that
>>>>>>>>>>> need yang-data should continue to use the version in
>>>>>>>>>>> 8040?  Or that
>>>>>>>>>>> people that need yang-data do not have a valid use case and
>>>>>>>>>>> they
>>>>>>>>>>> should do something else?
>>>>>>>>>> One option is that people use yang-data as defined in RFC 8040
>>>>>>>>>> until
>>>>>>>>> IMO, people should use plain YANG. With the new YANG library it
>>>>>>>>> will be
>>>>>>>>> possible
>>>>>>>>> to confine such non-NM schemas in a special datastore so that
>>>>>>>>> the
>>>>>> intention
>>>>>>>>> should be clear and multi-module schemas with all the additional
>>>>>>>>> data
>>>>>>>>> (versions,
>>>>>>>>>    features, deviations) can be used.
>>>>>>>>>
>>>>>>>> I don't see how yang-data interferes with "plain YANG" at all.
>>>>>>>> It is for data that is not in scope for plain YANG.
>>>>>>> My question is why this extension is needed in the first place.
>>>>>> For example, RFC 8040 could have used two modules instead of
>>>>>> "ietf-restconf", one with the contents of grouping "errors" and the
>>>>>> other with the contents of grouping "restconf". No extension.
>>>>>>
>>>>> This is true. We used to do this before yang-data was available.
>>>> If I remember correctly, the stuff was inside groupings that were not used
>>>> anywhere.
>>> Which doesn't quite work, since no namespace is attached to the nodes.
> Note that this is not what I was proposing. For RESTCONF errors, the module I
> had in mind could be like this:
>
> module ietf-restconf-errors {
>
>    container errors { // same content as in RFC 8040
>      ...
>    }
>
>    ...
>
> }
>
> Please explain why this cannot serve the given purpose, apart from the fact that
> it looks like configuration which it isn't - but this can be explained in the
> module description.
It is the "because it looks like configuration" that I don't like.

If the server supports and advertises this module then it is reasonable 
to expect that a client should be able to configure the errors 
container, since it is configuration ...

At least marking it as config false would be slightly better.

>
> With this module, one could validate error messages using generic YANG tools
> etc.
>
> (I am not proposing to update RFC 8040, just using it as an example.)
>
>> OK.  So, using plain groupings doesn't work.
>>
>> In cases where groupings are being used within a YANG defined RPC, then
>> presumably they do work OK?
>>
>> Is the specific problem scenario where the data is external to YANG
>> defined RPCs, but yet it is still desirable to use a YANG schema and one
>> of the associated YANG encodings to describe/encode the data?
>>
>>
>>>>>> What would be wrong with this solution? Instead, the reader is
>>>>>> overwhelmed with the complexity of the "yang-data" definition, and
>>>>>> most
>>>>>> tools cannot process the module.
>>>>> There are tools that can use yang-data.
>>>>> Not all use-cases involve a server to query for a yang-library.
>>>> Sure, but it is not necessary, I meant it just as an option. Such YANG
>>>> modules
>>>> can be passed straight to tools.
>>>>
>>>>> Offline tools need to know about the special data somehow.
>>>> Why? Let's say I want the ascii tree, and pyang will be able to generate
>>>> it. All
>>>> right, there will be "rw" labels that don't apply but it is not a big
>>>> deal.
>>>>
>>>>> The yang-data extension prevents data-def-stmts from being treated
>>>>> as if they were configuration or operational data.
>>>> This would be a problem if this yang-data is mixed with standard data in
>>>> the
>>>> same module. IMO this can be avoided, and then for it is essentially
>>>> irrelevant
>>>> for tools whether it is normal data or not.
>>>>
>>>>> I agree with you that unconstrained use of yang-data is questionable
>>>>> for a standard extension. The bar should be that all tools which choose
>>>>> to implement the extension should provide the user with the same
>>>>> behavior.
>>>>> Declaring that behavior out-of-scope does not help interoperability at
>>>>> all.
>>>> Yes, and so my proposal here is to silently misuse YANG somewhat where
>>>> necessary
>>>> rather than spend cycles on a Standard Track document that gives a false
>>>> impression of a general solution.
>>> I am strongly opposed to this.  IMO it is much better to put such
>>> structures in an extension, which tools that don't understand it will
>>> ignore, than relying on description statements in normal data nodes,
>>> which no tool can understand without hard coding special cases.
>> I'm also opposed to this.
>>
>> Stuff that looks like configuration should be configuration, and stuff
>> that looks like state should be state.  If this data is going to be
>> described in YANG then I think that there must be a programmatic way to
>> indicate that the resultant schema is not configuration or operational
>> state, but something else instead.  An extension seems to achieve this.
> YANG spec deals exclusively with configuration and state data, and using its
> statements inside an extension doesn't make this basic fact go away. Specifying
> all necessary changes properly inside a description of an extension is simply
> impossible.
If an implementation needs to support generating the error messages then 
they can support the yang-data extension if they want (or just hard code 
what they expect to receive).

Otherwise, devices can also ignore the yang-data extension and it 
doesn't seem to do any harm since its doesn't change the behaviour in 
any way.

Thanks,
Rob


>
> Lada
>
>> Thanks,
>> Rob
>>
>>
>>>
>>>> It would be great to remove NETCONF specifics from YANG and I'd be willing
>>>> to
>>>> contribute to this work.
>>> This is a different topic though.
>>>
>>>
>>> /martin
>>>
>>>
>>>> Lada
>>>>
>>>>>> Lada
>>>>>>
>>>>> Andy
>>>>>    
>>>>>>>> A plain client can ignore yang-data and not affect and RPC,
>>>>>>>> notification,
>>>>>> or
>>>>>>>> data
>>>>>>>> definitions in plain YANG.
>>>>>>> A plain (NC/RC) client should never see such data even if it is not
>>>>>> protected by
>>>>>>> yang-data in YANG. On the other hand, tools will be able to process
>>>>>>> such
>>>>>> schemas
>>>>>>> (generate the ascii tree, convert it to something else, generate
>>>>>>> sample
>>>>>>> instances etc.) without explicitly supporting yang-data.
>>>>>>>
>>>>>>> Lada
>>>>>>>
>>>>>>>>    
>>>>>>>>> Lada
>>>>>>>>>
>>>>>>>> Andy
>>>>>>>>    
>>>>>>>>>> there is a version of YANG that has a proper and complete
>>>>>>>>>> integrated
>>>>>>>>>> solution. (If for example yang-data is used to declare error
>>>>>>>>>> content
>>>>>>>>>> for RPCs, then more extensions are needed or a proper
>>>>>>>>>> integration
>>>>>> into
>>>>>>>>>> YANG. Is it really good to introduce augment-yang-data
>>>>>>>>>> (instead of
>>>>>>>>>> making augment work with say 'data' in YANG 1.2)? And then we
>>>>>>>>>> do
>>>>>>>>>> uses-yang-data etc.
>>>>>>>>>>
>>>>>>>>>> /js
>>>>>>>>>>
>>>>>>>>> -- 
>>>>>>>>> Ladislav Lhotka
>>>>>>>>> Head, CZ.NIC Labs
>>>>>>>>> PGP Key ID: 0xB8F92B08A9F76C67
>>>>>>>>>
>>>>>>>>> _______________________________________________
>>>>>>>>> netmod mailing list
>>>>>>>>> netmod@ietf.org
>>>>>>>>> https://www.ietf.org/mailman/listinfo/netmod
>>>>>>> -- 
>>>>>>> Ladislav Lhotka
>>>>>>> Head, CZ.NIC Labs
>>>>>>> PGP Key ID: 0xB8F92B08A9F76C67
>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> netmod mailing list
>>>>>>> netmod@ietf.org
>>>>>>> https://www.ietf.org/mailman/listinfo/netmod
>>>>>> _______________________________________________
>>>>>> netmod mailing list
>>>>>> netmod@ietf.org
>>>>>> https://www.ietf.org/mailman/listinfo/netmod
>>>> -- 
>>>> Ladislav Lhotka
>>>> Head, CZ.NIC Labs
>>>> PGP Key ID: 0xB8F92B08A9F76C67
>>>>
>>>> _______________________________________________
>>>> netmod mailing list
>>>> netmod@ietf.org
>>>> https://www.ietf.org/mailman/listinfo/netmod
>>>>
>>> _______________________________________________
>>> netmod mailing list
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>>> .
>>>
>>