XML schema evolution

For the rest of this section, we will describe the current state of the art in XML schema evolution as present in commercially available systems and research works. For each solution, in addition to comparing the solution against the requirements outlined in Sect. 2, we describe the classes of incremental changes that the solution supports and in what way it mitigates changes that must be made to either applications or instances. Table 6.2 shows the characteristics of the main approaches considered, which are discussed at the end of this section. 

Oracle offers two very different ways to evolve an XML schema (Oracle XML Schema Evolution 2008). The first is a copy-based mechanism that allows a great deal of flexibility. Data from an XML document collection are copied to a temporary location, then transformed according to a specification, and finally replaced in its original location. The second is an in-place evolution that does not require any data copying but only supports a limited set of possible schema changes. 

Gruber TR (1993) A translation approach to portable ontology specifications. In Knowledge acquisition, vol 5(2). Academic, London, pp 199-220 Guenini G, Mesiti M (2009) XML schema evolution and versioning current approaches and future trends. In Open and novel Issues in XML database applications. Future directions and advanced technologies. IDEA Group, pp 66-87... 

Mesiti M, Celle R, Sorrenti, MA, Guerrini G (2006) X-Evolution A system for XML schema evolution and document adaptation. In Proceedings of EDBT, 2006. Springer, Heidelberg Microsoft SQL Server 2008 R2 Data-Her Applications (2010) http //msdn.microsoft.com/en-us/ library/ee240739(SQL. 105).aspx... 

Oracle Edition-Based Redefinition (2009) Whitepaper. Available at http //www.oracle.com/ technology/deploy/availability/pdf/edition based redefinition.pdf Oracle XML Schema Evolution (2008) Chapter 9 of Oracle XML DB, Developer s Guide, 1 lg Release, May 2008... 

Schema matching aims at identifying semantic correspondences between metadata structures or models, such as database schemas, XML message formats, and ontologies. Solving such match problems is a key task in numerous application fields, particularly to support data exchange, schema evolution, and virtually all kinds of data integration. Unfortunately, the typically high degree of semantic heterogeneity reflected in different schemas makes schema matching an inherently complex task. Hence, most current systems still require the manual specification of semantic correspondences, e.g., with the help of a GUI. While such an approach is appropriate for...

Schema evolution is the ability to change deployed schemas, i.e., metadata structures formally describing complex artifacts such as databases, messages, application programs, or workflows. Typical schemas thus include relational database schemas, conceptual ER or UML models, ontologies, XML schemas, software interfaces, and workflow specifications. Obviously, the need for schema evolution occurs very often in order to deal with new or changed requirements, to correct deficiencies in the current schemas, to cope with new insights in a domain, or to migrate to a new platform.

Schema evolution has been an active research area for a long time and it is increasingly supported in commercial systems. The need for powerful schema evolution has been increasing. One reason is that the widespread use of XML, web services, and ontologies has led to new schema types and usage scenarios of schemas for which schema evolution must be supported. The main goals of this survey chapter are as follows ...

To provide an overview about the current state of the art and recent research results on schema evolution in three areas relational database schemas, XML schemas, and ontologies. For each kind of schema, we outline how and to what degree the introduced requirements are served by existing approaches. 

In Sect. 2, we introduce the main requirements for effective schema and ontology evolution. Sections 3 and 4 deal with the evolution of relational database schemas and of XML schemas, respectively. In Sect. 5, we outline proposed approaches for ontology evolution and conclude in Sect. 6. 

The W3C - the official owners of the XML and XML Schema recommendations - have a document describing a base set of use cases for evolution of XML Schemas (W3C 2006). The document does not provide any language or framework for mitigating such evolutions, but instead prescribes what the semantics and behavior should be for certain kinds of incremental schema evolution and how applications should behave when faced with the potential for data from multiple schema versions. For instance, Sect. 2.3 lists use cases where the same element in different versions of a schema contains different elements. Applications are instructed to ignore what they don t expect and be able to add extra elements without breaking the application. ... 

Oracle Microsoft SQL Server IBM DB2 Altova Diff Dog XEM Kramer (2001), Su et al. (20011 Model-based approaches (X-Evolution, CoDEX, UML) Temporal XML schema... 

The copy-based version of schema evolution is performed using the DBMS. XMLSCHEMA. copyEvolve stored procedure. The procedure takes as input three arrays a list of schema URLs representing the schemas to evolve, a list of XML schema documents describing the new state of each schema in the first list, and a list of transformations expressed in XSLT. Each transformation corresponds to a schema based on its position in the list so, the first transformation on the list is used to translate all instances of the first schema to conform to the first new schema definition, and so on. 

Once a schema collection has been created, it can be assigned to be the schema for any column whose type is XML. Also, once the collection is created, there are only two operations that can be done on it - drop it or alter it by adding new constructs. The ALTER statement is the only form of schema evolution that SQL Server allows without manually dropping the schema, manually translating instances, and reestablishing the schema. The ALTER statement has only one form ... 

Native XML databases, unlike relational systems, are built from the ground up to support XML storage. Relatively few of these systems support XML schemas or schema evolution. One notable exception is Tamino (Software AG 2006). 

As of the year 2000, the DTD was the predominant method for schematizing XML documents. As the decade progressed, XML Schema became the dominant schematizing technology for XML. That same trend has been mirrored in research schema evolution techniques introduced earlier in the decade focused more on changes to a DTD, while more recent publications cover the far more expressive XML Schema recommendation. 

XEM (Kramer 2001 Su et al. 2001) - XML Evolution Management - is a framework introduced by Worcester Polytechnic Institute in 2001 describing evolution management in DTDs. The approach predates schema evolution in any of the commercial systems introduced in the previous section. The work provides a sound and... 

Temporal XML Schema (Currim et al. 2009) - also referred to as xXSchema -is a way to formalize the temporal nature of schema and document versioning. The framework is assembled by the same research group that helped develop the temporal extensions to SQL. In all other frameworks discussed to date, the relationship between versions of schemas and documents are informal if they exist at all two versions of the same schema version are considered to be two separate schemas, related to each other only by whatever point-in-time script was used to perform the migration. xXSchcma makes evolution over time a first-class concept, modeling different versions of the same conventional XML schema in the same document. 

The evolution of XML schemas is easier than for relational schemas since the schemas can be extended by optional components that do not invalidate existing instances. Due to the absence of a standard schema modification language, schema changes are usually specified by providing a new version of the schema. In research approaches, schema matching and mapping techniques are being used... 

Miller R, Ioannidis YE, Ramakrishnan R (1994) Schema equivalence in heterogeneous systems Bridging theory and practice. Inform Syst 19(1) 3-31 Moto MM, Malaika S, Lim L (2007) Preserving XML queries during schema evolution. In Proceedings of WWW conference. ACM, NY, pp 1341-1342 Noy NF, Klein M (2004) Ontology evolution Not the same as schema evolution. Knowl Inform Syst 6(4) 428 140... 

Smith B, Ashburner M, Rosse C et al (2007) The OBO Foundry coordinated evolution of ontologies to support biomedical data integration. Nat Biotechnol 25(11) 1251-1255 Software AG (2006) Tamino XML schema user guide 4.4.1. http //documentation.softwareag.com/ cross vision/ins441 j/print/tsl. pdf... 

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