[webmap-discuss] OGC and google style tiling

Steven M. Ottens steven.ottens at geodan.nl
Thu Apr 27 14:08:27 EDT 2006

On Thu, 2006-04-27 at 10:22 -0400, Stephen Woodbridge wrote:
> Steven M. Ottens wrote:
> > Paul Spencer wrote:
> > 
> >>Steven
> >>
> >>there are a couple of very good reasons for this (in my opinion).  The
> >>approach you suggest is actually the first one I tried because I was
> >>focussing on the client side code at the time.  It became obvious that
> >>it would not actually work in a real world situation because:
> >>
> >>1) every request to mapserv starts the CGI, loads the map, renders the
> >>map and returns it.  This takes a substantial amount of time, even if
> >>your map file and data are highly optimized.  For instance, my test
> >>cases would take about 1-2 seconds per tile, with a HUGE load on the
> >>server.
> > 
> > The idea is to get serverside cache, we have caching working on
> > mapserver.cgi requests, which means it can can deliver lots and lots of
> > tiles in a second. The reason I'm developing tiling is because our
> > current setup can only handle 20 concurrent users and we need 300. If we
> > can server from cache we can do that, and tiling is a way to make sure
> > identical images are served to users and as such can be served from cache.
> > Also in your situation you need to run a tile.php, a mapserver.cgi and a
> > gd (why btw?) image process. If you multiply this 300 times it's also
> > quite a load on the server. I prefer to keep as much load on the client
> > side as possible.
> Steven,
> You mentioned that you can use apache to handle caching of mapserver 
> CGI, so this could also be done for tile.php.

That's true. Probably we can use the same techniques as we used to cache
mapserver to cache tile.php. The problem I have from a more
architectical point of view is that I don't want to add functionality to
mapbuilder that requires a serverside script and especially ties it to
one particular WMS server, if I can avoid it.
Ka-map is very nice client, but it's very much dependant on umn
mapserver. Mapbuilder has been a 'free' client with strong ogc
compliance and I like to keep it that way. Tiling is/will be a much
requested feature and as such I'm hoping for a solution which keeps
mapbuilder users the freedom to choose wms servers.
> Also I would consider, you mod_rewrite to convert the tile.php request 
> into a direct file hit first and only if the file does not exist, then 
> call tile.php to generate. This would significantly improve performance. 
> There are also tools with ka-map to pre generate all the tiles into the 
> tile cache.

That is indeed an interesting option. Would that work also with cached
mapserver requests, like we have now? On the pregeneration part: we have
right now about 30 maps which could/should be tiled and in the future
that amount will increase. I haven't done the math yet, but it probably
means a lot of diskspace and a lot of CPUtime. And especially the last
part is a problem. Some people here also suggest to pretile everything,
but I'm not sure it is feasible/desirable for all the data we have. In
my opinion the cache system should cache the common tiles and if needed
mapserver can generate more tiles. If mapserver only need to serve those
few which are not cached it can keep up with the 300 users I guess.

> -Steve W
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