[Mapserver-users] cascading map server

[Daniel Morissette]

Jean-Francois.Doyon at ccrs.nrcan.gc.ca wrote:
> 
> Heh, good question! I know that right now, you have to specifiy all the
> remote layers one by one, there's no way for MapServer to simply ingest a
> remote GetCapabilities and just figure it out on it's own ( Bugzilla ->
> feature enhancement ).
> 

Guys,

I see what you're talking about, but I'm not convinced that it would be
MapServer's job to do that.  To be honest, I find the concept of
cascading a whole server the way you describe nice and cool in theory,
but of very little use in practice.  Cascading WMS servers can only
result on lower performance.  Not to mention the fact that some servers
end up with over 1MB capabilities documents just because they cascade
about everything that's available on the internet... it makes it real
hard to deal with the capabilities of those servers and to look for the
data that we need.

The only benefit I can think of is that a cascading server can increase
the capabilities available for a given layer, for instance offer a
bigger list of SRS for a layer than the original server used to
(assuming the original server doesn't know how to reproject data).  But
is it worth bloating MapServer with full cascading capabilities just to
accomodate servers that don't have reprojection support?  Our time would
be better spent replacing those servers with MapServer instances!  ;)


> But this begs the question (I probably once knew the wanswer but I forget),
> when one does a GetCapabilities to a MapServer that has remote layers in it,
> does it re-fetch the remote GetCapabilities everytime ??? There's another
> one for the featur enhancement list, caching of remote capabilities for the
> purpose of cascading them.
> 

MapServer never fetches capabilities.  It is the client application
built on top of MapServer (such as MapBrowser, MapLab, CWC) that fetches
the capabilities and takes care of the caching.  The client application
defines layers and passes all the information that MapServer needs from
the capabilities via mapfile METADATA and the CONNECTION URL parameter.

-- 
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 Daniel Morissette               morissette at dmsolutions.ca
 DM Solutions Group              http://www.dmsolutions.ca/
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