[OpenLayers-Dev] offline caching issues

[Chris Berkhout]

Hi All,

I've been trying to get offline (localStorage) tile caching to work
like the example
    http://openlayers.org/dev/examples/offline-storage.html
and I've run into a couple of problems.

I'm restricting myself to a single layer, same origin via proxy and I
always set read from cache first (fall back to network).

The first problem is that after populating the cache and going
offline, failed tile fetches continue to populate the cache. These
appear as black squares in the example (non-cached failures are pink).
Manually turning off "Write to cache" in the example resolves this.
I've been able to avoid this by patching OpenLayers.Control.CacheWrite
to not write failed tiles to the cache, by checking the condition:
    !evt.tile.imgDiv.classList.contains("olImageLoadError")
I guess there's probably a better way to do that.

The other problem I found is that after populating the cache and going
offline, then panning away to non-cached areas and back to the cached
area, things around the edges of the cached areas are disappearing.
This was more obvious when working with my own layer (at detailed
zoom), but could also be replicated using the openlayers example. The
problem seems to be variation (rounding errors?) in the bbox
parameters of the WMS tile request.

For example, this URL was a cache miss:
http://m1.pozi.com/geoserver/gwc/service/wms?LAYERS=VicmapClassic&FORMAT=image%2Fpng8&SERVICE=WMS&VERSION=1.1.1&REQUEST=GetMap&STYLES=&SRS=EPSG%3A900913&BBOX=16133716.431963,-4554423.89271,16136162.416867,-4551977.9078052&WIDTH=256&HEIGHT=256

Even though the tile it refers to was cached with a URL of:
http://m1.pozi.com/geoserver/gwc/service/wms?LAYERS=VicmapClassic&FORMAT=image%2Fpng8&SERVICE=WMS&VERSION=1.1.1&REQUEST=GetMap&STYLES=&SRS=EPSG%3A900913&BBOX=16133716.431963,-4554423.89271,16136162.416868,-4551977.9078052&WIDTH=256&HEIGHT=256

That's a maxx (3rd bbox value) with last digit 7 instead 8.

I could avoid this by patching the cache read/write code to use an
optional URL normalization function, which the user could set to trim
off the last digits of the bbox parameters.

So, I think I've straightened out what's going on, but the only
solutions I can see involve various hacks in the openlayers code.

Has anyone else run into these issues or got some advice?

Cheers,
Chris