For those that are not familiar with it, cascading allows to expose layers coming from other WMS servers as if they were local layers. This provides for some interesting advantages:
- Clients connecting to your SDI need to care about less points of origin, which might be important for high security networks
- It is now possible to ask for maps in formats not supported by the original server, or to reproject the maps in projections not supported by the original server (GeoServer supports out of the box almost 5000 different coordinate reference systems)
- It is now possible to mix the layers with local ones to generate print oriented formats such as PDF
- It is now possible to provide more informations about the layer, such as a better description, more keywords, which will benefit all clients, in particular catalogs harvesting informations from your capabilites document
The configuration as usage of the cascaded layers follows GeoServer traditional ease of use.
The first step is to configure a new WMS server to be cascaded. In GeoServer terms that means configuring a new store, a WMS store in particular.
The only parameter needed is the capabilities document of the cascaded server, in this case, the DEMIS one (http://www2.demis.nl/wms/wms.asp?wms=WorldMap&request=getcapabilities&version=1.0.7)
Past this point it's business as usual, one just need to choose which layers to cascade using the normal layer configuration workflow:
Here is GeoServer map preview showing two layers, Topography and Borders:
Of course it is also possible to mix in some local vector layers, such as, for example, the Tasmania layer group:
As said, one of the nice things about cascading is that one can add a few features, such as reprojection, transparently. The original modis server does not advertise reprojection abilities, yet GeoServer can reproject the data into EPSG:3785 on the fly for us:
There is still a number of things that can be done vs cascading:
- Support for secured remote servers
- Support for vendor parameters in cascading, including custom styling
- Better networking support, with HTTP 1.1 persistent connections, limitations on the number of parallel requests to the cascaded server, and configurable timeouts
- Support for choosing the cascading format (right now PNG is used)
- Support for GetFeatureInfo cascading on servers that do not provide GML output (if the GML GetFeatureInfo output is there full cascading is already in place and working!)
- Cascading WMS 1.3 servers
- Auto configuration of the layers exposed by a remote server, so that new layers are automatically exposed by GeoServer without any manual intervention
Are you interested in any of the above features? Let us know!
the GeoSolutions team.