Re: [netmod] WHEN statement within mandatory objects doesn't ensure presence of the mandatory object
Ladislav Lhotka <lhotka@nic.cz> Wed, 10 October 2018 15:43 UTC
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From: Ladislav Lhotka <lhotka@nic.cz>
To: Michael Rehder <Michael.Rehder@Amdocs.com>, "netmod@ietf.org" <netmod@ietf.org>
Cc: "Walker, Jason (Jason_Walker2@comcast.com)" <Jason_Walker2@comcast.com>
Date: Wed, 10 Oct 2018 17:43:12 +0200
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Subject: Re: [netmod] WHEN statement within mandatory objects doesn't ensure presence of the mandatory object
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On Wed, 2018-10-10 at 15:23 +0000, Michael Rehder wrote: > Container "foo" would be mandatory if not for the "when" child element. > With the "when" child element, the logic becomes "inverted" and the constraint > is a negative one of "disallowed under certain condition". > > The UC is for enforcement in REST API payloads. > For a practical example: > > leaf AssignmentMechanism { > type enumeration { > enum "DHCP"; > enum "Static"; > } > mandatory true; > description "The address assignment mechanism."; > } > list IPAddresses { > when "../AssignmentMechanism = 'Static'"; > key Address; > min-elements 1; > > leaf Address { > type capit:IPv4Address; > description "An ipv4 address."; > } > } > > There is no way in the IPAddresses list to enforce that there is at least one > IP Address when the assignment method is "Static". Why do you think there is no way? For example, according to sec. 10.28 of RFC 6110, then "min-elements 1" statement is mapped to the RELAX NG pattern <oneOrMore> which enforces the presence of at least one entry of IPAddresses. >From the DSDL point of view, the problem is rather the opposite: at least one entry will be required during RELAX NG validation even if AssignmentMechanism is "DHCP". Lada > One could put a "must" on "AssignmentMechanism" to ensure at least one element > of the IPAddresses list when "Static", but I don't see this as a good schema > design, to have the controlling attribute check controlled attributes. > > I appreciate that this semantic can't be changed in YANG at this point. > Could the "when" statement have a modifying child element to state that the > mandatory status of the element is to be enforced? > Like > container foo { > when "condition" { > enforce-mandatory-status; > } > > There is already back-end for existential checks for mandatory choice so this > seems reasonably consistent to me. > I appreciate there are existing issues for "when" but I don't see why this > would make things any worse. > In fact by promoting a better dependency "direction" between schema > elements, think it could simplify things (so I naively think :) ). > > Thanks > Mike > > -----Original Message----- > > From: Ladislav Lhotka [mailto:lhotka@nic.cz] > > Sent: Wednesday, October 10, 2018 10:28 AM > > To: Michael Rehder <Michael.Rehder@Amdocs.com>; netmod@ietf.org > > Subject: Re: [netmod] WHEN statement within mandatory objects doesn't > > ensure presence of the mandatory object > > > > Michael Rehder <Michael.Rehder@Amdocs.com> writes: > > > > > I have a question about “when” and mandatory objects. > > > > > > It seems to me that the implemented semantics of “when” are really > > “optional when”, in that the enclosing object can be absent even though it > > is > > mandatory and the “when” clause holds true. > > > The RFC could be clearer about this. > > > > > > Example > > > > > > leaf color { > > > enumeration { > > > enum “blue”; > > > enum “black”; > > > } > > > mandatory true; > > > } > > > container foo { > > > when ../color = ‘blue’; > > > etc. > > > } > > > > > > “foo” is optional due to the presence of the “when” statement even > > > though the object is mandatory (same is true for mandatory leaf, > > > min-elements=1 list etc.). > > > > Maybe you intended to have, e.g., a "mandatory true" leaf inside "container > > foo"? > > > > > This is considered valid XML for the above > > > <color>blue</color> > > > > Yes, it is, under current YANG rules, no matter what "etc." stands for. Note > > that > > evaluation of the XPath expression in this case (with "foo" missing) > > requires the > > peculiar procedure of sec. 7.21.5 in RFC 7950. > > > > > In my view this makes conditionally variant schemas “loose” in their > > > enforcement (some scenarios can use choice but it doesn’t cover > > > everything). > > > > > > I think that mandatory should be respected for the enclosing objects > > > of a “when” statement. That is, a mandatory object must be present > > > when its “when” clause holds true and a Schematron statement should > > > enforce that. > > > > In fact, this is one case where the DSDL mapping (RFC 6110) deviates from > > YANG 1.0. Nodes that mandatory aren't enclosed in the RELAX NG <optional> > > pattern, and are then required no matter what any "when" > > statements say (because RELAX NG validation comes before Schematron). > > > > > What is the rationale behind the current YANG rules behavior, that the > > > “when” Schematron mapping doesn’t check for presence of the enclosing > > > mandatory object? > > > > FWIW, I have been repeatedly protesting against this behaviour but without > > much luck. See for example > > > > https://www.ietf.org/mail-archive/web/netmod/current/msg14012.html > > > > As a result, "when" is the trickiest feature in YANG by far. > > > > Lada > > > > > thanks > > > Mike Rehder > > > > -- > > Ladislav Lhotka > > Head, CZ.NIC Labs > > PGP Key ID: 0xB8F92B08A9F76C67 > “Amdocs’ email platform is based on a third-party, worldwide, cloud-based > system. Any emails sent to Amdocs will be processed and stored using such > system and are accessible by third party providers of such system on a limited > basis. Your sending of emails to Amdocs evidences your consent to the use of > such system and such processing, storing and access”. -- Ladislav Lhotka Head, CZ.NIC Labs PGP Key ID: 0xB8F92B08A9F76C67
- [netmod] WHEN statement within mandatory objects … Michael Rehder
- Re: [netmod] WHEN statement within mandatory obje… Sterne, Jason (Nokia - CA/Ottawa)
- Re: [netmod] WHEN statement within mandatory obje… Ladislav Lhotka
- Re: [netmod] WHEN statement within mandatory obje… Michael Rehder
- Re: [netmod] WHEN statement within mandatory obje… Robert Wilton
- Re: [netmod] WHEN statement within mandatory obje… Ladislav Lhotka
- Re: [netmod] WHEN statement within mandatory obje… Michael Rehder
- Re: [netmod] WHEN statement within mandatory obje… Juergen Schoenwaelder
- Re: [netmod] WHEN statement within mandatory obje… Michael Rehder
- Re: [netmod] WHEN statement within mandatory obje… Andy Bierman
- Re: [netmod] WHEN statement within mandatory obje… Ladislav Lhotka
- Re: [netmod] WHEN statement within mandatory obje… Michael Rehder
- Re: [netmod] WHEN statement within mandatory obje… Andy Bierman
- Re: [netmod] WHEN statement within mandatory obje… Michael Rehder
- Re: [netmod] WHEN statement within mandatory obje… Andy Bierman
- Re: [netmod] WHEN statement within mandatory obje… Michael Rehder
- Re: [netmod] WHEN statement within mandatory obje… Robert Wilton
- Re: [netmod] WHEN statement within mandatory obje… Michael Rehder
- Re: [netmod] WHEN statement within mandatory obje… Andy Bierman
- Re: [netmod] WHEN statement within mandatory obje… Robert Wilton
- Re: [netmod] WHEN statement within mandatory obje… Michael Rehder
- Re: [netmod] WHEN statement within mandatory obje… Juergen Schoenwaelder
- Re: [netmod] WHEN statement within mandatory obje… Michael Rehder
- Re: [netmod] WHEN statement within mandatory obje… Robert Wilton
- Re: [netmod] WHEN statement within mandatory obje… Michael Rehder
- Re: [netmod] WHEN statement within mandatory obje… Robert Wilton
- Re: [netmod] WHEN statement within mandatory obje… Ladislav Lhotka
- Re: [netmod] WHEN statement within mandatory obje… Michael Rehder
- Re: [netmod] WHEN statement within mandatory obje… Andy Bierman
- Re: [netmod] WHEN statement within mandatory obje… Alex Campbell
- Re: [netmod] WHEN statement within mandatory obje… Andy Bierman
- Re: [netmod] WHEN statement within mandatory obje… Alex Campbell