Re: [Netconf] [yang-doctors] YANG Doctor question: empty mandatory choice?

Martin Bjorklund <mbj@tail-f.com> Mon, 06 August 2018 18:00 UTC

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Date: Mon, 06 Aug 2018 20:00:27 +0200 (CEST)
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Subject: Re: [Netconf] [yang-doctors] YANG Doctor question: empty mandatory choice?
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Kent Watsen <kwatsen@juniper.net> wrote:
> 
> [top-posting for lack of time]
> 
> Hi Martin,
> 
> Interesting that you view the "transport" leaf as where the decision is 
> made.  I suppose that is the case now, but I always viewed the leaf as 
> a somewhat artificial "anchor" that was added only to support the "when"
> expression to assert homogeneity.  The 
> 
> On that, note that I merely said that I was sympathetic to the POV; I'm
> not convinced that there is a problem allowing heterogeneity, as what's
> been posted thus far for why it might be a [performance?] problem was 
> conjecture.
> 
> A fully cooked example would be helpful here, but I think you want:
> 
>   <receivers>
>     <receiver>
>       <name>foo</name>
>       <ex:param1>...</ex:param1>
>       <ex:param2>...</ex:param2>
>       <ex:param3>...</ex:param3>
>     </receiver>
>   </receivers>
> 
> whereas I was thinking that there would be a container, e.g.
> 
>   <receivers>
>     <receiver>
>       <name>foo</name>
>       <ex:udp-pub-sub>
>         <param1>...</param1>
>         <param2>...</param2>
>         <param3>...</param3>
>       </ex:udp-pub-sub>
>     </receiver>
>   </receivers>
> 
> which acts as the transport-selector, and hence homogeneity (if needed),
> could be via a "must" expression that ensures that there is only one
> such descendent, other than "name", across the receivers.  This is what
> that "must" expression I proffered last week was trying to do.  If we 
> can do this, I think the model simplifies, as then the [artificial?] 
> "transport" and "encoding" leaves, and the "when" expressions, wouldn't
> be needed to still assert the homogeneity.

Well, I don' think you can do what you want to do with a must
expression like that.

Also, I think the design w/ selecting a transport with an identity is
fine.  IMO both designs work; either use a discriminating leaf
(transport) and use augment with a "when" expression, or use a
mandatory choice where each case denotes which transport is used.

With the design choice that all receivers must use the same transport
+ encoding, the current design actually works better than a choice,
imo.


/martin



> 
> Thoughts?
> 
> Kent // contributor
> 
> 
> 
> 
> Robert Wilton <rwilton=40cisco.com@dmarc.ietf.org> wrote:
> > 
> > 
> > On 01/08/2018 17:09, Andy Bierman wrote:
> > >
> > >
> > > On Wed, Aug 1, 2018 at 9:01 AM, Robert Wilton <rwilton@cisco.com
> > > <mailto:rwilton@cisco.com>> wrote:
> > >
> > >
> > >
> > >     On 31/07/2018 21:31, Andy Bierman wrote:
> > >>
> > >>
> > >>     On Tue, Jul 31, 2018 at 12:39 PM, Eric Voit (evoit)
> > >>     <evoit@cisco.com <mailto:evoit@cisco.com>> wrote:
> > >>
> > >>         > From: Juergen Schoenwaelder, July 31, 2018 1:48 PM
> > >>         >
> > >>         > On Mon, Jul 30, 2018 at 08:41:42PM +0200, Martin Bjorklund
> > >>         wrote:
> > >>         > >
> > >>         > > The empty mandatory choice does provide value since it
> > >>         requires that
> > >>         > > some transport-specific parameters are configured. 
> > >>         However, can we
> > >>         > > assume that all transports require configuration
> > >>         parameters here?
> > >>         >
> > >>         > Can you have a receiver without any transport parameters?
> > >>         >
> > >>         > > It is probably safest to not have a mandatory choice, and
> > >>         instead
> > >>         > > ensure that each transport augements the proper params --
> > >>         and since
> > >>         > > this is YANG 1.1, the transport params that are augmented
> > >>         can actually
> > >>         > > be marked as mandatory.
> > >>         >
> > >>         > Frankly, an empty mandatory choice quite clearly says "this
> > >>         is incomplete and
> > >>         > unusable without an augmentation".
> > >>
> > >>         My read above is the YANG doctor's position is that we should
> > >>         *not* use the empty mandatory choice.  Let me know if I got
> > >>         this wrong.
> > >>
> > >>
> > >>     I do not think a consensus call has been done yet, but I agree
> > >>     with Juergen
> > >>     and already raised the point that YANG conformance does not handle a
> > >>     "MUST augment" use-case very well.
> > >     I think that "empty choice + mandatory true" it is OK from a
> > >     conformance perspective.  The concept seems similar to an
> > >     programmatic interface, abstract class, or even the abstract
> > >     identity idea that has been proposed for YANG.  If a server
> > >     implements the module but no augments of the choice then it cannot
> > >     be configured because the constraint will always fail.  Andy, is
> > >     your concern that tooling will warn that part of the model is
> > >     unusable?
> > >
> > >
> > > That is possible.
> > > I agree with Juergen that a mandatory empty choice clearly indicates
> > > that the module is incomplete
> > > and unusable on its own.  Is that a feature?
> > Yes, making that indication is the whole purpose of adding the
> > "mandatory: true" to the empty choice.  Note, that I see that the
> > "mandatory true" is there to say that every configured subscription
> > must have a transport configured, which if true, doesn't seem
> > unreasonable.
> 
> Note that the model already has a 'transport' leaf that is mandatory.
> 
> The choice is an explicit placeholder for transport-specific
> additional parameters.
> 
> This proposed design is slightly different than the design in
> ietf-interfaces; in interfaces we have:
> 
>    leaf type { ... }
>    // type-specific augmentations here
> 
> For example (from the RFC):
> 
>    augment "/if:interfaces/if:interface" {
>      when "if:type = 'ianaift:ethernetCsmacd'";
> 
>      container ethernet {
>        leaf duplex {
>          ...
>        }
>      }
>    }
> 
> 
> In the notif model the proposal is:
> 
>   leaf transport { ... }
> 
>   ...
>     choice transport-specific-params {
>       // transport-specific augmentations here
>     }
> 
> 
> Note that if the choice is not marked as mandatory, the resulting
> model will be less strict / useful compared to using a design like in
> the interfaces model (w/o the choice).  To demonstrate, suppose we
> have a transport 'example-udp' that needs a mandatory 'address' and an
> optional 'port'.  With the choice we'd have:
> 
>   augment '/sn:subscriptions/sn:subscription/sn:receivers/sn:receiver'
>         + '/sn:transport-specific-params' {
>     when 'derived-from(../../../../transport, "ex:example-udp")';
> 
>     case example-udp-params {
>       leaf address {
>         mandatory true;
>         ...
>       }
>       leaf port {
>         ...
>       }
>     }
>   }
>   
> If the choice is not mandatory, the model would allow a client to
> configure the transport leaf to 'example-udp', but not configure an
> address.
> 
> 
> Without the choice, we'd do:
> 
>   augment '/sn:subscriptions/sn:subscription/sn:receivers/sn:receiver'
>     when 'derived-from(../../../transport, "ex:example-udp")';
> 
>     leaf address {
>       mandatory true;
>       ...
>     }
>     leaf port {
>       ...
>     }
>   }
>   
> 
> In this case, or if the choice is mandatory, the model would require
> the client to configure an address if the transport is 'example-udp',
> which is what we want.
> 
> 
> But if the choice is marked as mandatory, *all* transports MUST define
> some transport-specific parameters, even if that is not needed
> (unclear if this will ever happen...)
> 
> 
> Thus, I prefer Eric's original model w/o the choice.  The choice is
> supposed to be clever, but might end up being confusing, and I don't
> think it adds any value anyway.
> 
> 
> 
> 
> /martin
> 
> > I.e. my main point is that I don't have an issue with
> > this generic YANG design.
> > 
> > In this particular instance, I'm also fine if "mandatory: true" is
> > left out, but I don't really agree with writing the equivalent of
> > "mandatory: true" in the description, that seems like a poor
> > compromise.
> > 
> > However, this is probably all bike-shedding.  I think that any of the
> > discussed solutions is acceptable, as long as it is obvious to the
> > readers of the YANG modules that a case statement must be provided for
> > it to be useful, and I make the assumption that sane vendors won't
> > enable the "configured" feature, if there is no actual way of
> > configuring usable subscriptions.
> > 
> > Perhaps Eric can propose his preferred choice, and we can see if
> > anyone still objects, otherwise maybe we can move on?
> > 
> > Thanks,
> > Rob
> > 
> > 
> > >
> > > Andy
> > >
> > >
> > >     I have to say that much prefer the option of putting "mandatory:
> > >     true" in the choice than "MUST provide an implementation" in the
> > >     description because the former is machine readable whilst the
> > >     latter is not.
> > >
> > >     However, I would also be fine not to have the "mandatory: true",
> > >     but with the choice description to state something along the lines
> > >     that the empty choice is to allow for augmentations of different
> > >     transports, and configured subscriptions may not be usable unless
> > >     at least one transport case statement is available."  But perhaps
> > >     some implementation will provide the flexibility of defining a
> > >     single transport for all subscriptions (if this is feasible).
> > >
> > >     One other observation that could affect the decision here is that
> > >     YANG allows "mandatory: true" to be removed in a future revision
> > >     in a backwards compatible way, but doesn't allow it to be added.
> > >
> > >     Thanks,
> > >     Rob
> > >
> > >
> > >>
> > >>     I prefer the MUST be in the description-stmt for the choice,
> > >>     instead of "mandatory true". (I prefer SHOULD but if the WG wants
> > >>     MUST)
> > >>
> > >>
> > >>     Andy
> > >>
> > >>
> > >>
> > >>         That would mean that each transport document supporting
> > >>         configured subscriptions would then augment transport
> > >>         specific parameters to
> > >>         "/subscriptions/subscription/receivers/receiver".  And
> > >>         (assuming the "single transport" decision of IETF100 isn't
> > >>         changed), that the identity "transport" could be leveraged to
> > >>         enforce that only a single transport specific set of
> > >>         credentials are associated with a receiver.
> > >>
> > >>         A sample YANG augmentation for NETCONF would then look like:
> > >>
> > >>         module ietf-netconf-subscribed-notifications {
> > >>
> > >>           prefix nsn;
> > >>
> > >>           import ietf-netconf-client { prefix ncc; }
> > >>           import ietf-subscribed-notifications { prefix sn; }
> > >>
> > >>           identity netconf {
> > >>             base sn:transport;
> > >>             base sn:inline-address;
> > >>             description
> > >>               "NETCONF is used as a transport for notification
> > >>         messages and
> > >>                state change notifications.";
> > >>           }
> > >>
> > >>           augment
> > >>         "/sn:subscriptions/sn:subscription/sn:receivers/sn:receiver" {
> > >>            when 'derived-from(../../../transport, "nsn:netconf")';
> > >>            description
> > >>               "This augmentation allows NETCONF specific parameters
> > >>         to be
> > >>               exposed for a receiver.";
> > >>             leaf netconf-endpoint {
> > >>               type leafref {
> > >>                 path
> > >>         "/ncc:netconf-client/ncc:initiate/ncc:netconf-server" +
> > >>                         "/ncc:endpoints/ncc:endpoint/ncc:name";
> > >>               }
> > >>               mandatory true;
> > >>               description
> > >>                 "Remote client which need to initiate the NETCONF
> > >>         transport if
> > >>                 an existing NETCONF session from that client is not
> > >>         available.";
> > >>             }
> > >>           }
> > >>         }
> > >>
> > >>         Which results in:
> > >>           +--rw subscriptions
> > >>              +--rw subscription*
> > >>                 +--rw transport         transport {configured}?
> > >>                 +--rw receivers
> > >>                    +--rw receiver*
> > >>                       +--rw nsn:netconf-endpoint leafref
> > >>
> > >>         Eric
> > >>
> > >>
> > >>         > /js
> > >>         >         > --
> > >>         > Juergen Schoenwaelder  Jacobs University Bremen gGmbH
> > >>         > Phone: +49 421 200 3587  Campus Ring 1 | 28759 Bremen | Germany
> > >>         > Fax:   +49 421 200 3103       
> > >>          <https://urldefense.proofpoint.com/v2/url?u=https-3A__www.jacobs-2Duniversity.de_&d=DwIFAw&c=HAkYuh63rsuhr6Scbfh0UjBXeMK-ndb3voDTXcWzoCI&r=9zkP0xnJUvZGJ9EPoOH7Yhqn2gsBYaGTvjISlaJdcZo&m=nvlGbzwaLflz8fPtIV-iMrmk_M-b_KUsBI5XkyR6OQA&s=HOUalUJdAOVqj49diEzGM-Z-35a5rPmj2Y4NfhsdovQ&e=
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> > >>
> > >>
> > >>
> > >>
> > >>     _______________________________________________
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