[Fwd: Re: [Context.RWG] why is FormatList optional?]

Daniel Morissette dmorissette at MAPGEARS.COM
Wed Nov 1 13:37:26 EST 2006


Bart van den Eijnden (OSGIS) wrote:
> This has some consequences for Mapserver WMS client running from a WMC 
> document.
> 
> Currently, no GetCapabilities request is used/needed, but for instance 
> Ionic exports WMC documents without a FormatList, so Mapserver WMS 
> client cannot use them.
> 
> This response on the Context.RWG list seems to indicate a WMS client 
> would need to do an extra GetCapabilities request in order to solve this 
> ....
> 
> Why are things made so difficult and inefficient ...... :-(
> 

See below, CC'ing the response to this question that I sent to the 
context.rwg list a few minutes ago.

Yes, that means that in theory MapServer should be modified to issue 
GetCapabilities all the time... but in practice I think I prefer the way 
it works now since it's more efficient, even if it's not 100% up to 
spec. I'd leave it up to the MapScript application developers to make 
sure that the mapfile/WMC that they feed into the mapObj contains fresh 
capabilities info.

Daniel


-------- Original Message --------
Subject: Re: [Context.RWG] why is FormatList optional?
Date: Wed, 01 Nov 2006 13:20:03 -0500
From: Daniel Morissette
To: Bart van den Eijnden (OSGIS)
CC: context.rwg

Bart van den Eijnden (OSGIS) wrote:
 > Hmm, not sure I get the logic behind this.
 >
 > Why having to do a GetCapabilities request and parse it, only to find
 > out the image type that can be used ..... seems very inefficient in my
 > opinion.
 >
 > Or are there other reasons why a GetCapabilities request would be needed
 > anyway? Only to do this for the image type seems a waste to me.
 >

Inefficient? Who cares about performance?   ;)

The point that was made in that meeting was that all of us (me included)
who chose to cache or store some Capabilities info locally (or in a WMC)
instead of going back to fetch the latest Capabilities everytime we
connect to a server were doing something bad, very very bad.

It seems that since the beginning of times in OGC it was agreed that the
correct practice when connecting to a server was to do a GetCapabilities
first and then GetMap... and that bypassing GetCapabilities was very
bad. Whether that hurts performance doesn't seem to matter.

I have to admit that storing capabilities info such as format list in a
WMC can lead to some problems if the server configuration changed after
the WMC was generated... that's why, in order to be consistent with the
WMS and other specs this change had to be made to the latest revision of
OWS context.

Daniel



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