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   RE: XML and Objects

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  • From: "Jonathan A. Borden" <jborden@mediaone.net>
  • To: <xml-dev@ic.ac.uk>, <xml-dev@ic.ac.uk>
  • Date: Mon, 28 Sep 1998 12:16:13 -0400

First, as Tim Bray suggests, the DOM goes a long way to represent objects in

We can add to this by

1) a DTD for a Simple Object Definition Language (SODL) this DTD is included
in a post to follow.
2) Use of both Base64 and multipart/related techniques.

	The advantage of SODL is that the object's data is represented in XML
itself. This works for most objects except those whose state is comprised of
large amounts of binary data. For moderate amounts of binary data base64
encoding is fine and certainly works. For large amounts e.g. video clips and
pictures, multipart/related MIME messages with Content-ID tagged binary
parts is more efficient. When representing an object by a multipart/related
compound document, the first part is an XML metadata header which contains
internal "cid:xxx" links.

	We are using these techniques in production for our XML/internet based
telemedicine system. I have developed a DOM for MIME which we use for this.
I would be happy to share more of these details if there is interest.

	Now to your specific question, if objects are represented in this fashion,
you can access members through interfaces (i.e. Java/C++) through get/set
pairs. A language XML interface layer is needed. This layer is identical to
COM's dispatch layer which allows COM objects to be used from within
Javascript and VBScript. COM uses a binary typelibrary as input. Our
technique takes the SODL document and

a) generates a typelibrary from it
b) employs a custom interface which is driven by the SODL document

	The advantage of (a) is that it is compatible with existing software
however the software is limited to Windows.
	XML-DEV would be an excellent place to develop an independent (b) layer
specification. This spec would certainly need to interface with DOM.

Jonathan Borden
JABR Technology
> I have 2 questions/problems:
> 1) A DTD describes a document which contains content specified by a URL, a
> local file name, or inline. Documents created using this DTD are assembled
> and transported across a network. How do you include the content? The 2
> ways we have discussed are:
> * Inline using Base64 encoding in a CDATA section
> * Wrap the document in a multipart/related MIME message
> and include the content as attachments
> I am leaning towards multipart/related, but would like to know of others
> experience in this area.
> 2) We desire to provide an API on the client side which exposes a simple
> mechanism for creating and modifying objects. These objects are serialized
> using XML and then transported to a server for further processing. The
> server then responds with another XML document that we then de-serialize
> into an object and present it to the API user. Here are some basic
> requirements:
> * Support for both Java and C++
> * API must be similar for both Java and C++
> * Object members are accessed via get/set methods
> * Adhere to JavaBean method naming patterns
> We are thinking of developing an application which takes a DTD and then
> generates Java and/or C++ code for each object. We would use a XML helper
> file to give more control over the generation process. Are we out in left
> field here? What are some of the other ways to do this? What are you
> experiences doing something like this?
> Gregory M. Messner
> gmessner@vsi.com

Jonathan Borden
JABR Technology Corporation
(fax) 617-557-5160

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