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RE: XML aggregation question?

• From: "Michael Kay" <mike@xxxxxxxxxxxx>
• To: "'Andrew S. Townley'" <ast@xxxxxxxxxxxx>,"'XML Developers List'" <xml-dev@xxxxxxxxxxxxx>
• Date: Sat, 26 Aug 2006 15:01:14 +0100

I think if you don't start of by using a database, there's a good chance
that you will end up in effect writing your own. I'd go for an XML DB.

Michael Kay
http://www.saxonica.com/ 

> -----Original Message-----
> From: Andrew S. Townley [mailto:ast@a...] 
> Sent: 26 August 2006 10:36
> To: XML Developers List
> Subject:  XML aggregation question?
> 
> Hi Folks,
> 
> I'm looking for some collective wisdom from the list on how 
> what I want can be done using only XML technologies.  I know 
> at least 3 different ways you could do it using databases of 
> various sorts, but I'm trying to see if there's a better way, 
> or if the RDBMS is the way to go.
> 
> What I'm trying to do is dynamically aggregate information 
> from XML instance documents without having to process all of 
> the instances every time I want the aggregate.  Maybe this is 
> a job for an XMLDB, but I'm not terribly familiar with them.  
> I'd also like to be able to keep my XML instance documents 
> stored on the filesystem rather than having them in a 
> database for easy access from a variety of tools, from text 
> editors to Web servers to other utilities written in various 
> languages.
> 
> Given something like a widget in an inventory or workflow 
> system where each instance represents a given widget, e.g.
> 
> <widget>
>   <status>XXX</status>
>   ...
> </widget>
> 
> What I would like to be able to do is get a view of the 
> collective status of my group of widgets in an on-demand 
> manner.  Other processes may be changing the status, so I 
> don't want to introduce a dependency on an 
> application-maintained static index updated when the status changes.
> 
> As I said, some of the ways I know are possible are:
> 
> 1 - Move the data from the XML instances into a database and 
> run queries.  When I need the data, either re-generate the 
> XML or store the XML as a blob.  Obviously, need to do 
> everything in the database or use ETL operations to do updates.
> 
> 2 - Keep the XML on the filesystem and periodically (via cron or
> similar) generate a static index based on the as-is state of 
> the information.  Aggregate info is only guaranteed to be as 
> fresh as the last batch job.  This also has the problem of 
> not scaling well as the number of instances increases.
> 
> 3 - Provide a centralized persistence layer to essentially do 
> what #1 is doing, but as the XML is modified, update the 
> static index.  This seems really cumbersome and error prone, 
> plus it means you can't have the flexibility of accessing the 
> "raw" instance documents with shell scripts, for example.
> 
> I'm sure I'm missing something obvious here, so any 
> pointers/suggestions would be appreciated.  This has to be a 
> common pattern, so I'm sure there are other solutions people 
> have come up with.  My goal is to keep whatever solution as 
> light as possible, but if I have to build or use 
> infrastructure, then that's what I'll have to do.
> 
> Thanks in advance,
> 
> ast
> --
> Andrew S. Townley <ast@a...>
> http://atownley.org
> 

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