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Re: [netmod] Y09: optional keys

Martin Bjorklund <mbj@tail-f.com> Thu, 20 November 2014 08:55 UTC

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Date: Thu, 20 Nov 2014 09:55:44 +0100
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To: lhotka@nic.cz
From: Martin Bjorklund <mbj@tail-f.com>
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References: <201411181937.sAIJbguL080247@idle.juniper.net> <20141119.104651.1930940910444913825.mbj@tail-f.com> <m27fyqqn64.fsf@nic.cz>
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Subject: Re: [netmod] Y09: optional keys
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Ladislav Lhotka <lhotka@nic.cz> wrote:
> Martin Bjorklund <mbj@tail-f.com> writes:
> 
> > Phil Shafer <phil@juniper.net> wrote:
> >> * OPEN :Y09: introduce optional keys
> >> 
> >> I don't understand the 'resolution' here.
> >> 
> >> My concern is that optional keys make interaction with databases
> >> more difficult.  Keys are mandatory for most databases.
> >
> > Implementation detail.  Most databases do not support hierarchical
> > structures; yet we have that in YANG.
> >
> >> The usage of 'not(key)' in the second solution is not compatible
> >> with xpath expressions
> >
> > [...]
> >
> >> not(optional-key) would be true when the value is a numeric zero
> >> or a zero length string.
> >
> > Whoops.  You are right.  This would work: not(count(key)) - but looks
> > extremely weird.
> 
> No, Phil is not right:

[...]

> not(key) evaluates to true only if no <key> element is present. The
> value doesn't matter.

Good!  Sorry for contributing to the confusion.

Actually, Phil wrote:

  not(optional-key) would be true when the value is a numeric zero
    or a zero length string.

which *is* true, but irrelevant.

What is important for Y09-02 is that we have a representation of an
instance-identifier for instances with unset optional keys, and that
this representation gives the correct instance when evaluated as an
XPath expression.



/martin

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