MAP file parsing

[Ed McNierney]

Bart -

I agree that there could be - in some cases - a big performance
improvement here.  But it's also understandably difficult (not
impossible) for MapServer to try to figure out in advance which symbols
might possibly be used in a specific request, and to try to check for
and load only those symbols.  In my situation I have found that MAP file
optimization (trimming down MAP files to minimal ones) combined with
creating lots of little MAP files, works well.  But that's mainly
because my maps are mostly raster and the majority of my GetMap requests
are for only one layer.  This is probably not a common arrangement, and
many other users would benefit from such an improvement.

I am not a parser guru, but I can see how the "straight up the middle"
MAP file parsing approach used now is simple to implement.  The approach
used is "load up the MAP file and get ready for any possible GetMap
request, then look at the request", and I think you're suggesting
reversing that to something like "look at the request and parse/load
only those portions of the MAP file absolutely necessary to serve that
request".

	- Ed

Ed McNierney
President and Chief Mapmaker
TopoZone.com / Maps a la carte, Inc.
73 Princeton Street, Suite 305
North Chelmsford, MA  01863
Phone: +1 (978) 251-4242
Fax: +1 (978) 251-1396
ed at topozone.com

-----Original Message-----
From: UMN MapServer Developers List [mailto:MAPSERVER-DEV at LISTS.UMN.EDU]
On Behalf Of Bart van den Eijnden (OSGIS)
Sent: Wednesday, October 11, 2006 8:08 AM
To: MAPSERVER-DEV at LISTS.UMN.EDU
Subject: [UMN_MAPSERVER-DEV] MAP file parsing

Hi list,

can anybody explain to me why Mapserver tries to open up all symbols
when doing a GetMap request on a totally different layer? I think Ed
came to the same conclusion a while back.

Why is this step done during MAP file parsing?

I think a lot of performance can be won here, especially for large MAP
files (with large symbolsets) like we maintain.

msLoadMap(): General error message. Undefined overlay symbol "loofbos"
in class 9, style 0 of layer TOP10VLAKKEN. <br>
msAddImageSymbol(): Unable to access file. Error opening image file
/data/home/bart/performance/loofbos. <br>

Best regards,
Bart

--
Bart van den Eijnden
OSGIS, Open Source GIS
http://www.osgis.nl