request for comment: adding xslt processing to mapserver

[Daniel Morissette]

Sorry for the late reply, I know you already added notes about this in 
your document but I just wanted to add that I think Steve has some very 
good points here and I am also worried about the performance impact of a 
complete switch to libxml2, especially on large XML responses (e.g. 
large GML responses, etc.).

Another point that has not been mentioned yet is that converting all 
OGC-related code from msIO_printf to libxml2 comes with a high risk of 
introducing new bugs in something that's currently very stable (I'm 
mostly thinking about the WMS support). Yes I agree that using printf to 
generate XML is clunky, but this proposal will take thousands of lines 
of stable and tested code and rewrite them to use libxml2. This is 
likely to introduce new bugs in the OGC specs support and this should be 
taken into account as you plan this proposal.

Daniel


Steve Lime wrote:
> Paul: I did look at libxml2/libxslt and it's XSLT support before doing any of the 
> work on GML transformation last summer. At that time I decided against it 
> because:
> 
>  - performance, libxslt required an entire document be memory resident
> before transforming. You had to write vanilla GML to memory first (as DOM
> object) and for large datasets that wouldn't be straightforward). Streaming 
> transformations were  needed and, at the time, only available via java (and 
> experimental at that), I don't know if that has changed or not.
> 
>  - with WFS, what you produce you need to document. So for the transformed
> output you'd need to produce a corresponding schema, yet another XSLT file.
> Referencing an external schema (e.g. some standard application schema) is
> one workaround but I don't know that it is a perfect solution.
> 
>  - with WFS, what you produce you also need to support queries against, so
> now you need to transform an incoming filter into something that that can be
> applied against the native data store, basically inverting the transformation 
> applied in the first place. Then you also have to deal with non-xml request 
> encoding.
> 
> In the end it was all too much to swallow, not to mention that XSLT isn't exactly
> trivial for users. Not that configuring GML transformations through metadata is
> easy- it ain't, it sucks and overly complicates a mapfile. 
> 
> Still there's an elegance to only having to write construct (and support!) vanilla 
> GML and then using external transformations. If the issues above could be addressed
> I would support the idea and might go one step further and suggest yanking out
> the metadata configured tranformation.
> 
> Steve
> 
> 
>>>> Paul Spencer <pspencer at DMSOLUTIONS.CA> 03/22/07 2:50 PM >>>
> MapServer devs ...
> 
> DMSG has a contract with Environment Canada/Tom Kralidis to implement  
> some changes to the Sensor Observation Service.
> 
> Part of this contract is to write a technical report on the  
> feasibility of introducing the ability to transform XML coming out of  
> MapServer's various OGC interfaces (specifically WMS GetFeatureInfo,  
> WFS GetFeature and SOS GetObservation) using an XSLT directly in  
> MapServer.  I (with help from Assefa) have prepared a draft of this  
> report structured more-or-less like an RFC.
> 
> One of the requirements is to 'gauge the reaction of the TSC to this  
> proposal', which is the purpose of this email. If the general  
> feedback from you is positive, I believe Tom will then submit this as  
> an RFC for approval and, if approved, will seek funding to implement  
> the changes.
> 
> Please give the attached proposal a read over and let me know:
> 
> * would you be for or against this capability being introduced into  
> mapserver?
> 
> * if against, would any rewording or additional detail change your mind?
> 
> * is there enough technical detail in the proposal?
> 
> * is there anything obvious missing (I know I may have missed some  
> files that would need to be modified)?
> 
> * any other comments?
> 
> I know this is last minute, but the report is due for mid next week  
> so getting your feedback ASAP is highly desirable.
> 
> Cheers
> 
> Paul
> 
> +-----------------------------------------------------------------+
> |Paul Spencer                          pspencer at dmsolutions.ca    |
> +-----------------------------------------------------------------+
> |Chief Technology Officer                                         |
> |DM Solutions Group Inc                http://www.dmsolutions.ca/ |
> +-----------------------------------------------------------------+
> 

-- 
Daniel Morissette
http://www.mapgears.com/