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ensure that the various packages adhere to a common external standard for sharing models, rather
than engaging in 'feature creep' to try to get an edge over rivals and tying users into their own
proprietary system.
One concept that will almost certainly emerge is that of workflow. Some tools produce a model
as output, others input a model, while others do both. We can therefore envisage a chain of
operations, constituting a higher-level task. For example, a user might wish to routinely compare
two models (perhaps the latest versions from two collaborating institutes), produce a new model
combining the recent changes, transform this into a simpler model, then display this as an HTML
page and run it under standard conditions. Workflow languages and design systems exist, and
they can be applied to a sequence of modelling tasks. For example, WSFL (the Web Services
Flow Language) [see URLs] has been designed for doing just this task for web services.
7.8 Publishing the models
I have suggested that XML be used as the metalanguage for representing models. Now, XML is
intended for distributing machine-readable content over the web, just as HTML is intended for
distributing human-readable documents. It therefore follows that we can 'publish' models
represented in XML, just as we can publish web pages.
We can therefore envisage that, in the future, you will be able to search for models just as you
now search for web pages, perhaps using a search engine like an XML-enabled Google. Clicking
on the model could automatically load it into your favourite modelling environment, or you could
load a model in your modelling package by entering its URL rather than a local file name.
Specialist web crawlers could build up model catalogues automatically by searching out models,
and some people would build up their own catalogues manually just as people currently construct
lists of web sites. Building a model could involve selecting submodels published in a variety of
countries with a few mouse clicks.
The published models can be viewed directly in a standard browser - both Netscape and Internet
Explorer are able to display XML using a Windows Explorer-type interface. This is unlikely to
be particularly digestible for the average user, since it will contain much technical information on
the model structure. More usefully, the time will come shortly when XML can be viewed
through the filter of an XSLT document (the eXtensible Stylesheet Language for Transformations
- see Appendix A2.2), enabling it to be simplified and displayed as HTML or diagrammatically in
your browser, or even converted into an audio presentation (Section 8.3).
7.9 Conclusions
The architecture for a declarative modelling approach is unusual, in that the emphasis is placed
firmly on the specification of a model-representation language, rather than on the design of a
software package. The design of the language is a challenging task, requiring a balance to be
struck between a number of competing objectives. Once it has been specified, tools for
supporting the modelling process can be developed in a piecemeal fashion, to satisfy a wide
variety of modelling requirements.
The next Section illustrates the general ideas presented here, using the Simile visual modelling
environment as a proof-of-concept demonstrator.