Re: 20040414: Attribute modification in Java2.1 NetCDF API

Steve Loch wrote:

> John,
>
> I think I was overly brief in my query. If the attribute doesn't
> require additional space then it should, in my view, be possible to
> update the file without regenerating from scratch. [I have always been
> surprised that the APIs (or rather the part of the NetCDF APIs that are
> documented for public consumption) don't allow the addition of free
> space within the header at definition time which would ameliorate the
> problem. Perhaps it's the complexities of garbage collection that led to
> the simpler choice.]
i completely agree. the reason the java API doesnt allow it was a design 
agrument about immutability, by the implementor of the version 1 library 
(ucar.netcdf). the 2.1 version still depends on that for the actual 
reading of the file, and so cant allow it either. the 2.2 version will 
replace that code, and at that point will allow attribute modification.

> If required the other language APIs will rewrite the file as and when
> needed but in a manner which is largely transparent to the user.
the transparency can be problematic. i would probably design the API so 
that the user optionally can be notified, and possibly even veto, a 
change requiring a file rewrite. also, there would be a way to specify 
extra room in a file when the file is being written, when you know that 
you will be doing that.

> In the BODC applications the attributes are normally fixed in size but,
> inter alia, represent summary information and need to be updated.
> Because of the immediacy of our requirements I'll need to hack the
> source code as a temporary expedient. I have done this once before, for
> other reasons,  with the C library (finding the address of the attribute
> within the file) . Any hints (or code) gratefully accepted.
im afraid i dont have much guidance at this point, because i didnt write 
that code. good luck..

> Regards, 
> Steve Loch
> BODC
> http://www.bodc.ac.uk
>  
>
> > Is there any prospect of a later release permitting the update of the
> > attributes within a NetCDF file without having to regenerate the
> > complete file?
> >
> > The APIs I have used to date including Fortran, C and Matlab don't
> >    
> have
>  
>
> > this restriction.
> >
> > Steve Loch
> > British Oceanographic Dta Centre
> >
> >
> >
> >
> >    
> we will support attribute modification in the next version of 
> netcdf-java (2.2), which may be ready by end of year (?)
>
> as to whether that can be done without rewriting the file depends on
> the 
> file; we are constrained by the version 1 file format. what would you 
> need in that regard?
>
> netcdf-4 (on top of HDF5) should be able to modify attributes without 
> rewriting entire file.
>
>
>
>
>