XML-schema 'best practice' question

Frank Millman frank at chagford.com
Sat Sep 20 07:59:55 CEST 2008

On Sep 18, 8:28 am, Frank Millman <fr... at chagford.com> wrote:
> Hi all
> This is not strictly a Python question, but as I am writing in Python,
> and as I know there are some XML gurus on this list, I hope it is
> appropriate here.
> XML-schemas are used to define the structure of an xml document, and
> to validate that a particular document conforms to the schema. They
> can also be used to transform the document, by filling in missing
> attributes with default values.
> Or maybe the best practice is to *always* validate a document before
> processing it.

I have realised that my question was irrelevant.

xml's raison d'etre is to facilitate the exchange of information
between separate entities. If I want to use xml as a method of
serialisation within my own system, I can do what I like, but there
can be no question of 'best practice' in this situation.

When xml is used as intended, and you want to process a document
received from a third party, there is no doubt that you should always
validate it first before processing it. Thank you, Lorenzo, for
pointing out the obvious. It may take me a while to catch up, but at
least I can see things a little more clearly now.

As to why I am using xml at all, I know that there is a serious side
to Skip's light-hearted comment, so I will try to explain.

I want to introduce an element of workflow management (aka Business
Process Management) into the business/accounting system I am
developing. I used google to try to find out what the current state of
the art is. After several months of very confusing research, this is
the present situation, as best as I can figure it out.

There is an OMG spec called BPMN, for Business Process Modeling
Notation. It provides a graphical notation, intended to be readily
understandable by all business users, from business analysts, to
technical developers, to those responsible for actually managing and
monitoring the processes. Powerful though it is, it does not provide a
standard method of serialsing the diagram, so there is no standard way
of exchanging a diagram between different vendors, or of using it as
input to a workflow engine.

There is an OASIS spec called WS-BPEL, for Web Services Business
Process Execution Language. It defines a language for specifying
business process behavior based on Web Services. This does have a
formal xml-based specification. However, it only covers processes
invoked via web services - it does not cover workflow-type processes
within an organisation. To try to fill this gap, a few vendors got
together and submitted a draft specification called BPEL4People. This
proposes a series of extensions to the WS-BPEL spec. It is still at
the evaluation stage.

The BPMN spec includes a section which attempts to provide a mapping
between BPMN and BPEL, but the authors state that there are areas of
incompatibility, so it is not a perfect mapping.

Eventually I would like to make sense of all this, but for now I want
to focus on BPMN, and ignore BPEL. I can use wxPython to design a BPMN
diagram, but I have to invent my own method of serialising it so that
I can use it to drive the business process. For good or ill, I decided
to use xml, as it seems to offer the best chance of keeping up with
the various specifications as they evolve.

I don't know if this is of any interest to anyone, but it was
therapeutic for me to try to organise my thoughts and get them down on
paper. I am not expecting any comments, but if anyone has any thoughts
to toss in, I will read them with interest.



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