WMS DescribeLayer does not work as expected with coverages and groups of layers

For coverage the thing is a little confusing, as we'd have to drop the wfs attribute and use owsType, owsURL instead. From the DTD:

Looking at the dtd, my interpretation is that group layer just should return
<LayerDescription name="tasmania"/>

The description says that LayerDescription has one or more Query elements, but the dtd definitely says (Query*), not (Query+).

And a raster layer should return something like
<LayerDescription name="sf:dem" owsType="WCS" owsURL="http://localhost/geoserver/wcs"/>

The result for groups is sintactically valid, but has no meaning, since you're not pointing to any source of description (the very objective of a Describelayer call).
A service exception is a better response, it does not make sense to make the call on group layers to start with... I have to check the standard on what the behaviour is in the case a layer cannot be described (a WMS layer is fully opaque, so description cannot be expected).

For rasters yep, that looks like the expected syntax and makes sense as well.

The problem with a service exception is that if you issue a DescribeLayer request with multiple layers (e.g. "topp:states,tasmania", you would get a service exception although there would be useful information for topp:states.

Since Query is 0..infinity, the group layer could also return a Query element for the wfs capable layers it consists of:

<LayerDescription name="tasmania" owsType="wfs" owsURL="whatever">
<Query typeName="topp:tasmania_roads"/>
<Query typeName="topp:tasmania_cities"/>
<!-- ... -->
</LayerDescription>

Only question is: how do raster layers that are part of a group layer fit in here?

And for rasters in general: I don't know the WCS specs, but if there is something like a typeName that can be queried, Query elements could be included for raster layers as well.

Hmmm... given a group that mixes vector and raster layers we would not have any good way to report back the description since the owsType would be different for the sub-parts. The spec did not consider groups, and they actually cannot make sense to start with... The reason for DescribeLayer is to allow a client to retrieve enough information to build a dynamic SLD for the layer. But layer groups are opaque and cannot be restyled (only the default style is valid).

So I guess a wrong, but still acceptable behaviour, is to just do what you suggested earlier, or just skip the group layers if the description request targets multiple layers.

It seems worth noting that GeoServer trunk already uses owsType and owsURL properly for layers that aren't layergroups.

For layergroups, it seems that returning an error implies that the layer does not exist at all, as opposed to returning a description with no queries, which would indicate that the layer is known, but not available via WFS or WCS. This snippet from the prose docs for DescribeLayer in SLD 1.1.0 seems relevant:

For each named layer, the description should indicate if it is indeed based on feature data
and if so it should indicate the WFS/WCS (by a URL prefix) and the feature/coverage
types. Note that it is perfectly valid for a named layer not to be describable in this way.
Annex B gives theXML Schema for the response.

David, yep, correct, the SLD 1.0 specification says the same. So ok, let's go down this path. Given the amount of discussion I gather this is switching from minor to critical priority?

Yes, please. If this could be critical priority that would be wonderful.

Has this one gone anywhere? Executing the call against a raster layer seems to have the desired affect... i get wcs reference back. But layer groups just throws an exception that the layer is not found...

Nope. I guess it was not so critical in the end?

I have tested trunk and it seems like this may be fixed now. In 1.7.7 the request (a wcs layer):

http://localhost:8080/geoserver/ows?service=wms&layers=nurc:mosaic&request=describelayer&version=1.1.1

resulted in:

<WMS_DescribeLayerResponse version="1.1.1">
<LayerDescription name="nurc:mosaic" wfs="http://localhost:10080/geoserver/wfs/WfsDispatcher?">
<Query typeName="nurc:mosaic"/>
</LayerDescription>
</WMS_DescribeLayerResponse>

the same request sent to Geoserver 2.0.1 and trunk results in:

<WMS_DescribeLayerResponse version="1.1.1">
<LayerDescription name="nurc:mosaic"
owsURL="http://localhost:10080/geoserver/wcs?"
owsType="WCS">
<Query typeName="nurc:mosaic"/>
</LayerDescription>
</WMS_DescribeLayerResponse>

That seems to be the correct semantics.

Works for coverages but still does not work for layer groups