Re: [netmod] Circular dependency in 'when'
> -Original Message-
> From: netmod [mailto:netmod-boun...@ietf.org] On Behalf Of Ladislav
> Lhotka
> Sent: Wednesday, September 7, 2016 8:22 PM
> To: Vladimir Vassilev
> Cc: netmod@ietf.org
> Subject: Re: [netmod] Circular dependency in 'when'
>
>
> > On 07 Sep 2016, at 19:44, Vladimir Vassilev
> wrote:
> >
> > On 09/07/2016 02:18 PM, Martin Bjorklund wrote:
> >> Hi,
> >>
> >> Your example is not circular, and it is legal. However, the 'when'
> >> expression refers to the node in which the when expression is defined.
> >> Note that this expression will always evaluates to 'false' (see the
> >> third bullet in 7.21.5 in RFC 7950).
> > This is true for YANG 1.1 due to that 3rd bullet in 7.21.5.
> >
> > However it should be noted that in YANG 1.0 nothing says the data node
> for which the "when" statement is defined will be replaced with dummy
> node
> without a value so that this expression always evaluates to 'false'. In
> YANG 1.0 the expression can evaluate to 'true' when a+b=100.
>
> Using "when" expressions that refer to the context node or its descendants
> could easily lead to deadlocks in YANG 1.0, so this part was underspecifed
> and ill-defined. The new 1.1 procedure for evaluating "when" expressions
> is thus a bug fix. Although it is somewhat tricky and sometimes (as in
> your example) counter-intuitive at first sight, it leads to unambiguous
> and stable results.
>
> >>
> >> Take a step back and consider what the 'when' statement means - it is
> >> used to indicate if the node can be present or not. As such, it
> >> doesn't make any sense to refer to the node itself in the xpath
> >> expression.
> >
> >> In your case, you probably want to use a 'must' expression. This is
> >> evaluated once the node is present, in order to enforce some
> >> constraint.
> >>
> > I agree. I think the conclusion can be generalized to: there is no point
> in using "when" expressions directly depending on the value of the data
> node for which the "when" statement is defined or its children when the
> parent is not 'augment', 'uses', 'case' or 'choice' statement in YANG 1.1
> because the 'when' statement will always evaluate to 'false' and the data
> node will never exist.
>
> Not necessarily. The following expression is always true:
>
> leaf foo {
> when ".";
> ...
> }
>
> >
> > Would be helpful if model validation tools could issue at least warning
> in such cases.
>
> This is rather difficult to detect in general. XPath is a complicated
> beast.
We've implemented partial evaluation of XPath expressions that operates on an
"accessible tree skeleton" (schema of the accessible tree as defined by modules
in scope).
For the example given by Vladimir, our compiler would complain with:
WARNING; test@2016-09-07:8; when: XPath construct '.' (at 2) references the
initial context node or one of its descendants in a when expression
since it is referring to the "leaf" for which the "when" statement is defined
and this was discouraged by one of the 6087bis drafts. It would detect this
(and other issues) no matter how "obfuscated" the expression is.
Implementing it was mostly problematic due to poorly defined accessible trees
or initial context nodes in some cases (YANG 1).
Jernej
>
> Lada
>
> >
> > Vladimir
> >>
> >> /martin
> >>
> >>
> >> Vladimir Vassilev wrote:
> >>> Hi,
> >>>
> >>> Is there any practical value of 'when' statements with circular
> >>> dependency to the value of the parent (in case it is a leaf) or any
> >>> children of the parent?
> >>>
> >>> container circular-dependency-when {
> >>> leaf a {
> >>> when "(. + ../b) = 100";
> >>> type uint16 {
> >>> range "0 .. 100";
> >>> }
> >>> }
> >>> leaf b {
> >>> type uint16 {
> >>> range "0 .. 100";
> >>> }
> >>> }
> >>> }
> >>>
> >>>
> >
> > ___
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> > netmod@ietf.org
> > https://www.ietf.org/mailman/listinfo/netmod
>
> --
> Ladislav Lhotka, CZ.NIC Labs
> PGP Key ID: E74E8C0C
>
>
>
>
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Re: [netmod] Circular dependency in 'when'
On Wed, Sep 7, 2016 at 11:21 AM, Ladislav Lhotka wrote:
>
> > On 07 Sep 2016, at 19:44, Vladimir Vassilev
> wrote:
> >
> > On 09/07/2016 02:18 PM, Martin Bjorklund wrote:
> >> Hi,
> >>
> >> Your example is not circular, and it is legal. However, the 'when'
> >> expression refers to the node in which the when expression is defined.
> >> Note that this expression will always evaluates to 'false' (see the
> >> third bullet in 7.21.5 in RFC 7950).
> > This is true for YANG 1.1 due to that 3rd bullet in 7.21.5.
> >
> > However it should be noted that in YANG 1.0 nothing says the data node
> for which the "when" statement is defined will be replaced with dummy node
> without a value so that this expression always evaluates to 'false'. In
> YANG 1.0 the expression can evaluate to 'true' when a+b=100.
>
> Using "when" expressions that refer to the context node or its descendants
> could easily lead to deadlocks in YANG 1.0, so this part was underspecifed
> and ill-defined. The new 1.1 procedure for evaluating "when" expressions is
> thus a bug fix. Although it is somewhat tricky and sometimes (as in your
> example) counter-intuitive at first sight, it leads to unambiguous and
> stable results.
>
> >>
> >> Take a step back and consider what the 'when' statement means - it is
> >> used to indicate if the node can be present or not. As such, it
> >> doesn't make any sense to refer to the node itself in the xpath
> >> expression.
> >
> >> In your case, you probably want to use a 'must' expression. This is
> >> evaluated once the node is present, in order to enforce some
> >> constraint.
> >>
> > I agree. I think the conclusion can be generalized to: there is no point
> in using "when" expressions directly depending on the value of the data
> node for which the "when" statement is defined or its children when the
> parent is not 'augment', 'uses', 'case' or 'choice' statement in YANG 1.1
> because the 'when' statement will always evaluate to 'false' and the data
> node will never exist.
>
> Not necessarily. The following expression is always true:
>
> leaf foo {
> when ".";
> ...
> }
>
> >
> > Would be helpful if model validation tools could issue at least warning
> in such cases.
>
> This is rather difficult to detect in general. XPath is a complicated
> beast.
>
>
I think it helps that "real" YANG developers are attempting to create
operationally useful data structures. This is much different than a tool
developer
trying to construct a test case for a YANG compiler.
The when-stmt says "add foo to bar when XXX is true"
It seems obvious to real developers that XXX has to do with testing the
values in bar, not foo,
> Lada
>
Andy
>
> >
> > Vladimir
> >>
> >> /martin
> >>
> >>
> >> Vladimir Vassilev wrote:
> >>> Hi,
> >>>
> >>> Is there any practical value of 'when' statements with circular
> >>> dependency to the value of the parent (in case it is a leaf) or any
> >>> children of the parent?
> >>>
> >>> container circular-dependency-when {
> >>> leaf a {
> >>> when "(. + ../b) = 100";
> >>> type uint16 {
> >>> range "0 .. 100";
> >>> }
> >>> }
> >>> leaf b {
> >>> type uint16 {
> >>> range "0 .. 100";
> >>> }
> >>> }
> >>> }
> >>>
> >>>
> >
> > ___
> > netmod mailing list
> > netmod@ietf.org
> > https://www.ietf.org/mailman/listinfo/netmod
>
> --
> Ladislav Lhotka, CZ.NIC Labs
> PGP Key ID: E74E8C0C
>
>
>
>
> ___
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> https://www.ietf.org/mailman/listinfo/netmod
>
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Re: [netmod] Circular dependency in 'when'
> On 07 Sep 2016, at 19:44, Vladimir Vassilev wrote:
>
> On 09/07/2016 02:18 PM, Martin Bjorklund wrote:
>> Hi,
>>
>> Your example is not circular, and it is legal. However, the 'when'
>> expression refers to the node in which the when expression is defined.
>> Note that this expression will always evaluates to 'false' (see the
>> third bullet in 7.21.5 in RFC 7950).
> This is true for YANG 1.1 due to that 3rd bullet in 7.21.5.
>
> However it should be noted that in YANG 1.0 nothing says the data node for
> which the "when" statement is defined will be replaced with dummy node
> without a value so that this expression always evaluates to 'false'. In YANG
> 1.0 the expression can evaluate to 'true' when a+b=100.
Using "when" expressions that refer to the context node or its descendants
could easily lead to deadlocks in YANG 1.0, so this part was underspecifed and
ill-defined. The new 1.1 procedure for evaluating "when" expressions is thus a
bug fix. Although it is somewhat tricky and sometimes (as in your example)
counter-intuitive at first sight, it leads to unambiguous and stable results.
>>
>> Take a step back and consider what the 'when' statement means - it is
>> used to indicate if the node can be present or not. As such, it
>> doesn't make any sense to refer to the node itself in the xpath
>> expression.
>
>> In your case, you probably want to use a 'must' expression. This is
>> evaluated once the node is present, in order to enforce some
>> constraint.
>>
> I agree. I think the conclusion can be generalized to: there is no point in
> using "when" expressions directly depending on the value of the data node for
> which the "when" statement is defined or its children when the parent is not
> 'augment', 'uses', 'case' or 'choice' statement in YANG 1.1 because the
> 'when' statement will always evaluate to 'false' and the data node will never
> exist.
Not necessarily. The following expression is always true:
leaf foo {
when ".";
...
}
>
> Would be helpful if model validation tools could issue at least warning in
> such cases.
This is rather difficult to detect in general. XPath is a complicated beast.
Lada
>
> Vladimir
>>
>> /martin
>>
>>
>> Vladimir Vassilev wrote:
>>> Hi,
>>>
>>> Is there any practical value of 'when' statements with circular
>>> dependency to the value of the parent (in case it is a leaf) or any
>>> children of the parent?
>>>
>>> container circular-dependency-when {
>>> leaf a {
>>> when "(. + ../b) = 100";
>>> type uint16 {
>>> range "0 .. 100";
>>> }
>>> }
>>> leaf b {
>>> type uint16 {
>>> range "0 .. 100";
>>> }
>>> }
>>> }
>>>
>>>
>
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> https://www.ietf.org/mailman/listinfo/netmod
--
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PGP Key ID: E74E8C0C
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Re: [netmod] Circular dependency in 'when'
On 09/07/2016 02:18 PM, Martin Bjorklund wrote:
Hi,
Your example is not circular, and it is legal. However, the 'when'
expression refers to the node in which the when expression is defined.
Note that this expression will always evaluates to 'false' (see the
third bullet in 7.21.5 in RFC 7950).
This is true for YANG 1.1 due to that 3rd bullet in 7.21.5.
However it should be noted that in YANG 1.0 nothing says the data node
for which the "when" statement is defined will be replaced with dummy
node without a value so that this expression always evaluates to
'false'. In YANG 1.0 the expression can evaluate to 'true' when a+b=100.
Take a step back and consider what the 'when' statement means - it is
used to indicate if the node can be present or not. As such, it
doesn't make any sense to refer to the node itself in the xpath
expression.
In your case, you probably want to use a 'must' expression. This is
evaluated once the node is present, in order to enforce some
constraint.
I agree. I think the conclusion can be generalized to: there is no point
in using "when" expressions directly depending on the value of the data
node for which the "when" statement is defined or its children when the
parent is not 'augment', 'uses', 'case' or 'choice' statement in YANG
1.1 because the 'when' statement will always evaluate to 'false' and the
data node will never exist.
Would be helpful if model validation tools could issue at least warning
in such cases.
Vladimir
/martin
Vladimir Vassilev wrote:
Hi,
Is there any practical value of 'when' statements with circular
dependency to the value of the parent (in case it is a leaf) or any
children of the parent?
container circular-dependency-when {
leaf a {
when "(. + ../b) = 100";
type uint16 {
range "0 .. 100";
}
}
leaf b {
type uint16 {
range "0 .. 100";
}
}
}
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Re: [netmod] Circular dependency in 'when'
Hi,
Your example is not circular, and it is legal. However, the 'when'
expression refers to the node in which the when expression is defined.
Note that this expression will always evaluates to 'false' (see the
third bullet in 7.21.5 in RFC 7950).
Take a step back and consider what the 'when' statement means - it is
used to indicate if the node can be present or not. As such, it
doesn't make any sense to refer to the node itself in the xpath
expression.
In your case, you probably want to use a 'must' expression. This is
evaluated once the node is present, in order to enforce some
constraint.
/martin
Vladimir Vassilev wrote:
> Hi,
>
> Is there any practical value of 'when' statements with circular
> dependency to the value of the parent (in case it is a leaf) or any
> children of the parent?
>
> container circular-dependency-when {
> leaf a {
> when "(. + ../b) = 100";
> type uint16 {
> range "0 .. 100";
> }
> }
> leaf b {
> type uint16 {
> range "0 .. 100";
> }
> }
> }
>
> I notice none of the tools known to me complain about this example
> model however some will not allow the user to interactively configure
> 'a' (even if he intends to use value which would make the 'when'
> statement evaluate as "true").
>
> Did not find any 'when' statements depending on the value of their
> parent or the value of children of that parent in the standard and
> draft models known to me but this is valid YANG according to the YANG
> RFC text.
>
> I believe if there is consensus this indeed qualifies as circular
> dependency and it has no value to allow such cases it can at least
> give a signal to model designers they should avoid using such 'when'
> statements until this is explicitly noted in a follow up YANG version.
>
> Vladimir
>
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