Re: [thredds] unable to do ncml aggregation

Hi Upendra,

On 6/8/2010 12:11 PM, Upendra Dadi wrote:
> Hi Ethan,
>  It does help. Thank you very much. Just to confirm if my understanding
> is correct- the client software deduces whether the URL is an OPeNDAP
> URL  or not by making additional requests to the server and if it is, it
> requests the data in uncompressed format from OPeNDAP  server which the
> OPeNDAP server would be able to handle whereas other servers wouldn't.

Yes. OPeNDAP is a data access protocol that allows clients to request
descriptions of the dataset and subsets of the actual data values (more
at The server must therefore be able to read,
understand, and extract the data from the file.

Access through the "fileServer" URLs are simple file transfer over HTTP.
So the server doesn't have to understand the data file it is serving.
(The netCDF-java library actually uses HTTP byte range requests to not
get the entire data file. But the server doesn't know anything about the
file except for the bytes it contains.)


> Thanks,
> Upendra
> Ethan Davis wrote:
>> Hi Upendra,
>> Are the data files accessible from through OPeNDAP? If so, try
>> using the OPeNDAP URLs in the NcML rather than the file server URLs.
>> With OPeNDAP, whether the source data file is compressed or not
>> shouldn't matter as that is all handled on the server.
>> When accessed through the file server, the client software is reading
>> the bytes from the compressed file over HTTP. When the netCDF-Java
>> library tries to open a non-OPeNDAP HTTP URL, it assumes the resource is
>> an uncompressed file (for efficiency and other reasons). When it tries
>> this on a compressed file, it doesn't recognize the first few bytes of
>> the file as the netCDF header and so fails to open the dataset.
>> Hope that helps,
>> Ethan
>> On 6/7/2010 4:05 PM, Upendra Dadi wrote:
>>> Hi,
>>>  I am trying to aggregate some netcdf files thought ncml. I am getting
>>> an error like this when aggregating:
>>>       Cant read
>>> not a valid NetCDF file.
>>> But I am able to open the same file on THREDDS server. Any suggestions
>>> on what might be going wrong?
>>> By the way, does THREDDS download the entire netcdf file before
>>> aggregating?  How do I control how long the temporary dataset stays on
>>> the server?
>>  On 6/8/2010 11:10 AM, Upendra Dadi wrote:
>>> Hello,
>>>  Can someone please help me here with my ncml issue? My THREDDS server
>>> is unable to read remote compressed netcdf files present in the ncml
>>> files. However, it is able to recognize remote uncompressed files as
>>> well as local compressed files. Does ncml working with remote compressed
>>> files? Is there a TDS setting I should be looking at? Thanks!
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