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From Craig McClanahan <>
Subject Re: [Shale] SCXML as an alternative for describing dialogs
Date Tue, 11 Oct 2005 04:33:45 GMT

On 10/10/05, Rahul Akolkar <> wrote:
> Having looked at Shale dialogs, they stand out as a very good
> candidate for use of State Chart XML (SCXML), IMO. A proof of concept,
> including a usecases war wherein the same dialogs are described using
> SCXML, is available at the URL below (since the email was getting
> lengthy, I was tempted to wrap some HTML tags around the content):
> *time-sensitive URL*
> Any objections to moving this content to the Shale section of the
> wiki? As one of a growing number of people who otherwise use SCXML for
> describing dialogs, this will add significant value to the Shale
> adoption story. I'm extremely interested in feedback from the Struts
> Shale team, and as a Commons SCXML developer, I'd be more than happy
> in taking this work to completion, if there is interest.

You definitely piqued my interest when I saw a couple of "Shale related" SVN
commits on the Commons developer list :-). It's pretty interesting stuff --
and the set of state diagrams that can be described is quite a lot larger
than the scope I originally had in mind for Shale's dialogs (which were
intended to be as simple as could possible to support the basic dialog
facilities, integrated with JSF's expression language semantics). It would
be interesting to compare the expressive power of SCXML to the configuration
file format for Spring Web Flow as well.

I wouldn't object to posting this stuff on the Shale Wiki ... although it
might be better as a link to the Commons SCXML pages (and I can add a link
to the Dialog part of the Shale website as well).

A couple of quick questions, as this was my first exposure to SCXML.

* A quick skim through the SCXML working draft didn't point me at any
definition of the syntax and semantics of the expressions being used.
Is that totally up to the implementation?

* It looks like you are using a mixture of JSP 2.0 EL (${...}) and JSF 1.1EL
(#{...}) expressions. If so, it might make sense to standardize on only
one of them throughout -- probably the JSF one because it knows what
a method binding expression is, to reduce confusion.

I need to do more research before I have any real opinion on the potential
applicability of SCXML inside Shale -- in particular, one concern is that
for the simple cases the XML is somewhat more verbose. This could be
mitigated by tools support that generated the things from a real state
diagram, and/or some sort of XSLT transformation that converts an existing
Shale Dialog configuration file into the SCXML format automatically. The
latter sort of tool would be very useful for someone who started with the
simpler Shale version, then needed extra power.

What do you think?



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