Hi Bob,
We're doing some work with ERDDAP and running into an issue using NetCDF-Java
to access files served by ERDDAP. I think I understand the issue and know how
to address it, so I'm passing the info onto you all so it can be addressed. So
here goes:
1) ERDDAP allows one to download a NetCDF file by building a link appended with
'.nc'. The link URL for the netcdf file would be something like
http://beach.mbari.org:8180/erddap/griddap/erdRyanSST.nc. This works great for
downloading the files. However, it does NOT work with the NetCDF-Java API;
NetCDF-Java can normally read NetCDF files from arbitrary non-opendap http urls.
2) The reason it fails is because NetCDF-Java needs to know the size of the
file being served. This requires that the HTTP response for a URL like
http://beach.mbari.org:8180/erddap/griddap/erdRyanSST.nc to contain a
'Content-Length' field. ERDDAP is not sending that … here's a response header
from ERDDAP (notice there's no 'Content-Length':
HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
Date: Thu, 11 Oct 2012 15:44:19 GMT
Last-Modified: Thu, 11 Oct 2012 15:44:19 GMT
xdods-server: dods/3.7
erddap-server: 1.38
Content-Disposition: attachment;filename=erdRyanSST_8571_f367_229e.nc
Content-Encoding:
Content-Type: application/x-download
Transfer-Encoding: chunked
3) Since ERDDAP is running on Tomcat, the only way I know of to set the
'Content-Length' is to explicitly call 'response.setBufferSize()' in the
servlet that returns the NetCDF file. Note that once the response size goes
beyond the bufferSize, Tomcat will fallback to 'Transfer-Encoding: Chunked'
(which we don't want). So make sure you're setting the buffer size to the
correct value.
Hope that helps!
p.s. I cc'd this to the netcdf-java mailing list in case I got something wrong.
Hopefully someone will correct me.
Cheers
-- B
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
Brian Schlining
MBARI
Software Engineer, Research and Development
brian@xxxxxxxxx (mailto:brian@xxxxxxxxx)
(831) 775-1855
http://www.mbari.org/staff/brian
