Transformation Techniques and Model Driven Process

An overview of all model driven activities currently investigated by the UWE research team is presented in [Koch06b]. The proposed global UWE process is depicted in Figure 17 as a stereotyped UML activity diagram. This work realizes by transformations a subset of the proposed global UWE process which is essentially the global UWE process minus adaptation modeling, architecture modeling and big picture modeling. The realization of the missing points of the global process is part of the future work of the UWE research group, see also the conclusions chapter.

Models are represented with object nodes and transformations as stereotyped activities (special circular icon). A chain of transformations then defines the control flow. The proc- ess starts by defining a requirements model or business model, called computation inde- pendent model (CIM) in the terms of MDA. Platform independent design models (PIM) are derived from the requirements model, see [Koch06a]. The set of design models repre- sents the different concerns of Web applications. It comprises the content, the navigation, the process, the presentation and the adaptation concern of Web applications. The next step in the global approach is to integrate the design models mainly for the purpose of verifica- tion into a so-called big picture model by graph transformations using the AGG tool (cf. 2.3.1.5). The big picture model is based on UML state machines, which can be checked by the tool Hugo/RT, a UML model translator for model checking and theorem proving [Knapp06]. In a joined work with the author of the WebSA (Web software architecture) approach (cf. 3.4.1) the inclusion of a separate architecture model for capturing the archi- tectural features of Web applications was investigated [Meliá05a]. In the global approach it is proposed to integrate the architecture model with the big picture model to an integrated platform independent model covering functional and architectural aspects. Architecture modeling is further future work of the UWE research group. It is not considered in this work because the author claims that the architecture of a Web application is tightly coupled to the target platform and implicitly encoded in the transformations to the platform specific models, and therefore it is more important to provide a way for transformation and plat- form modularization to support different architectures (or platforms). The last proposed step in the global process is the transformation of the integration model to platform specific models, just like it is realized in this work.

Model Driven Software Engineering for Web Applications Business Models (CIM) Platform Independent Models (PIM) Platform Specific Models (PIM) Business Models (CIM) Platform Independent Models (PIM) Platform Specific Models (PIM)

Figure 17. Global UWE process overview (from [Koch06b])

3.4.2 WebSA

The Web Software Architecture (WebSA) approach [Meliá06a] is not a stand-alone ap- proach for model driven Web engineering. It complements other Web design approaches by providing an additional viewpoint for the architecture of a Web application and a model driven development process based on the Unified Process [Jacobson99]. The author claims that the approach can be used in combination with the functional models of every other approach for Web design that is based on a MOF metamodel such as for example UWE, see also 3.4.1.3.

The architecture of a Web application is defined by the means of two architecture models, the subsystem model and the configuration model. The former defines the architectural layers of a Web application and the latter an architecture of implementation components.

The architectural models are integrated with the functional models to a so called integra- tion model by means of a transformation at the platform independent level. The integration model is then transformed to a platform specific model by another transformation.

The metamodels of WebSA are based on MOF and additionally a corresponding UML 2 profile is defined. For the transformation to the integration model a proprietary transforma- tion language called UPT (UML Profile for Transformations) [Meliá06b] is defined which allows the specification of transformations by using a UML profile. For the transformation to the platform specific implementation the transformation language MOFScript is used.

As already stated in 3.4.1.3, the proposal of the integral UWE approach is to integrate the functional models of UWE with the architecture models provided by WebSA. This stands in contrast to this work because the author claims that the architecture of a Web application is tightly coupled to the target platform and implicitly encoded in the transformations to the platform specific models, and therefore it is more important to provide a way for transfor- mation and platform modularization to support different architectures (or platforms). Most important, the resulting integration model in the WebSA approach is enriched with low- level artifacts for the architecture, which still have to be refined by the developer. This hinders a translationist approach by having to complement one more model for the trans- formation to code.

3.4.3 MIDAS

MIDAS [Cáceres04] is another model driven approach for Web application development based on the MDA approach. For analysis and design it is based on the content, navigation and presentation models provided by UWE and it uses therefore UML with UML profiles as notation. For the platform specific implementation it relies on object-relational tech- niques for the implementation of the content aspect and on XML techniques for the im- plementation of the navigation and presentation aspects. A process aspect is not supported. The transformations for mapping the design models to the specific target platform are not defined formally.

3.4.4 WebML

WebML [Ceri02] is a data-intensive approach based on entity relationship modeling. Until now WebML does not use an explicit metamodel. The corresponding tool WebRatio inter- nally uses a Document Type Definition (DTD) for storing content and navigation models, i.e. a grammar-like definition for the structure of XML documents. DTDs do not have the same expressiveness as MOF and lack an easily understandable notation. The XML trans- formation language XSLT (cf. 2.3.1.8) is used for model-to-code transformations support-

Model Driven Software Engineering for Web Applications

ing presently transformations to Java and JSPs. XSLT is not suitable for more complex transformations and the development of XSLT programs is difficult and error prone.

Currently, efforts are made for enhancing the interoperability of WebML with other model driven Web engineering approaches. In [Schauerhuber06b] an interesting approach is pre- sented to semi-automatically transform a DTD to a MOF compatible metamodel. The transformation uses a set of transformation rules and heuristics, but still requires some user interaction for improving the semantics of the generated metamodel. This approach is ap- plied to WebML, thus enabling the use of standardized MDE technologies for WebML. Additionally, in [Moreno06] a first step towards a UML 2.0 profile for WebML is pre- sented that would also enable standardized MDE technologies.

3.4.5 OOWS

OOWS [Fons03] is an extension of the object-oriented software development method OO- Method [Pastor01] for Web application development. Similar to this work a navigation model represents the navigational aspects of a Web application as views of classes from a class diagram which is similar to the content model. The presentation aspect is integrated with the navigation aspect, a dedicated presentation model for further abstraction of the user interface is not available.

Recently, a model driven extension of OOWS to support business processes has been pro- posed with emphasis on the integration of external applications, the development of special dedicated user interfaces that guide through processes and the consideration of automatic as well as manual tasks [Torres06]. Therefore, the navigation model of OOWS has been extended by the inclusion of graphical user interface elements to allow for the interaction between users and business processes using a UML-like notation.

Processes are captured in the business process model using an extended version of the Business Process Modeling Notation (BPMN) [OMG06c] and a corresponding extended metamodel for business modeling. BPMN stems from the B2B (business-to-business) field and is similar (but not identical) to UML activities, which stem from the software engi- neering field. It provides concepts such as flow objects (corresponding to activity nodes or events), connecting objects (corresponding to activity edges), swimlanes and artifacts (cor- responding to object nodes). In contrast to this work also manual tasks are considered in the process model, i.e. tasks that are manually carried out by humans and not automatically by the system by invoking operations or Web services.

The business process model is the starting point for model transformations. Operational Mappings, the imperative part of QVT (see 2.3.2.2), is used as transformation language.

The transformations are implemented with Borland Together Architect 2006 for Eclipse (see 2.3.2.3). The process model (in BPMN) is transformed to a (platform independent) default navigation model for the user interaction with the process, which then has to be re- fined by the designer. The navigation model is then transformed to a concrete Web tech- nology, i.e. a platform specific model. The proposal will be integrated in the ONME tool9 for automatic code generation.

In contrast to the approach of this work processes are not represented by a dedicated model, but distributed over the BPMN process model and the navigation model. The latter contains a lower level view of the process model, where the constructs of the process model are resolved into navigation constructs. The approach of this work defines a clear separation of the process and the navigation aspect and specifies how processes are inte- grated in the navigation model, see also 4.5. Further, the strength of the approach of this work is also the use of standards for all aspects of Web application development. Finally, although the use of the imperative part of QVT is comparable to the use of ATL, it is un- clear how much of the model driven approach is already realized.

3.4.6 HyperDE

In contrast to the heavyweight approach presented in this work there is an increasing inter- est in the use of small domain specific languages that can be used for agile development [Cockburn01] and for quick prototyping.

The open source Web framework Ruby on Rails (or short Rails) [Thomas06] is especially suited for the agile development of Web applications. It is based on the reflective and ob- ject-oriented programming language Ruby, which provides extensive metaprogramming possibilities and facilitates the use of internal domain specific languages. The Rails frame- work allows the development of Web applications following the Model/View/Controller (MVC) pattern. Following its two guiding principles called “don’t repeat yourself” and “convention over configuration” much less code and configuration data than with other Web frameworks is necessary. For example the mapping between classes and database ta- bles is derived automatically from class and field names. A technique called scaffolding allows rapid prototyping by quickly providing most of the logics and views for common operations, such as CRUD (create, read, update and delete database operations).

Model Driven Software Engineering for Web Applications

A modification of the Ruby on Rails framework called HyperDE is presented in [Nunes06]. HyperDE is based on SHDM, a method for the design and implementation of Web applications for the semantic Web [Lima03]. The MVC implementation of Ruby on Rails is extended by navigation primitives of SHDM and the persistence layer is modified to operate on a RDF database [Lassila99] where the user defined navigation model and ap- plication instance data is stored. Additionally, HyperDE provides a domain specific lan- guage by which model instances can directly be manipulated. HyperDE itself provides a Web interface, so that Web applications can directly be created or modified.