[netcdf-java] NcML location attribute and IOSPs
I've been developing a few AbstractIOServiceProvider-s (with NetCDF Java
4.1) to use in my NcML for some custom datasets. I've run into a few
issues (some of which I've already discussed with the developers) that
I'd like to get some insight on.
I have an IOSP that reads data from our relational database. The first
issue I ran into was setting the "location" attribute in my NcML. There
is no file and anything that wasn't a file made netcdf unhappy. I
eventually found that "/dev/null" got me around that problem.
That got me thinking about the possibilities of "location" that weren't
tied to a RandomAccessFile. I'd like to think that a "location" could be
any URI (or URN?) that the IOSP would know how to deal with.
I then thought that the "location" could be a file that contains the URI
that the IOSP could then read. I didn't like the idea of maintaining yet
another file, so I figured I could use the ncml file itself.
Unfortunately, netcdf is too clever to let me use an ncml file that
refers to itself. Could/should that restriction be relaxed when an
"iosp" is defined?
On a related note, there are often other bits of metadata that I need
for my IOSP when I am serving some data files (e.g. flat binary with no
metadata). Again the ncml file itself is the most practical source of
the metadata my IOSP needs to define variables and such, so I don't have
to keep a redundant set of information. In this case, I am using the
"location" for the data file. The only option I found, beyond a naming
convention, is using the "iospParam" attribute to encode the location of
the ncml so my IOSP can read it. This hack is less than ideal. I'd like
to think that the NetcdfFile that the netcdf code provides me in the
"open" method would have a handle on the ncml file that defines it.
After all, it is an instance of NcMLReader$NcMLNetcdfFile. Do you see
any problem exposing the ncml location (or even the netcdf Element)? It
looks trivial to add an arg to the NcMLNetcdfFile constructor and a
getter to retrieve it.
Any other thoughts or workarounds to give the IOSP more flexibility to
serve datasets that can't easily be represented by single location that
is mapped to a RandomAccessFile?
Thanks,
Doug
P.S. This may also be relevant to the thredds thread: "OPENDAP dataset
via a RESTFUL service"
