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Subject: further design considerations for 2.0 (prototype '-sdg-2' attached)

It seems that we still have to prove, with prototype(s),
a suitable design for the next version (2.0) of UBL
so I have been trying to prototype a set of possible
2.0 schemas and prove what design is necessary
in these to still allow polymorphism in 2.1 and even
in a theoretical 2.2 minor version.
The first step, already demonstrated, was to show that
there are problems introduced with the local declarations
of IDs and Codes in 1.0, hence the decision to remove
this design aspect by moving to a 2.0 major version.
The main problem introduced with these local elements
was in the fact that the instances for 1.0 would, if a
minor version used polymorphism with imports of these
schemas, require a change to the local elements from
default namespace without prefixes (e.g. in:Invoice/ID)
to element names with prefixes (e.g. in:Invoice/in:ID) for
tehm to be valid against the new schemas. This is
considered to be an undesirable compromise of backwards
compatibility in one sense of our understanding of it.
I've tried to test this a bit further by making the change
to the IDs and Codes and then using the substitutionGroup
mechanism in subsequent prototypic minor versions
and I've found that this change may not quite be enough.
To allow minor version schemas to correctly validate the
major version instances with only minimal changes (e.g. just
schema location and the like) to the instances it appears
(as the attached prototype demonstrates) that we have to
make two further changes to the document schemas:
Firstly there are in UBL 1.0 some BBIEs declared in the
document schemas rather than in the common (CBC)
schema and these would have to be moved to CBC;
Secondly there are in UBL 1.0 some ABIEs declared in
the document schema and these too would have to be
moved to the common schema (CAC).
[ the attached file is a zipped file renamed .zzz from .zip ]
For those not too familiar with the UBL schemas, here
is an example:
In the UBL 1.0 Invoice we have:
<xsd:complexType name="InvoiceType">
      <xsd:element ref="LineItemCountNumeric"
          minOccurs="0" maxOccurs="1"/>
      <xsd:element ref="AdditionalDocumentReference"
         minOccurs="0" maxOccurs="unbounded"/>
<xsd:element name="AdditionalDocumentReference"
<xsd:complexType name="LineItemCountNumericType">
      <xsd:extension base="udt:NumericType"/>
and this would, I believe, in 2.0, have to become something like:
<xsd:complexType name="InvoiceType">
      <xsd:element ref="cbc2-0:LineItemCountNumeric"
           minOccurs="0" maxOccurs="1"/>
      <xsd:element ref="cac2-0:AdditionalDocumentReference"
          minOccurs="0" maxOccurs="unbounded"/>
with both the document-specific BBIEs such as LineItemCountNumeric
moved to the CBC schema module and the
document-specific ABIEs such as AdditionalDocumentReference
moved to the CAC schema module.
As the examples in the prototype ('instances' directory)
attempt to demonstrate, this allows instances to be
valid against all major and following minor versions
when using the substitutionGroup
polymorphism mechanism for the minor version schemas.
In producing this prototype I came across a number
of issues relating to the changes needed for 2.0
1. Moving two Codes, ReasonCode and IdentificationCode
to the common module (CBC) there may need
to be a renaming of the elements since the names are
far less specific in 1.0 than the datatypes which
are tied to quite specific codelists (namely
AllowanceChargeReasonCode and CountryIdentificationCode).
There is also a general principle here to which we'd
need to adhere more in modeling with this new design:
that the more specific the datatype (e.g. a more
specific codelist), the less generic should be the element
2. We need to clarify whether an ID element should
have a complexType named IDType or IdentifierType
There may be other improvements to such naming
rules we'd like to make too (some type names seem
too arbitrary generally).
3. Does an xxxxCode element have to have a
complexType name xxxxCodeType or is just CodeType sufficient?
Is this covered by NDR?
4. I take it that each code for which we have a codelist
schema does still have to have a separate complexType
(such as AllowanceChargeReasonCodeType) declared
for it in the CBC schema which then extends its
codelist schema datatype. Is this so? Is there a rule that
covers it (the rules may not have considered Codes
with codelists before in the context of the CBC schema
since the codes were all locally declared).
5. The NDR and other schema module diagrams will need
reworking since the patterns of imports will change.
I hope the attached prototype will help with this.
6. The 1.0 schemas have many fixed attributes in the
datatypes (UBLAmount and the schema-defined codelists)
and these could break the requirements for backwards
compatibility of minor versions where they might need
to be changed (e.g. the UBL urls).
7. If the reasoning about the need to define all of the
document BIEs in CAC and CBC schema modules are accepted
this may need to be explicit in the NDR
8. If polymorphism with the limited substitutionGroup
mechanism is accepted there would need to be a consideration
of how to provide for both restriction and extension of the
same types (e.g. to introduce an intermediate type suitably
named) - see previous prototype (sdg-1) but this is omitted
from the attached prototype
9. There will likely be frequent occasions when a type to
be extended was not explicitly declared in the previous version
(if only the types we changed in that version were explicitly
included, as required for modular rather than blanket revisions).
This does then require (as in the 2.2 schema of the attached
prototype - Delivery example) the import of earlier version
schemas such as the previous major version and with it the
older namespaces and suitable prefixes. This then means that
we are likely to need rules to define separate suitably named
prefixes for each version and these should ideally include
some indication of the version number (perhaps even for the
major version). For example, I have used cac2-0, cac2-1, etc.
10. I do not forsee problems incorporating the ATG2 CCTS
schemas into the prototype (though I haven't done so here)
- see previous mail with such a design prototyped. But the
complexities of the polymorphic versioning may be quite a
challenge, it seems, to keep it in step with the ATG2 design -
mainly considering possible namespace aspects (as seen
in point 9 above). This concerns whether instances with the
necessary multiple namespaces/prefixes for the UBL design
instances could be catered for in an ATG2-type set of schemas. 
Whether instances valid to the ATG2 schemas could have
the necessary complexity of namespaces to validate against
corresponding UBL schemas is what seems an increasingly
challenging prospect as the real details are considered.
All the best
Stephen Green


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