[Tilecache] out of range bbox requests and Tilecache failures

[Christopher Schmidt]

On Thu, Nov 22, 2007 at 10:00:19AM +0000, David Herbert wrote:
> Apologies for cross-posting this - not sure quite where it belongs.
> 
> While using Firebug (Firefox 2.0.0.9 WinXP) to look at performance and
> client-side memory usage of my OpenLayers 2.5 app (which uses Tilecache
> 1.9 in standalone server mode with a seeded cache of WMS layer images),
> I noticed a large number (50+) failed 404 responses for images e.g:
> 
> http://10.48.1.76:8084/?LAYERS=Coastline%20and%20bathymetry&STYLES=sggis_bathymetry_polygon%2Csggis_coastline&FORMAT=image%2Fpng&TRANSPARENT=off&SERVICE=WMS&VERSION=1.1.1&REQUEST=GetMap&EXCEPTIONS=application%2Fvnd.ogc.se_inimage&SRS=EPSG%3A2007056&BBOX=-82000,-43494.81471999999,-30705.185279999998,7800.000000000015&WIDTH=256&HEIGHT=256
> 
> Now the BBOX values being asked for in this request are way outside the
> bounds I have specified in tilecache.cfg - here is the extract for the
> above layer:
> 
> [Coastline and bathymetry]
> type=WMSLayer
> url=http://10.48.1.76:8080/geoserver/wms?transparent=off
> layers=sggis:bathy_poly,sggis:coastline2007
> extension=png
> #extent_type=loose 
> bbox=-82000,7800,80000,116000
> maxResolution=200.370370
> srs=EPSG:2007056
> metaTile=true
> 
> (The coordinates are a metre projection for South Georgia in the
> Southern Ocean). Tilecache correctly reports this with the following
> error:
> 
> An error occurred: Lower left corner (-82000.000000, -43494.814720) is
> outside layer bounds [-82000.0, 7800.0, 80000.0, 116000.0]. 
> To remove this condition, set extent_type=loose in your configuration.

What is the maxextent of your OpenLayers Map? OpenLayers shouldn't
request tiles outside your maxExtent: If it is, I would suggest rounding
off your maxResolution (in TC and OpenLayers) -- floating point errors
can cause OL to request tiles it shouldn't.

(I'm hoping you're not using 'displayOutsideMaxExtent'.)

I think you've hit a couple things: the bug in TileCache, below, and
what sounds to me like a configuration change needed in your OpenLayers
setup. I'd recommend we omit the latter for now -- it's annoying, but
*shouldn't* be affecting you -- and concentrate on how to fix your OL
configuration issue -- which means followups should go to OL-Users.

> An HTTP 404 response is generated.  If I do as the error suggests and
> set extent_type=loose (uncomment the line above) then I notice a marked
> drop in performance - one layer takes half a minute to load from the
> cache for some reason.  There are also 20-30 HTTP 500 responses which
> generate the error:

Right. This is a bug in Tilecache that I haven't fixed: metaTiling
results in requests outside the bounding box failing, so each tile is
requested, others wait to get the lock, then they're requested, etc. in
sequential order. 

Regards,
-- 
Christopher Schmidt
MetaCarta