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QVT

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In the model-driven architecture, QVT (Queries/Views/Transformations) is a standard for model transformation defined by the Object Management Group. Several model transformation languages like VIATRA by the University of Budapest or ATL by INRIA are presently available, with different levels of compliance to the QVT standard. Other implementations are announced by Compuware, Borland or IBM and should be available in the future. The most advanced open source implementation seems presently (May 2006) to be the INRIA ATL implementation with a large user community and a significant number of examples available from an open source library. Also announced is a forthcoming implementation from the Tata Consultancy Services, India. There is also a version called MT available from [the Department of Computer Science at King's College, London]. Other implementations are also on the way like Tefkat and IBM'S MTF.

Contents

Description

Model Transformation is the process of converting a model Ma conforming to metamodel MMa into a model Mb conforming to metamodel MMb. If MMa=MMb, then the transformation is endogeneous else it is an exogeneous transformation. Model transformation is a critical component of MDA. Recognizing this, an RFP (Request for proposal) has been issued by OMG on MOF Query/Views/Transformations to seek a standard compatible with the MDA recommendation suite (UML, MOF, OCL, etc.).

Several replies were given by a number of companies and research institutions that evolved during three years to produce a common proposal that was submitted and approved.

Presently there are several products (commercial or open source) that claim compliance to the QVT standard. QVT defines a standard way to transform source models into target models. There are several ideas in this proposal. One is that the source and target models may conform to arbitraries MOF metamodels. Another one is that the transformation program is considered itself as a model, and as a consequence also conforms to a MOF metamodel. This means more precisely that the abstract syntax of QVT should conform to a MOF 2.0 metamodel.

As a matter of fact, this is a bit more complex. First the QVT language integrates the OCL 2.0 standard and also extends it to imperative OCL. Second QVT defines not one but three Domain Specific Languages named Relations, Core and Operational Mappings and these languages are organized in a layered architecture. Relations and Core are declarative languages at two different levels of abstraction, with a normative mapping between them. The Relations language has a graphical concrete syntax. The QVT/OperationalMapping language is an imperative language that extends both QVT/Relations and QVT/Core. The syntax of the QVT/OperationalMappings language provides constructs commonly found in imperative languages (loops, conditions, etc.).

Finally a mechanism called QVT/BlackBox for invoking transformation facilities expressed in other languages (XSLT, XQuery) is also an important part of the specification. It is especially useful for integrating existing non-QVT libaries.

For the time being the QVT standard only addresses model to model transformations, model meaning some entity conforming to any MOF 2.0 metamodel. All transformations of type model to text or text to model, whatever the text is (XML, Code, SQL, etc.), are presently outside the scope of QVT and possibly subject to other standardization initiatives. They may be viewed as alternative transformation DSLs in the MDA technical space.

Criticisms

Although QVT is an OMG recommendation, it is criticized for the following deficiencies:

  1. No complete implementation yet. It seems quite difficult to achieve full implentation of QVT in a reasonable delay. It is very likely that QVT 1.0, like UML 1.x will never see any full compliant implementation because the specification is much too large.
  2. Fuzzy requirements. The requirements for QVT are mainly the translation of PIM to PSM. Since the specification of what is really a PIM and a PSM was never achieved in the OMG MDA guide, the QVT requirements remains quite fuzzy.
  3. Limited usability. QVT is for XMI to XMI transformations only. Since the quantity of real data natively encoded in XMI is very limited (and rarely found), the potential applicability of QVT in industry is also and consequently rather limited. In order to be practically used, QVT tools need to be complemented by Model to Text or Text to Model tools. This is a very severe limitation.
  4. Huge Specification (or the UML 2.x/SysML2.x Syndrom). It is very likely that no single tool will be ever able to implement all of this huge specification in a reasonable time.
  5. Premature standardization This is typically a domain with little to no previous large scale industrial experience. The decision to publish at this point in time such a huge (and somewhat arbitrary) recommendation with twelve precise compliant points may seem a bit arrogant or at the least pretentious to certain. The reason to rush was probably the competition from W3C XML-based standards like XSLT or XQuery.
  6. Committee-defined. QVT is composed of several parts contributed by different partners. This committee-defined syndrom is not specific to QVT, but it seems here to have achieved maximum effect.
Several of these criticisms may take less importance if we see broadly-used wide-scale full QVT implementations in the future. They will then compete with XSLT or XQuery proposals. The much awaited QVT-tools by Borland, IBM or Compuware may rapidly confirm this.

See also

References



External links













ATL: Atlas model transformation language






UMT: UML model transformation tool





Related model-driven development approaches























 




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