abstraction of mapogcsos.c common ogc bits

[Kralidis,Tom [Burlington]]

> -----Original Message-----
> From: UMN MapServer Developers List 
> [mailto:MAPSERVER-DEV at LISTS.UMN.EDU] On Behalf Of Daniel Morissette
> Sent: 18 October, 2006 1:52 PM
> To: MAPSERVER-DEV at LISTS.UMN.EDU
> Subject: Re: [UMN_MAPSERVER-DEV] abstraction of mapogcsos.c 
> common ogc bits
> 
> Kralidis,Tom [Burlington] wrote:
> > 
> > What do people think about a generic mapowscommon.c / mapowscommon.h
> > 
> 
> I think that grouping all OWS Common stuff in a 
> mapowscommon.c file would be a good idea.
> 
> > 
> > Maybe Assefa/others can provide comments on using libxml2 
> compared to 
> > msIO_printf for XML.  If it were up to me, I'd go with doing things 
> > the
> > libxml2 way, as msIO_printf can be quite error prone for XML output 
> > and well formedness.  This would of course mean that any future 
> > development of OGC stuff in MapServer would be best to 
> align with the 
> > libxml2 way of doing things.
> > 
> 
> It seems that using libxml2 for future OGC service work is 
> probably a good idea, but I think we'd need some comments on 
> whether libxml2 brought real benefits for the SOS 
> implementation by simplifying the code, and also it would be 
> interesting to see a comparison of the performance impact of 
> using libxml2 vs msIO_printf.
> 

Assefa would be the best judge here, sice he primarily did mapogcsos.c.

> I guess before deciding to go with libxml2 all the way I'd 
> like to make sure that we are not trading an ugly mechanism 
> for another ugly one with just poorer performance.
> 
> For instance, from a quick browse of the mapogcsos.c code, it 
> seems that everytime something is set with libxml2 there is a 
> call to xmlNewNs() with hardcoded namespace and uri. There is 
> a total of 110 calls like this in mapogcsos.c, most of them 
> with the same hardcoded gml or ows namespace info, are these 
> really necessary? Do these repeated allocations of objects 
> not add up and eventually impact on performance? 
> If they cannot be avoided then at least we should use a 
> #define to avoid duplicating the same namespace and uri 
> strings 39 times (for ows) and 25 times (for gml)... at least 
> to make the code look better than the former msIO_printf version.
> 

Very true.  Say hello to mapowscommon.h :)

I think we need to look into libxml2 a lot deeper and what the API does.

For example, Assefa used the tree module of libxml2.  And we are
outputting the OGC Exceptions with the ows: namespace explicitly defined
in the output (e.g. ows:ExceptionReport).

This is more commonly done by setting a global namespace and NOT
explicitly defining a namespace foreach element, i.e.:

<ExceptionReport xmlns="http://www.opengis.net/ows" ...>
 <Exception exceptionCode="Foo">
...
</ExceptionReport>
 
...type of thing.

So we may be able to save namespaces declarations there.

> Please don't get me wrong, I'm not saying that libxml2 is not 
> a good way to go, I'm just thinking out loud.
> 
> Daniel
> --
> Daniel Morissette
> http://www.mapgears.com/
>