XML schemas

The different dialects of XML (XHTML, KML) are constrained by XML schemas (W3C, 2004). These schemas are critical to the success of XML. They are used to ensure that an XML file adheres to a well-defined structure. Schemas are themselves XML files, which must conform to the XSD specification. Schema designers are free to develop constraints to varying degrees. Forcing an XML file to be compatible with a tightly-constrained schema frees developers from having to write their own data validation procedures. This leads to a great simplification of data manipulation software. 

All modern relational databases include the ability to export tables as XML files. It is usually possible to apply an XSLT transformation to the data as part of the export procedure. In the interest of simplicity and compatibility across different databases, no special transformation was applied to the tables extracted from the New Brunswick till database. Therefore, after exporting data out of MS Access in a generic XML format, the first XSLT transformation involves restructuring the data to conform to a Geochemical Survey XML schema, developed at the GSC (Adcock 2009b). The second transformation produces a set of files which conform to the GML schema (OGC, 2007). KML shares many features with GML, and hence the third and final GML-to-KML transformation is very simple. 

W3C 2004. XML Schema Part 0 Primer Second Edition W3C Recommendation 28 October 2004... 

The technological foundations of this vision currently consist of extensible Markup Language (XML) [76], XML Schema [77], the Resource Description Framework (RDF) [61], RDF Schema [62], and the Web Ontology Language [63]. These technologies are interdependent and can thus be arranged in the form of a semantic layer cake (Fig. 9). 

D.C. McArthur, An Extensible XML Schema Definition for Automated Exchange of Protein Data PROXIML (PROtein extensible Markup Language). University of California, Santa Cruz, CA, 2001, available from http //xml.coverpages.org/ proximl.xml. 

Because CIF predates XML, this definition is not machine-processable (i.e., it acts as a specification for a human programmer, but it cannot be used to generate software automatically). XML schemas provide mechanisms to overcome this. 

There are many ways to write schemas for XML documents, but only a few are commonly used. XML designers usually select from this triumvirate of schema languages Document Type Definition (DTD), W3C XML Schema, and RelaxNG. Sometimes they augment their validation with Schematron rules. 

DTD is the oldest XML schema language. Its rules are declared in the same standard as the XML language (Bray et al. 2006b), and many software tools support DTD. It is the only schema language that can be used directly inside XML documents. DTD declarations are used not only for validation purposes, but also as vehicles for providing names to uncommon characters or inclusion of external files. 

W3C XML Schema is a schema language defined by the W3C (Thompson et al. 2004 Biron and Malhotra 2004). The choice of name for this schema language was very unfortunate. As discussed above, several schema languages can be used for validation of XML documents, and W3C XML Schema is just one of them. Unfortunately, many people do not understand this nuance and often insist on W3C XML Schemas in situations where another schema language would be more suitable. 

W3C XML Schema is a powerful validation language that is well supported by software tools, but it has some disadvantages that are important to keep in mind. Its specifications are very complex and difficult to thoroughly comprehend. This unfortunate fact led to many corrections to the original specifications, and many problems are still not fully resolved. Although people are shielded from this complexity by schema-authoring software, they become hostages to their tools. If a tool does not offer some validation rule, it may be very difficult to write it correctly by hand if... 

RelaxNG is a very powerful schema language that is much simpler to use and comprehend then W3C XML Schema. It is particularly suitable for validation of flexible data and for fast prototyping. This makes it the validation language of choice in data mining applications where diverse sources are processed and combined. 

In the heart of the language lies a flexible core applicable to many analytical techniques, as described by the W3C XML Schema. Specific needs of individual techniques are addressed in Analytical Technique Definition Documents that are XML files specifying particular parameters. The attempt to separate core features from particularities in a modular way is laudable. Other specification authors would benefit from using this approach. 

Biron, P. V., and Malhotra, A. 2004. XML Schema Part 2 Datatypes Second Edition. Available at http //www.w3.org/TR/xmlschema-2/. 

Julian, R. Proposal for a Multi-Layer XML-Schema Approach to Data Interchange Standards, paper presented at Pittcon, Orlando, FL, March 11, 2003. 

Matching large XML schemas, e.g e-business standards and message formats (Rahm et al. 2004 Smith et al. 2009)... 

Coma++ is available for free for research purposes, and hundreds of institutes worldwide have used and evaluated the prototype. Surprisingly, the default match workflow of Coma++ (combining four metadata-based matchers) proved to be competitive in many diverse areas, particularly for matching XML schemas (Algergawy et al. 2009), web directories (Avesani et al. 2005), or even meta-models derived from UML (Kappel et al. 2007). Coma- — - successfully participated in the ontology matching contest OAEI 2006. 

Amano S, Libkin L, Murlak F (2009) XML schema mappings. In PODS. ACM, NY, pp 33—42 An Y, Borgida A, Miller R, Mylopoulos J (2007) In Proceedings of the 23rd International Conference on Data Engineering, ICDE 2007, April 15-20, 2007, The Marmara Hotel, Istanbul, Turkey... 

---