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buildingSMART / NextGen-IFC

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WHERE rules #39

Open berlotti opened 4 years ago

berlotti commented 4 years ago

Since nobody else is putting this on github, here we go....

Description of the proposal: The ISG questionaire last year clarified that vendors are not parsing WHERE rules. It also clarified that WHERE rules are almost never needed in practical use cases (with a few exceptions). Since the WHERE rules are not being used anyway, and are typical STEP/EXPRESS heritage, I suggest to remove them from the schema. Specific cases that need additional restrictions will be looked at case-by-case.

Describe how it contributes to the objectives set in https://github.com/buildingSMART/NextGen-IFC/wiki/Towards-a-technology-independent-IFC: Makes it STEP/EXPRESS independent.

What do we win: Interoperability

What do we loose: According to the ISG Questionnaire nothing in practice.

Schema impact: Simpler

Instance model impact: none

Backwards compatible: no in theory, yes in practice because it is not used

Automatic migration possible: yes, because files don't use it

berlotti commented 4 years ago

WHERE Rules are actually documentation about restrictions. They are not parsed / evaluated at run-time because that is too late. The WHERE rules semantics should be in documentation in a different way.

There is a desire to have machine readable restrictions to facilitate automatic checkers (additional to documentation). mvdXML is not capable of doing this.

TLiebich commented 4 years ago

WHERE rules are used for file validation, e.g. during the certification process. Therefore they do have a purpose. Also being able to declare restrictions in a computer-interpretable way (rather then my pure text documentation) is a benefit for the consistency check of IFC exchanges.

Hence I see it as premature to generally remove WHERE rules (while I agree that several could be removed).

klacol commented 4 years ago

I like WHERE rules. They are implemented in XBim very well and that teached me a lot about the rules in valid IFC files. The fact, that implementers to not respect the standard, is for me not a reason to leave them out (and I am myself an implementer 😀 ).

For me it would be ok, when I would know, that I have a file that does not respect the where rules.

Quality level 1: file that does not check WHERE rules on import or export Quality level 2: file that checks WHERE rules on import or export