Re: Strict and lax schema validation (PR#87)

Hi Erik,

The working group accepts that we must specify how schemas are applied to
instance data based on the namespace involved and the available schema
declarations for that namespace.  The goal will be to document what current
practice is achieving in effect, rather than a specific means of implementing it
and rather than adding another feature at this time to provide the form author
with control over the process.  We intend, with your help of course, to increase
both form author control and overall flexibility of schema processing in a
future release of XForms.

Thank you,
John Boyer

> 
> All,
> 
> XForms 1.1 mentions in 4.3.5:
> 
>      "the node satisfies all applicable XML schema definitions
>      (including those associated by [...] an external or an inline
>      schema [...])
> 
> I would like a precision on what "applicable" means for schema types
> when types are not assigned with xforms:bind or xsi:type. If I do a
> correct job below, then you will see that things are currently not
> clear.
> 
> Assume a schema defining types in the:
> 
>     xmlns:foo="http://example.org/schema/foo"
> 
> namespace (that schema does not import other schemas). I import the
> schema into a model with three instances:
> 
>     <xforms:model schema="foo.xsd">
> 
>       <xforms:instance id="instance-1">
>         <foo:form>
>           ...
>         </foo:form>
>       </xforms:instance>
> 
>       <xforms:instance id="instance-2">
>         <bar:form>
>           ...
>         </bar:form>
>       </xforms:instance>
> 
>       <xforms:instance id="instance-3">
>         <foo:undefined>
>           ...
>         </foo:undefined>
>       </xforms:instance>
> 
>     </xforms:model>
> 
> Assume foo.xsd defines only a complex type for foo:form, but no type
> for foo:undefined, and no type for bar:form (bar maps to a different
> namespace, and there is no schema for "bar").
> 
> It seems to me that foo:form for sure has an "applicable" schema
> definition. So no problem here.
> 
> Now what's the algorithm for bar:form? It certainly doesn't have any
> schema definition: there is no schema for namespace "bar". So I guess
> this means there is no "applicable" definition for bar:form. However,
> you could decide to recurse the tree and validate further, or not.
> 
> Now what about foo:undefined? There is a schema for namespace "foo",
> but none for the type foo:undefined. Does this mean there there is an
> applicable definition or not? Same thing, you could decide to recurse
> the tree and validate further, or not.
> 
> Now I shouldn't even have to discuss this in these terms, because XML
> schema provides options already, see "3.10.1 The Wildcard Schema
> Component" in [1].
> 
> There are three ways you can process a subtree in schema:
> 
>     *strict*
> 
>       There must be a top-level declaration for the item available, or
>       the item must have an xsi:type, and the item must be ?valid? as
>       appropriate.
> 
>     *skip*
> 
>       No constraints at all: the item must simply be well-formed XML.
> 
>     *lax*
> 
>       If the item has a uniquely determined declaration available, it
>       must be ?valid? with respect to that definition, that is,
>       ?validate? if you can, don't worry if you can't.
> 
> So it seems to me that here the question is whether we process
> instances with "strict" or "lax" processing (and possibly "skip" as
> well).
> 
> XSLT 2.0 solves the problem by providing a "validation" attribute [2]
> which can have value "strict", "lax", and two others ("preserve" and
> "strip") which are probably not relevant here.
> 
> XSLT 2.0 also provides a "type" attribute, exclusive with
> "validation". In XForms, we have a similar situation: we can
> explicitly bind a type to the root element of an instance with
> xforms:bind or with xsi:type. If we do, everything is fine. If we
> don't, THEN we want to specify whether we want strict or lax
> validation.
> 
> It seems to me then, if my understanding is correct, that there is an
> omission in XForms at this point regarding how validation is
> performed. We should:
> 
> 1. Explicitly specify whether instance validation is performed in
>      "strict" or "lax" mode when no type is assigned to the root element
>      of the instance.
> 
> 2. Ideally, provide an option to specify whether "strict", "lax" or
>      even "skip" mode is applied. This could be done with a simple
>      attribute on xforms:instance:
> 
>        <xforms:instance validation="lax">
> 
> 3. Possibly, also provide the option for "skip" mode, to specifically
>      exclude an instance from validation.
> 
> Comments on this are of course welcome.
> 
> -Erik
> 
> [1] http://www.w3.org/TR/xmlschema-1/
> [2] http://www.w3.org/TR/xslt20/#validation
> 
> -- 
> Orbeon Forms - Web Forms for the Enterprise Done the Right Way
> http://www.orbeon.com/
> 
>

Received on Wednesday, 31 October 2007 17:17:13 UTC