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- From: "Stephen D. Williams" <firstname.lastname@example.org>
- To: Oren Ben-Kiki <email@example.com>
- Date: Sun, 28 Mar 1999 11:31:09 -0500
Oren Ben-Kiki wrote:
> Stephen D. Williams <firstname.lastname@example.org> wrote:
> >One other subject that I haven't mentioned, but need for another
> architecture that I designed
> >a while ago is a mechanism for 'parallel inheritance' overlay tree
> processing. Has anyone
> >else worked on this? The idea is to have one or more base trees and work
> with a delta tree
> >which represents changes from the underlying trees. This last part is a
> basic data structure
> >for a rule engine and metadata application environment I designed last
> For general XML trees, I think you'll find that the only way to describe a
> 'delta' on a tree is using an XSL stylesheet, or something as complex, so
> you might as well stick with XSL. We use "delta trees" very heavily, but in
> a somewhat specialized form suitable for our application - the input trees
> have to be in a very strict format and the set of operations is much
> narrower then allowed in XSL.
I don't understand how to use XSL in a general way to acheive a 'delta tree' architecture. I
have a vague idea, but nothing that I could see being automated sufficiently. Can you
In my case I'm really talking about a specialization also. Certain processing or data
interpretation rules would have to be used, although these could be specified with attributes
to allow a full range of possibilities.
The situation that I am solving is where you have a base XML document and want to treat it as
a read-only base where changes are made to an overlayed read-write layer (or layers).
'Lookups' would traverse a series of trees to determine the current state.
The problems are related to ambiguous situations such as whether a read-write entity replaces
or adds to an underlying layer, how to handle deletes, etc. There are a number of possible
partial solutions, but it's difficult to find a completely general solution. For instance,
using unique ID's creates a problem of managing and assigning unique ID's.
This kind of thing really does have real-world application. A year ago I designed a rule
engine for business rule processing in a web application that used this kind of data
structure. The rulebase could have thousands of entries for structure and metadata where the
session state for each user would only consist of a few fields that were modified or had
values. Obviously a great optimization. Actually developing this is still on my short list.
I don't know whether this 'delta tree' aspect has solid prospects for becoming commonly used,
but I need it.
> Have fun,
> Oren Ben-Kiki
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