Working Draft 03, 04/04/03
- Document identifier:
wd-cmsc-cmguidelines-v01.3-03
Matthew Gertner <matthew@acepoint.cz>
Eduardo Gutentag, Sun Microsystems, Inc. <eduardo.gutentag@sun.com>
- Status:
This is a draft document and is likely to change on a weekly basis.
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Copyright © 2003 OASIS Open, Inc. All Rights Reserved.
Table of Contents
- 1. Introduction
- 2. Background
- 3. Compatibility: Customization through Derivation
- 3.1. Use of XSD Derivation
- 3.2. Extensions
- 3.3. Restrictions
- 4. Compatibility: Context and Documentation
- 4.1. Context chains
- 5. Customization through other means
- 5.1. Use of Ur-Types
Appendixes
With the first public release of UBL, version 0p70, users can begin to gain experience using the library in their applications for interchange of business data among trading partners. Although the library will be subject to change and extension as it approaches the final version, it already contains important document types informed by the broad experience of members of the UBL Technical committee, including EDI and XML experts.
One of the most important lesson learned from previous standards is that no business library is sufficient for all purposes. Requirements differ significantly amongst companies and industries, and a customization mechanism is therefore needed in many cases before the document types can be used in real-world applications. A primary motivation for moving from the relatively inflexible EDI formats to a more robust XML approach is the existence of formal mechanisms for performing this customization while retaining maximum interoperability and validation.
In order for this interoperability and validation to be achieved, care must be taken to adhere to strict guidelines when customizing UBL document types. Although the UBL TC intends to produce a customization mechanism that can be applied as an automatic process in the future, in the meantime, and as part of this Phase I effort, the following guidelines are offered.
This document aims to describe the procedure for customizing UBL, with three distinct goals.
The first goal is to ensure that UBL users can extend UBL schemas in a manner that:
allows for their particular needs,
can be exchanged with trading partners whose requirements for data content are different but related, and
is UBL compatible.
The second goal is to provide a couple of canonical escape mechanisms for those whose needs extend beyond what the compatibility guidelines can offer. Although the product of these escape mechanisms cannot claim UBL compatibility, at least it can offer a clear description of its relashionship to UBL, a claim that cannot be made by other ad hoc methods.
The third goal is to gather use case data for the future UBL context extension methodology, an automatic mechanism for creating customized UBL schemas.
The major output of the UBL TC is encapsulated in the UBL Schemas. It is assumed that in many cases users will need to customize these schemas for their own use. In accordance with [ebXML] the UBL TC expects this customization to be carried out only in response to contextual needs (see [xxx]) and by the application of any one of the eight identified context drivers and their possible values.
It must be noted that the UBL schemas themselves are the result of a theoretical customization:
Behind every UBL Schema, an Ur-schema exists in which all elements are optional and all types are abstract. As mandated in the XSD specification, abstract types cannot be used as written; they can only be used as a starting point for deriving new, concrete types. Ur-types are modelled as abstract types since they are designed for derivation.
The first set of derivations from the abstract Ur-types is the UBL Schema Library itself, which is assumed to be usable in 80% of business cases. These derivations contain additional restrictions to reduce ambiguity and provide a minimum set of requirements to enable interoperable trading of data by the application of one context, Business Process. The UBL schema may then be used by specific industry organizations to create their own customized schemas. When the Schema is used, conformance with UBL may be claimed. When a Schema that has been customized through the UBL sanctioned derivation processs is used, conformance with UBL may also be claimed.
It is assumed that in many cases specific businesses will use customized UBL schemas. These customized schemas contain derivations of the UBL types, created through additional restrictions and/or extensions to fit more precisely the requirements of a given class of UBL users. The customized UBL Schemas may then be used by specific organizations within an industry to create their own customized schemas.
Due to the extensiblilty of W3C Schema, this process can be applied over and over to refine a set of schemas more and more precisely, depending on the needs of specific data flows.
In other words, there is no theoretical limit to how many times a Schema can be derived, leading to the possible equivalent of infinite recursion. In order to avoid this, we have developed the Rule of Once-per-Context, as presented later, in "Context Chains "
Central to the customization approach used by UBL is the notion of schema derivation. This is based on object-oriented principles, the most important of which are inheritance and polymorphism. The importance of the latter can be gleaned from its linguistic origin: poly, meaning "many", and morph, meaning "shape". By adhering to these principles, document formats with different "shapes" can be used interchangeably.
The UBL Naming and Design Rules Subcommittee (NDRSC) has decided to use XSD, the standard XML schema language produced by the World Wide Web Consortium (W3C), to model document formats. One of the most significant advances of XSD over previous XML document description languages like DTDs is that it has built-in mechanisms for handling inheritance and polymorphism. It therefore fits well with the real-world requirements for business data interchange and our goal of interoperability and validation.
There are three different scenarios (not all of which can be accomplished through direct XSD derivation) in which derivations could be performed on existing types:
An existing UBL type fits the requirements for the application with modifications supported by XSD derivation. These modifications can include extension, adding new information to an existing type, and/or refinement, restricting the set of information allowed to a subset of that permitted by the existing type.
An existing UBL type is close to the requirements of the application, but the changes needed go beyond those allowed directly by XSD derivation. For example, the new type might need to broaden the set of information allowed to a superset of that permitted by the existing type.
No existing UBL type is found that can be used as the basis for the new type. Nevertheless, the base library of core components that underlies UBL can be used to build up the new type so as to ensure that interoperability is at least possible on the core component level.
These Guidelines will deal with each of the above scenarios, but we will first and foremost concentrate on the first, as it is the only one that can produce UBL compatible schemas.
XSD derivation allows for type extension and restriction. These are the only means by which one can customize UBL schemas and claim UBL compatibility. All other possible means, even if allowed by XSD itself, are not allowed by UBL.
XSD extension is used when additional information must be added to an existing UBL type. For example, a company might use a special identification code in relation to trading partners. This code should be included in addition to the standard information used in a BuyerParty description (AccountCode, PartyName, Address, etc.) when purchasing goods. This can be achieved by creating a new type that references the existing type and adds new the information:
<xsd:complexType name="MyBuyerPartyType">
<xsd:extension base="cat:BuyerPartyType">
<xsd:element name="InternalSupplierCode" type="xsd:string"/>
</xsd:element>
</xsd:extension>
</xsd:complexType>Some observations:
Notice that derivation can be applied only to types and not to elements that use those types. This is not a problem; UBL uses explicit type definitions for all elements, in fact disallowing XSD use of anonymous types that define a content model directly inside an element declaration,
This derived type can be used anywhere the original type is allowed. The instance document should use the xsi:type attribute to indicate that a derived type is being used. This does not enforce the use of the new type inside a given element, however, so in this example Orders could still be created using the standard UBL BuyerParty type. If the user wishes to require the use of the derived type, a new derived type must be created from the Order type using refinement and specifying that the MyBuyerPartyType is used.
UBL defines global elements for all types, and these elements, rather than the types themselves, are used in aggregate element declarations. It is therefore recommended that the same procedure be used for derived types, so a MyBuyerParty element should be created based on the MyBuyerPartyType.
All derived types should be created in a separate namespace (which might be tied to the user organization) and reference the UBL namespaces as appropriate.
In other cases, the user may wish to use the existing UBL type but restrict the information in some way. This is accomplished through XSD restriction. For instance, the UBL BuyerPartyType permits the inclusion of any number of addresses or none. If a specific organization wishes to allow exactly one address, this is achieved as follows (note that the annotation fields are removed from the type definition to make the example more readable):
<xsd:complexType name="MyBuyerPartyType">
<xsd:restriction base="cat:BuyerPartyType">
<xsd:sequence>
<xsd:element ref="ID" id="UBL000090">
</xsd:element>
<xsd:element ref="AccountCode" id="UBL000091" minOccurs="0">
</xsd:element>
<xsd:element ref="PartyName" id="UBL000092" minOccurs="0" maxOccurs="unbounded">
</xsd:element>
<xsd:element ref="Address" id="UBL000093" minOccurs="1" maxOccurs="1">
</xsd:element>
<xsd:element ref="PartyTaxScheme" id="UBL000094" minOccurs="0" maxOccurs="unbounded">
</xsd:element>
<xsd:element ref="BuyerContact" id="UBL000095" minOccurs="0">
</xsd:element>
</xsd:sequence>
</xsd:restriction>
</xsd:complexType>Note that the entire content model of the base type, with the appropriate changes must be repeated when performing restriction.
Every time a derivation is performed on an UBL- or UBL-derived Schema, the context and the context value used must be documented. If this is not done, then by definition the derived Schema is not UBL-compliant.
It is planned that a context extension methodology will be designed to enable automatic customization of UBL types for specific purposes. This methodology works by using a formal specification of the reasons for customizing the type, known as the context. By expressing the context formally and specifying rules for adapting types based on context, most of the changes that need to be made to UBL in order for it to fit in a given usage environment can be generated by the context engine rather than performed manually. In addition, significant new flexibility can be gained, since rules from two complementary contexts can be applied simultaneously, yielding types appropriate for, say, the automobile industry and the French geopolitical entity.
UBL has not yet progressed to this stage of development. For now, one of the main goals of the UBL Context Methodology Subcommittee is to gather as many use cases as possible to determined what types of customizations are performed in the real world, and on what basis. Another important goal is to ensure that types derived at this point from UBL's version 1 can be still used in the future, intermixed with types derived automatically in the future.
Context is expressed using a set of name/value pairs, where the names are one of a limited set of context drivers established by the UBL TC:
Business process
Official constraints
Product classification
Business process role
Industry classification
Supporting role
Geopolitical
System capabilities
Context should be included as an element Context (in the UBL namespace) inside the documentation for each customized type, with the name of the context derived expressed as in the list above, but using capitalized camel case. The Context element has two attributes, driver and value. For example, if the type is to be used in the French automobile industry, the Context documentation would appear as follows:
<xsd:annotation>
<xsd:documentation>
<ubl:Context driver="IndustryClassification" value="Automotive"/>
<ubl:Context driver="Geopolitical" value="France"/>
</xsd:documentation>
<xsd:annotation>If a customization is made that does not fit into any of the existing context drivers, it should be described in prose form inside the Context element:
<xsd:annotation>
<xsd:documentation>
<ubl:Context>Used for jobs performed on weekends to specify additional data required by the trade union</ubl:Context>
</xsd:documentation>
<xsd:annotation>Note
Any issues with the set of context drivers currently defined or the taxonomies to be used for specifying values should be communicated to the UBL Context Driver Subcommittee.
As mentioned in "Customization of Customization", there is a risk that derivations may form extremely long and unmanageable chains. In order to avoid this problem, the Rule of Once-per-Context was formulated: no context can be applied, at a given hierarchical level of that context, more than once in a chain of derivations.
There are two important types of customization that XSD derivation does not allow. The first can be summarized as the deletion of required components (that is, the reduction of a component's cardinality from x..y to 0..y). The second is the ad hoc location of an addition to the content model through extension. There may be some cases where the user needs a different location for the addition, but XSD extension only allows addition at the end of a sequence.
Because XSD derivation does not allow these types of customization, any attempts at enabling them must by necessity produce results that are not UBL compatible. However, in order to allow users to customize their schemas in a UBL-friendly manner, the notion of an Ur-schema was invented: for each UBL Schema, an Ur-schema exists in which all elements are optional and all types are abstract. The use of abstract types is necessary because Ur-types can never be used as is; a derived type must be created, as per the definition of abstract types in the XSD specification.
XSD derivation is sufficient for most cases, but in some instances it might be necessary to perform changes to the UBL types that are not handled by standard mechanisms. In this case, the UBL Ur-types should be used. Remember, an Ur-ype exists for each UBL standard type and differs only in that all elements in the content model are optional, including elements that are required in the standard type. By using the Ur-type, the user can therefore make modifications, such as eliminating a required field, that would not be possible using XSD derivation on the standard type.
For instance, suppose an organization would like to use the UBL BuyerPartyType, but does not want to use the required ID element. In this case, normal XSD refinement is used, but on the ur type rather than the standard type:
<xsd:complexType name="MyBuyerPartyType">
<xsd:restriction base="ur:BuyerPartyType">
<xsd:sequence>
<xsd:element ref="ID" id="UBL000090" minOccurs="0" maxOccurs="0">
</xsd:element>
<xsd:element ref="AccountCode" id="UBL000091" minOccurs="0">
</xsd:element>
<xsd:element ref="PartyName" id="UBL000092" minOccurs="0" maxOccurs="unbounded">
</xsd:element>
<xsd:element ref="Address" id="UBL000093" minOccurs="0" maxOccurs="unbounded">
</xsd:element>
<xsd:element ref="PartyTaxScheme" id="UBL000094" minOccurs="0" maxOccurs="unbounded">
</xsd:element>
<xsd:element ref="BuyerContact" id="UBL000095" minOccurs="0">
</xsd:element>
</xsd:sequence>
</xsd:restriction>
</xsd:complexType>The new type is no longer compatible with the UBL BuyerPartyType, so standard processing engines that know about XSD derivation will not recognize the type relationship. However, some level of interoperability is still preserved, since both UBL BuyerPartyType and MyBuyerPartyType are derived from the BuyerPartyType Ur-type. If this additional flexibility is required, a processor can be implemented to use the Ur-type rather than the UBL type. It will then be able to process both the UBL type and the custom type, since they have a common ancestor in the Ur-type.
Once again, changes to the Ur-type do not enforce changes in the enclosing type, so the UBL OrderType has to be changed as well if the user organization wants to ensure that only the new MyBuyerPartyType is used. In fact, the new OrderType will not be compatible with the UBL OrderType, since MyBuyerPartyType is no longer derived from UBL BuyerPartyType. However, the new OrderType can be derived from the OrderType Ur-type to achieve maximum interoperability.
Sometimes no type can be found in the UBL library or ur type library that can be used as the basis of a new type. In this case, we should still strive for maximum interoperability by building up the new type using types from the core component library that underlies UBL.
For example, suppose a user organization needs to include a specialized product description inside business documents. This description includes a unique ID, a name and the storage capacity of the product expressed as an amount. The type definition should appear as follows:
<xsd:complexType name="ProductDescriptionType">
<xsd:sequence>
<xsd:element name="ID" type="cct:IdentifierType"/>
<xsd:element name="Name" type="cct:TypeType"/>
<xsd:element name="Capacity" type="cct:AmountType"/>
</xsd:sequence>
</xsd:complexType>It goes without saying that all new names defined when creating custom types from scratch should also conform to the UBL Naming and Design Rules.
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Normative
[RFC 2119] S. Bradner. RFC 2119: Key words for use in RFCs to Indicate Requirement Levels. IETF (Internet Engineering Task Force). 1997.