Understanding STEP, step by step

With our information technology background we are trained to mistrust everything on paper - because the information is incorrect, obsolete and definitely computer-insensible. Product databases to support repeated reporting in an application oriented form, is the preferred solution. STEP provides a methodology and a data architecture including an industry foundation in the form of application protocols or data models. As debated in many media, the data specification language Express is dissimilar from other methodologies and data definition languages. However, the strength of STEP is that all parts with an implementation focus, provides an unambiguous and computersensible specification that enables the implementation of a dictionary driven tool arch-itecture.

Unfortunately, STEP and Express have been poorly supported by tools. Further-more, the tools available seem to only have implemented those parts of Express where the language is in overlap with other, more commercial and possibly light-weight solutions. The real power of Express becomes evident when the language is implemented in full and supporting the execution engine for derived attributes, local and global rules, and other constructs making use of executable statements and the strong type definitions. The fact that tools do not exploit all the features of Express, make both STEP and Express less commercial without any foundation in the technology - and with no good alternatives.

Instead of discussing the eligibility of a language that a unison industry has defined as supporting their requirements, the EXPRESS Data Manager has been under development since 1989. The objective is to provide a platform for the implementation of all inform-ation requirements defined in Express supported by high level functionality to reduce implementation and deployment cost.

The Express virtual machine

Similar to Sun and Java, EPM has implemented EXPRESS Data Manager as an Express virtual machine environment. It is not a new concept, but was introduced in version EDM V3.6. Since one important aspect of STEP and its implementation is mapping, the virtual machine is also designed to support Express-X which from the Express code aspect is the same.

The most significant change in EXPRESS Data Manager version 3.6 is the introduction of the EDMdebugger. It has been a necessary development for EPM's continuous effort to check the quality of ISO 10303/SC4 standards. Data structure, data types and inverse constraints are one aspect to be checked. Another is the more explicit constraint mechanism provided by local and global rules, which validates to either true or false. It is of a certain value to get information if an instance is correct or not, but in most cases the user wants to find out what is wrong, and possibly to correct the value at fault. The EDMdebugger enables stepping into all expressions and capturing the status and values including the built-in functions and procedures.

A print mechanism is provided to enable the configuration of a more detailed report of the Express and Express-X execution.

Derived attributes are also fully supported, and needless to say this will reduce redundancy in the database and repeated implementation by the applications.

Potential use of an Express platform

A dictionary driven approach as provided by the Express virtual machine is a cost efficient way of both developing specifications such as the SC4 standards, as well as implementing them. The P21 format to populate any database accord-ing to an Express schema is only one example.

In EXPRESS Data Manager functionality is schema-independent. The EDMmodelChecker that checks instances against the constraints given by a data storage schema and optionally one or more rule and value schemas to verify the data quality. (See EXPRESSway No. 1/98 for more information). Using the EDMinstanceBrowser, the instance in question can be queried and updated. Such a module is mandatory in the exchange process in and between enterprises.

Both creating and verifying test suites for an Express schema can be done very well in EXPRESS Data Manager. Schema driven functionality will ease the work of populating the schema and the EDMmodelChecker will check the dataset for all constraints for all occurrences, all instances of a certain type, or one specified instance for one specified constraint. The schema driven approach works well for small datasets or when the dataset exists in a supported format such as e.g. P21. For efficient production of a large dataset, an application may be needed and using either a mapping or the STEP compliant access interface.

The support for a formal mapping specification is a solution that today is used in production systems for the aerospace, shipbuilding and process industry. For implem-entation, mapping between various levels of models is mandatory and the availability of predefined mappings is a solution to interoperability between standards and between versions of the standard. In STEP mapping tables could be replaced by an Express-X specification to use the application objects as the interface to the Integrated Resources.

EDMvisualExpress - the graphical modelling application in EXPRESS Data Manager, is capable of defining and storing all Express constructs according to an Express schema, and to present data automatically onto paper or the web. If the supplementary directives in STEP were changed to include the information requirements as Express specifications, applications can better support the correct population of that part as well as exchange within the development team and publishing of the results. Quality control of the part can also be highly automated using Express constraints instead of manual inspection.