NXDL: group cardinality in NXDL vs. in HDF5

[woutdenolf]

In NXDL we have:

<xs:complexType name="groupType">
    <xs:attribute name="minOccurs" use="optional" default="0" type="nx:nonNegativeUnbounded"/>
    <xs:attribute name="maxOccurs" use="optional" type="nx:nonNegativeUnbounded"/>
    <xs:group ref="nx:groupGroup" minOccurs="0" maxOccurs="unbounded"/>
</xs:complexType>

<xs:group name="groupGroup">
    <xs:sequence>
        <xs:element name="group" type="nx:groupType" minOccurs="0" maxOccurs="unbounded"/>
    </xs:sequence>
</xs:group>

The groupGroup definition just controls how many <group> entries can appear in an NXDL definition file, not how many groups exist in an HDF5 file.

In XML Schema (W3C), minOccurs and maxOccurs default to 1 when not specified
(https://www.w3.org/TR/xmlschema-1/#declare-element). This does not apply however to the maxOccurs attribute in NXDL which has not default. So it is undefined.

So in:

<definition name="NXentry" type="group">
    <group type="NXuser"/>
    <group type="NXsample"/>
    <group type="NXinstrument"/>
    ...
</definition>

these lines define which group types are allowed in an NXentry. We can add as many as we want because

<xs:element name="group" type="nx:groupType" minOccurs="0" maxOccurs="unbounded"/>

But the cardinality in HDF5 is undefined because maxOccurs in NXDL has no XML default value defined in its attribute declaration.

<xs:attribute name="maxOccurs" use="optional" type="nx:nonNegativeUnbounded"/>

That is not at all what we intended right?

[woutdenolf]

The words minOccurs/maxOccurs are very confusing. They come from XML Schema (XSD), where they describe element repetition, but in NXDL they are reused as attributes of the NXDL language itself to describe NeXus data-model cardinality.

This creates a two-layer interpretation:

• In XSD: minOccurs/maxOccurs control repetition of XML elements.
• In NXDL: they describe how many times a NeXus concept (e.g. NXuser, NXinstrument) may appear in an HDF5 file when interpreted through NXDL.

[woutdenolf]

I would expect we need

	<xs:complexType name="groupType">
		        <xs:attribute name="minOccurs" use="optional" default="0" type="nx:nonNegativeUnbounded">

-               <xs:attribute name="maxOccurs" use="optional" type="nx:nonNegativeUnbounded">
+               <xs:attribute name="maxOccurs" use="optional" default="unbounded" type="nx:nonNegativeUnbounded">

[woutdenolf]

Related: #320

Seems like default="unbounded" got removed in 2014: 6572f4b

[woutdenolf]

In addition

<xs:attribute name="minOccurs" use="optional" default="0" type="nx:nonNegativeUnbounded"/>

What would minOccurs = "unbounded" mean? I think xs:nonNegativeInteger would be the correct XSD type.