Denmark has adopted XML as a key to the information architecture in support of E-Government. The philosophy of the Danish E-Government strategy is based on government wide co-operation and reuse - and support also cooperation with both the private sector and with a wider international community. The Danish XML-project focuses on coordination and supports the development and standardization of XML interfaces and vocabularies. A national XML Committee is responsible for ensuring coherence and momentum in the standardization of XML-based interfaces and vocabularies. A number of handbooks provide rules and advice for developing XML vocabularies and application integration solutions "the right way". A national XML registry provides a number of tools, including a XML Schema repository, a community module which supports collaboration, shared data modeling and Schema development, re-use of data definitions, interfaces, and a UDDI registry for implemented services.

A characteristic of the Danish approach is that messages are composed from multiple namespaces. The Danish XML Committee and the related Technical Committees will only develop XML Schemas from core information objects. Private companies and public authorities are encouraged to develop and submit their own vocabularies to the XML Committee for public hearing and approval. Provided that one or more vocabularies exist in a given domain - public authorities whishing to develop XML interfaces in that domain - are required to re-use existing XML Schema fragments from these vocabularies. Reinventing XML Schemas is strictly prohibited.

Reuse from different namespaces requires that the same XML Schema Naming and Design Rules are used. XML Schema Naming and Design Rules (NDR) are key to successful application integration in E-Government. The Danish approach is to rely heavily on reuse of atomic schema fragments from a large number of namespaces related to different authorities and international standardization initiatives, such as the OASIS Universal Business Language (UBL). The Danish naming and design rules differ in a number of ways from the naming and design rules of UBL. The paper will present the naming and design rules and the rationale leading to the differences with UBL naming and design rules.

An advantage of W3C XML Schema is its introduction of strong data typing capabilities. These advantages can not be exploited in schema vocabularies such as UBL, simply because it is impossible to agree on strong data types that are relevant to all parties. The consequence is that much of the validation must be performed by back end systems. In E-Government there is a substantial benefit to be gained at a national level by expressing all elements with strong data types. The resulting messages will convey much of the integrity rules of the exchange, and the XML Schemas can be used as part of a contract between two parties. Otherwise the integrity rules have to be expressed only in the supporting documentation. This approach is off course a lot more error prone that the strong data typing approach, as the definition of the data is separated from the structure.

The Danish XML Committee has decided to embrace and re-use ebXML and UBL Core Components. This decision is a challenge since these vocabularies are composed from weakly defined elements and types. The approach taken is to define a number of national namespaces which are strongly typed versions of the ebXML and UBL namespaces. The criteria is that for each element in the national namespace - it must be possible to transform the element into the corresponding ebXML or UBL namespace. This approach and its applicability for other national XML initiatives is discussed.

Defining a unanimous and publicly available standard for communicating, with and within the government, opens the market for vendors to create applications that comply with the standard. This should create a better competition on the marked and better interoperability between different products.