Re: [netmod] yang-data-ext issues

Ladislav Lhotka <lhotka@nic.cz> Fri, 27 April 2018 11:04 UTC

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From: Ladislav Lhotka <lhotka@nic.cz>
To: Robert Wilton <rwilton@cisco.com>, Martin Bjorklund <mbj@tail-f.com>
Cc: netmod@ietf.org
Date: Fri, 27 Apr 2018 13:03:58 +0200
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Subject: Re: [netmod] yang-data-ext issues
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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.

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.

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
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> > > 
> > 
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> > .
> > 
> 
> 
-- 
Ladislav Lhotka
Head, CZ.NIC Labs
PGP Key ID: 0xB8F92B08A9F76C67