Concurrent Engineering http://cer.sagepub.com
Exchanging STEP Data through XML-Based Mediators Stephen C. F. Chan, Tharam Dillon and Vincent T. Y. Ng Concurrent Engineering 2003; 11; 55 DOI: 10.1177/1063293X03011001006 The online version of this article can be found at: http://cer.sagepub.com/cgi/content/abstract/11/1/55
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CONCURRENT ENGINEERING: Research and Applications Exchanging STEP Data Through XML-based Mediators Stephen C. F. Chan,1,* Tharam Dillon2 and Vincent T. Y. Ng1 1
Department of Computing, Hong Kong Polytechnic University, Hung Hom, Hong Kong
2
Department of Computer Science and Computer Engineering, La Trobe University, Melbourne, Australia
Abstract: Standard for the Exchange of Product Model Data (STEP) provides a systematic approach for user communities to model and exchange data on specific types of products and manufacturing technologies. With the increasing popularity of the Extensible Mark-up Language (XML) on the Internet, mapping STEP data into XML seems a logical way to make STEP data more accessible through the Internet. On the other hand, Internet-based information systems generally consist of data sources and clients that access the data sources; and mediators have been proposed as a middle layer that provides added value by converting the basic data into information required by the clients. To work effectively, mediators require machine-readable data models and standardized interfaces, which can be developed based on XML. In this paper we discuss the ways of automated conversion from STEP into XML, and the exchange of STEP data through the XML-based mediator architecture. Key Words: STEP, XML, mediators.
1. Introduction Standard for the Exchange of Product Model Data (STEP) is the dominant technology for product data distribution and sharing. It is a series of standards consisting of these major parts: 1. a formal data model language, EXPRESS [10], 2. conceptual data models for specific applications represented in EXPRESS called Application Protocols (APs) (e.g., AP for Configuration-Controlled Design [16] and many more similar ones), 3. a file format for exchanging instances of data entities specified in APs [11], 4. a programming interfaces for accessing shared data called Standard Data Access Interface, SDAI [12] and SDAI bindings to several popular languages. STEP is an International Standards Organization (ISO) standard, having a long history, a large community of researchers, developers and users, and a proven track record of widely-used standards. The first ISO standard in the family was published in 1994, and the continuing work is supported by a rich official web site [9]. It provides a systematic approach for well-established user communities to share data on specific types of products and manufacturing technologies.
*Author to whom correspondence should be addressed. E-mail:
[email protected]
When the first STEP standards were developed, the World-Wide Web was in its infancy. Since then, the development of Web technology and its popularity have opened up further opportunities in the application of STEP, linking STEP data to a much wider range of users and data sources that may have little previous exposure to STEP technology. Many researchers and developers have recognized that, as evidenced by the furry of activities in this direction in recent years [3,14,24]. Much effort have gone into making STEP technology compatible with the Internet. Initially, efforts to map STEP into the Hypertext Mark-up Language (HTML) did not quite produce satisfying results because HTML is too display-oriented, in-extensible, and lacking in semantic structures. For example, the tag set and tag semantics in HTML are fixed [28]. Standard Generalized Markup Language (SGML) does not have these short-comings of HTML but it is too complex and too difficult to implement for the purpose of viewing structured documents over the Web [28]. In the mean time, the Extensible Mark-up Language (XML) is rapidly becoming the next-generation replacement for HTML [21], and becoming an obvious choice. On the other hand, the popular two-tier client-server model for information systems has undergone major revisions. One of its problems is that it does not scale well. For example, the introduction of new clients or servers may trigger cascading changes to existing servers and other clients because of new or changed requirements. Among other technologies, mediators
Volume 11 Number 1 March 2003 1063-293X/03/01 0055–10 $10.00/0 DOI: 10.1177/106329303032817 Downloaded from http://cer.sagepub.com at PENNSYLVANIA STATE UNIV on April 17, 2008 ß reserved. 2003 Sage © 2003 SAGE Publications. All rights Not forPublications commercial use or unauthorized distribution.
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have been proposed as a middle layer that provides added value by converting the basic data into information required by the clients. At the same time, mediators localize the disturbances caused by changes in either the clients or data sources. In order for the mediators to work effectively, we need to develop machine-readable data models for the mediators, as well as interfaces between clients and mediators and between mediators and data sources. The data models and interfaces can be developed based on XML. In the short run, the popular choice for defining XML data models is to use Document Type Definitions (DTD). But XML Schema [27] may be a better choice in the longer term, since, among other advantages, it will permit the schema to be interchanged in XML form. Related issues, such as the automated conversion from data formats popularly used by data sources (such as STEP) into XML and efficient processing of XML-based information, also need to be resolved. Further, because of the evolution of data sources, and because of large differences between the technologies used in the data sources and those used in popular client programs such as World-Wide Web browsers, the functions performed by the mediators can only be partially automated. Mechanisms must be developed for human experts to perform maintenance on the mediators efficiently. An interface to the mediators is needed, so that human operators can perform necessary management and maintenance. The mediator-based architecture, together with XML, could provide such a link. Relevant product models and their schema developed for STEP can be converted into XML and schema, and appropriate mediators will be designed and implemented. Client applications that access the STEP data can be tested against the mediator interface. It is expected that the development of machine-readable schemas for the mediator tier will help to validate the mediator-based three-tier architecture for information systems based on the Internet, and improve efficiency in the development and maintenance of such systems. The implementation of such a system for the distribution of product data will make such data much more accessible through the Web, and facilitate the deployment of Internet-based manufacturing information systems and virtual enterprises.
2. STEP and XML Standard for the Exchange of Product Model Data provides a systematic approach for well-established user communities to share data on specific types of products and manufacturing technologies. STEP predates the rapid development of the World-Wide Web, which can link STEP data to a much wider range of users and data sources that may have little previous
exposure to STEP technology. Recently there is an increasing awareness of the potential of the Internet/ Web infrastructure in the manufacturing community and a concomitant demand for access to manufacturing product data through the Web. For example, Burkett [3] describes an initial effort by the US Department of Defense (DoD) to develop a Product Data Markup Language (PDML), essentially a mapping of selected data models (defined in the data modeling language EXPRESS) for STEP data to and from XML, to support the deployment of product data on the Internet. The mapping is a kind of early-binding approach (see below). Subsequently, PDES Inc., an international industry and government consortium, has started to develop STEPml, a library of XML specifications – DTDs and/or XML schemas – for product data based on STEP [23]. The effort seems to be concentrated on product data management; in particular, a specification (DTDs) for product data identification and classification has been produced [24]. The approach seems to be based on a mapping from EXPRESS to XML that is driven entirely by EXPRESS [25], kind of a late-binding mapping method (see below). Officially, the ISO is also working rigorously on developing standard methods to represent STEP data in XML. STEP Part 28 [14] defines a mapping of EXPRESS schema and data into XML, currently using XML DTDs, but will probably be moving towards XML schemas. It is developing an EXPRESS-driven late binding DTD as the architecture that can be used to develop multiple early-binding DTDs. On the other hand, STEP Part 25 [13] specifies a mapping of EXPRESS constructs to the UML Interchange Metamodel. The structural parts of EXPRESS schemas can be mapped into static structural diagrams of UML, and then exchanged in XML format. The mapping is a one-way mapping and only a subset of EXPRESS concepts are mapped, as UML does not support everything that EXPRESS supports [13]. These limitations make this mapping unsuitable for the general interchange of EXPRESS schemas and UML models for information modeling purposes. Extensible Mark-up Language and mediators can be used as the key components in an architecture for the distribution and sharing of product data on the Internet. Relevant data models developed for the international standard for product model data, STEP, can be converted into XML, and appropriate mediators will be designed and implemented. Client applications that access the STEP data can access STEP data through the mediator interface. Here we expect to be able to make use of standards such as STEP Parts 28 and 25, as well as related projects such as the PDML [3], and STEPml [23]. PDML and STEPml provide direct mappings between STEP and XML. In contrast, XMI is a multilayer system for defining data models, meta-
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Exchanging STEP Data Through XML-based Mediators
models and instances of these models. It was designed mainly for exchanging UML models [19]. Two approaches can be taken in mapping STEP data into XML: early binding and late binding. In the early binding approach, such as used in PDML, the entity types and attribute names in the STEP data models as defined in the EXPRESS language are mapped into XML element types and attribute names. Each EXPRESS schema will be mapped into a unique DTD, and the actual data describing the product will then be represented as a XML file conforming to the DTD. For example, consider the following EXPRESS schema and corresponding product information represented in the STEP file exchange format [11] in Figure 1. In an early binding mapping of the example EXPRESS schema in Figure 1, the corresponding XML DTD may look like the following in Figure 2. Note that references from one XML element (EXPRESS entity) to another XML element are made with XML ID references, one of XML’s built-in referential
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(a) ENTITY product; prod_name: STRING; prod_owner : designer; END_ENTITY; ENTITY designer; designer_name : STRING; … END_ENTITY;
…
(b) #1 = designer ("Paul Johnson", …); #2 = product ("ModelX", #1); #3 = product ("ModelT", #4);
…
Figure 1. (a) An example EXPRESS schema; (b) A sample exchange file conforming to the EXPRESS schema.
(a) … …
(b) ModelX "d1" ModelT … Paul Johnson …
Figure 2. (a) DTD of an early binding mapping into XML; (b) A sample XML file conforming to the DTD. Downloaded from http://cer.sagepub.com at PENNSYLVANIA STATE UNIV on April 17, 2008 © 2003 SAGE Publications. All rights reserved. Not for commercial use or unauthorized distribution.
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mechanisms. For example, see the definition of the ELEMENT designer_ref and its use in the definition of the ELEMENT product.prod_owner. On the other hand, in a late binding mapping approach, the XML elements correspond to generic EXPRESS constructs. Information specific to each EXPRESS schema is represented as attributes or subelements. A single DTD can be used for all possible schemas. For example, the corresponding DTD may look like the following in Figure 3. Noting the advantages of XML Schema over DTDs and the apparent momentum building towards wider deployment of XML Schemas, we propose to develop the mediator-based architecture for manufacturing data distribution and sharing based on XML Schemas. The mediator-based architecture is flexible enough to support both early- and late-bindings between STEP and XML. It is mainly a matter of choosing the appropriate mediators. A sample early binding mapping into XML Schema may look like the following in Figure 4. Both the early- and late-bindings have their advantages and disadvantages. The mapping from earlybinding XML data to the STEP domain is more straightforward. On the other hand, the mediator that processes the XML data must possess knowledge of the mapping between the specific EXPRESS schema and
the XML, e.g., whether attributes in an EXPRESS entity maps into an attribute of a XML element, or a sub-element, or an element elsewhere. In the case of late-binding, the names of the XML elements correspond not to names of the EXPRESS entities (such as ‘‘product’’ or ‘‘designer’’). Rather, they correspond to components of the meta-model of EXPRESS, i.e., ‘‘entity’’, ‘‘attribute’’, etc. A latebinding generally involves fewer things to define and the mediators for processing may be easier to implement. The performance of the processing mediators may suffer, however, as the translation between the XML domain and the STEP domain is indirect. Based on the discussions in the STEP community, it appears that demand for both mappings exist. Fortunately, the mediator-architecture is flexible enough to support both kinds of mappings. A related issue of interest is the interoperability between related or overlapping data schemas. The STEP project has developed data models (called application
a)
a)
type_name CDATA #REQUIRED>
…
…
b) ModelX
…
