Re: [netmod] Interaction of 'when' and 'default' statements

t.petch <ietfc@btconnect.com> Fri, 24 March 2017 17:34 UTC

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From: "t.petch" <ietfc@btconnect.com>
To: William Ivory <wivory@Brocade.com>, Phil Shafer <phil@juniper.net>
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Date: Fri, 24 Mar 2017 17:30:54 +0000
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Archived-At: <https://mailarchive.ietf.org/arch/msg/netmod/kRa0QEsg7m9RryQjijroLD3QMi8>
Subject: Re: [netmod] Interaction of 'when' and 'default' statements
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----- Original Message -----
From: "Phil Shafer" <phil@juniper.net>
Sent: Thursday, March 23, 2017 8:24 PM

> William Ivory writes:
> >Yes, I'd noticed that.  Does this make the behaviour 'undefined' in
YANG 1.0?
>
> No, this was a clarification.  The text in 6020 was reasonably clear:
>
>    The "when" statement makes its parent data definition statement
>    conditional.  The node defined by the parent data definition
>    statement is only valid when the condition specified by the "when"
>    statement is satisfied.
>
> And no default should be provided for any invalid node.  If the node
> can't exist, the default can't either.

William

Having tracked the discussions that led to RFC7950, the sense I got was
that if you can avoid using 'when' then avoid using it.  If you cannot
avoid using 'when' avoid using it.  Not something that is ever likely to
appear in an I-D but the 'when' statement does have complex interactions
that may not be intuitive and although RFC7950 does spell them out, yet
they may not be apparent; which this thread also spells out to me:-)

I would not say this of any other YANG statement.

Tom Petch

> Thanks,
>  Phil
>
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