Re: [netmod] yang-data-ext issues
Ladislav Lhotka <lhotka@nic.cz> Wed, 25 April 2018 12:38 UTC
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From: Ladislav Lhotka <lhotka@nic.cz>
To: Martin Bjorklund <mbj@tail-f.com>
Cc: nite@hq.sk, netmod@ietf.org
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Date: Wed, 25 Apr 2018 14:38:41 +0200
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Subject: Re: [netmod] yang-data-ext issues
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Martin Bjorklund <mbj@tail-f.com> writes: > Ladislav Lhotka <lhotka@nic.cz> wrote: >> Robert Varga <nite@hq.sk> writes: >> >> > On 23/04/18 18:51, Juergen Schoenwaelder wrote: >> >> Some people will say that the cost of a new language version is high. >> >> (Well, when we did 1.1, some people said it will never be deployed.) >> >> Anyway, not bumping the YANG version number but having instead several >> >> (optional) language extensions is just hiding the version number >> >> change under the carpet. >> > >> > Not quite, as extensions allow for modular/incremental evolution, as an >> > implementer I do not have to go through a long development cycle where I >> > need to rewire language aspects just to get the features my users need >> > for their models... >> >> But what prevents you from forking YANG, indicating it in the >> "yang-version" string and implementing your new statements as built-ins? >> >> It is tempting to think about extensions as a magic for adding modular >> extensions but it breaks down as soon as such extensions interfere with >> the YANG core (with and each other, or both). > > Sure, nothing new here. This is how the extension mechanism is > designed. Do you feel that there are so many standard extensions that My objections are also not new. Implementations are permitted to ignore extensions, but if an extension affects the resulting schema, then implementation A that supports the extension may end up working with another schema than implementation B that doesn't support the extension. This situation has to be avoided. > interfere with each other that we actually have a problem? I strongly Perhaps not yet, but we do have extensions (or proposals thereof) that interfere with core YANG. And my impression is that OpenConfig is going to use extensions for creating their own YANG silo. > agree that we need to be careful with standard extensions, and I > strongly encourage vendors to not define extensions that interfere > with YANG core (been there, done that, bad idea). > > In the case of yang-data, the fact is that it *is* used in standard > models (in ways not originally anticipated), and vendors have similar > proprietary extensions (we have a tailf:structure, and I think others > have reported similar things). I don't know how much effort those vendors invested into the specification of such extensions, but the text of the current draft and module descriptions leave quite a few questions unanswered. Here are just a few that come to my mind: - What is the accessible tree for XPath evaluations and leafrefs? In particular, error messages might want to refer to state data. Is it possible? - This sentence makes no sense: "The XPath document root is the extension statement itself, ..." A YANG statement cannot be an XPath node. - What is the meaning of leaf and leaf-list default values? In RFC 7950 it is defined in terms of server behaviour but there is no server for yang-data. - What does it mean if an action is defined inside yang-data? - If a module is imported without revision and its groupings/typedefs are used inside yang-data, how is the exact revision of the imported module determined? I suspect that users of such extensions accept the fact that some amount of hand-waving is necessary. But then, why all this hassle? Why cannot normal YANG be used for specifying a schema that is not intended for an NM protocol? It requires just a little bit more hand-waving. The draft doesn't explain why the extension is needed. Also, I am quite sure that YANG experts can use it carefully so that it makes sense. But as a Standard Track document it becomes an official part of YANG that seems to give unsuspecting users enough rope for hanging themselves. Lada > > > > /martin -- Ladislav Lhotka Head, CZ.NIC Labs PGP Key ID: 0xB8F92B08A9F76C67
- Re: [netmod] yang-data-ext issues Kent Watsen
- Re: [netmod] yang-data-ext issues Joe Clarke
- Re: [netmod] yang-data-ext issues Kent Watsen
- [netmod] yang-data-ext issues Martin Bjorklund
- Re: [netmod] yang-data-ext issues Joe Clarke
- Re: [netmod] yang-data-ext issues Andy Bierman
- Re: [netmod] yang-data-ext issues Martin Bjorklund
- Re: [netmod] yang-data-ext issues Martin Bjorklund
- Re: [netmod] yang-data-ext issues Andy Bierman
- Re: [netmod] yang-data-ext issues Robert Wilton
- Re: [netmod] yang-data-ext issues Andy Bierman
- Re: [netmod] yang-data-ext issues Martin Bjorklund
- Re: [netmod] yang-data-ext issues Robert Wilton
- Re: [netmod] yang-data-ext issues Andy Bierman
- Re: [netmod] yang-data-ext issues Ladislav Lhotka
- Re: [netmod] yang-data-ext issues Kent Watsen
- Re: [netmod] yang-data-ext issues Martin Bjorklund
- Re: [netmod] yang-data-ext issues Andy Bierman
- Re: [netmod] yang-data-ext issues Martin Bjorklund
- Re: [netmod] yang-data-ext issues Andy Bierman
- Re: [netmod] yang-data-ext issues Kent Watsen
- Re: [netmod] yang-data-ext issues Robert Varga
- Re: [netmod] yang-data-ext issues Ladislav Lhotka
- Re: [netmod] yang-data-ext issues Martin Bjorklund
- Re: [netmod] yang-data-ext issues Andy Bierman
- Re: [netmod] yang-data-ext issues Martin Bjorklund
- Re: [netmod] yang-data-ext issues Andy Bierman
- Re: [netmod] yang-data-ext issues Robert Varga
- Re: [netmod] yang-data-ext issues Juergen Schoenwaelder
- Re: [netmod] yang-data-ext issues Robert Varga
- Re: [netmod] yang-data-ext issues Juergen Schoenwaelder
- Re: [netmod] yang-data-ext issues Martin Bjorklund
- Re: [netmod] yang-data-ext issues Martin Bjorklund
- Re: [netmod] yang-data-ext issues Andy Bierman
- Re: [netmod] yang-data-ext issues Martin Bjorklund
- Re: [netmod] yang-data-ext issues Andy Bierman
- [netmod] Extensions vs new YANG versions [was Re:… Robert Wilton
- Re: [netmod] yang-data-ext issues Robert Wilton
- Re: [netmod] Extensions vs new YANG versions [was… Ladislav Lhotka
- Re: [netmod] yang-data-ext issues Ladislav Lhotka
- Re: [netmod] yang-data-ext issues Ladislav Lhotka
- Re: [netmod] yang-data-ext issues Martin Bjorklund
- Re: [netmod] yang-data-ext issues Martin Bjorklund
- Re: [netmod] yang-data-ext issues Martin Bjorklund
- Re: [netmod] yang-data-ext issues Ladislav Lhotka
- Re: [netmod] yang-data-ext issues Andy Bierman
- Re: [netmod] yang-data-ext issues Kent Watsen
- Re: [netmod] yang-data-ext issues Martin Bjorklund
- Re: [netmod] yang-data-ext issues Martin Bjorklund
- Re: [netmod] yang-data-ext issues Andy Bierman
- Re: [netmod] yang-data-ext issues Andy Bierman
- Re: [netmod] yang-data-ext issues Ladislav Lhotka
- Re: [netmod] yang-data-ext issues Juergen Schoenwaelder
- Re: [netmod] yang-data-ext issues Ladislav Lhotka
- Re: [netmod] yang-data-ext issues Andy Bierman
- Re: [netmod] yang-data-ext issues Ladislav Lhotka
- Re: [netmod] yang-data-ext issues Ladislav Lhotka
- Re: [netmod] yang-data-ext issues Andy Bierman
- Re: [netmod] yang-data-ext issues Ladislav Lhotka
- Re: [netmod] yang-data-ext issues Martin Bjorklund
- Re: [netmod] yang-data-ext issues Robert Wilton
- Re: [netmod] yang-data-ext issues Ladislav Lhotka
- Re: [netmod] yang-data-ext issues Robert Wilton
- Re: [netmod] yang-data-ext issues Ladislav Lhotka
- Re: [netmod] yang-data-ext issues Juergen Schoenwaelder
- Re: [netmod] yang-data-ext issues Ladislav Lhotka
- Re: [netmod] yang-data-ext issues Andy Bierman
- Re: [netmod] yang-data-ext issues Juergen Schoenwaelder
- Re: [netmod] yang-data-ext issues Ladislav Lhotka
- Re: [netmod] yang-data-ext issues Kent Watsen
- Re: [netmod] yang-data-ext issues Andy Bierman
- Re: [netmod] yang-data-ext issues Kent Watsen
- Re: [netmod] yang-data-ext issues Juergen Schoenwaelder
- Re: [netmod] yang-data-ext issues Kent Watsen
- Re: [netmod] yang-data-ext issues Juergen Schoenwaelder
- Re: [netmod] yang-data-ext issues Martin Bjorklund
- Re: [netmod] yang-data-ext issues Ladislav Lhotka
- Re: [netmod] yang-data-ext issues Martin Bjorklund
- Re: [netmod] yang-data-ext issues Martin Bjorklund
- Re: [netmod] yang-data-ext issues Martin Bjorklund
- Re: [netmod] yang-data-ext issues Ladislav Lhotka
- Re: [netmod] yang-data-ext issues Martin Bjorklund
- Re: [netmod] yang-data-ext issues Robert Wilton
- Re: [netmod] yang-data-ext issues Martin Bjorklund
- Re: [netmod] yang-data-ext issues Juergen Schoenwaelder
- Re: [netmod] yang-data-ext issues Ladislav Lhotka
- Re: [netmod] yang-data-ext issues Robert Varga
- Re: [netmod] yang-data-ext issues Robert Varga
- Re: [netmod] yang-data-ext issues Kent Watsen
- Re: [netmod] yang-data-ext issues Juergen Schoenwaelder
- Re: [netmod] yang-data-ext issues Andy Bierman
- Re: [netmod] yang-data-ext issues Kent Watsen
- Re: [netmod] yang-data-ext issues Ladislav Lhotka