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Summarizing some discussions started on the mailing list related to this topic:
In the Metoc/EO world, layers are based on complex NetCDF/Grib files, and the protocols we have to output them, WCS in particular, are setup to allow slice, rescale, reproject data, but not exactly to preserve the original data as is.
As the term suggests, "Direct download" would allow to directly download the raw data for that layer instead of getting them served by a Service which does some kind of processing.
So, for these layers, we would like to provide a direct download of the raw data from CSW, by adding specific entries within the "term-references" entry in dublin core metadata, and OnlineResource in ISO
We create a link pointing back to the CSW service, to a vendor request that will assemble the files for the requested resource, zip them, and send the result back to the requestor.
A typical link would be like
http://host:port/geoserver/ows?service=CSW&version=2.0.1&request=DirectDownload&resourceId=<nameSpace:layerName>&file=<fileID>&time=from/to&elevation=from/to&bbox=...
time, elevation, bbox and additional dimensions would be added when dealing with multidimensional datasets in order to provide domain information of the available resource.
You may think about NetCDF/GRIB datasets containing multiple times/elevations within the same file. Moreover a link would be provided for each different resource file composing the multidimensional data. (Think about a mosaic of NetCDF files).
Of course we would not automatically create those links, we'll have configurations at the CSW level, and at the layer level as an override, to allow direct download to be available.
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