Ph.D. Thesis - Business Informatics Group

2.2 DTDs and Ecore at a Glance
Amongst the approaches not yet in line with MDE, WebML [CFB + 03] is one of the most
elaborated web modeling languages stemming from academia and is supported already
over several years by the commercial tool WebRatio as already explained in Chapter 1.
WebML’s language concepts are partly defined in terms of XML document type definitions
(DTDs) [W3C06], i.e., a grammar-like textual definition, specifying an XML document’s
structure, and partly hard-coded within the corresponding modeling tool. In contrast to
MOF, DTDs represent a rather restricted mechanism for describing modeling languages,
e.g., with respect to expressiveness, extensibility as well as readability and understandability
for humans. Furthermore, since WebRatio internally represents models in XML
[W3C06], it uses XSLT for generating code directly from WebML models. In contrast to
dedicated MDE code generation technologies, e.g., MOFScript 1 , writing XSLT programs
for code generation, however, is difficult and error-prone. Concerning these problems, a
metamodel-based approach allows expressing transformation rules in a more compact and
readable way by using existing MDE-conform code generation techniques or model transformation
languages such as QVT [OMG05b] and ATL [JK06] in order to produce platformspecific
models in an additional step before generating code.
In order to define WebML’s language concepts in an MDE-suitable way and thus to bridge
WebML to MDE, a MOF-based metamodel for WebML is a fundamental prerequisite. Considering
the language’s size, however, we refrain from re-modeling WebML from scratch,
since this would be a cumbersome and error-prone process. Instead, the existing DTD-based
language specification as well as constraints hard-coded within WebRatio shall be reused in
a semi-automatic process for MOF-based metamodel generation from DTDs. This process
is illustrated in Figure 2.2 and encompasses three phases, whereby the first two phases concern
the semi-automatic generation of the basic WebML language concepts defined within
the WebML DTD. During the first phase a preliminary version of the metamodel is automatically
generated from the available DTD, while in the second phase this preliminary
version is manually validated and refactored according to constraints captured within the
WebRatio tool support.
The following section explains the concepts of DTDs and MOF and points out the aforementioned
DTD deficiencies.
2.2 DTDs and Ecore at a Glance
As a first step towards bridging WebML to MDE, this section elaborates on the expressiveness
of DTDs, i.e., the concepts used to describe the WebML language, with respect to MOF.
In the context of OMG’s meta-level architecture [OMG05e], this means that a WebML model,
which is represented by an XML document, relates to the model level (M1) (cf. Figure 2.3).
1 www.eclipse.org/gmt/mofscript
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