Re: [netcdfgroup] [CF-metadata] [Hdf-forum] Detecting netCDF versus HDF5 -- PROPOSED SOLUTIONS --REQUEST FOR COMMENTS

  • To: "John Caron" <jcaron1129@xxxxxxxxx>
  • Subject: Re: [netcdfgroup] [CF-metadata] [Hdf-forum] Detecting netCDF versus HDF5 -- PROPOSED SOLUTIONS --REQUEST FOR COMMENTS
  • From: "Pedro Vicente" <pedro.vicente@xxxxxxxxxxxxxxxxxx>
  • Date: Fri, 22 Apr 2016 08:57:50 -0500
John

>>>i have written various blogs on the unidata site about why netcdf4 != hdf5, 
>>>and what the unique signature for shared dimensions looks like, in >>>case 
>>>you want details.

yes, I am interested, I had the impression by looking at the code some years 
ago that netCDF writes some unique name attributes somewhere

----------------------
Pedro Vicente
pedro.vicente@xxxxxxxxxxxxxxxxxx
https://twitter.com/_pedro__vicente
http://www.space-research.org/



  ----- Original Message ----- 
  From: John Caron 
  To: Pedro Vicente 
  Cc: cf-metadata@xxxxxxxxxxxx ; Discussion forum for the NeXus data format ; 
netcdfgroup@xxxxxxxxxxxxxxxx ; Dennis Heimbigner ; NetCDF-Java community 
  Sent: Thursday, April 21, 2016 11:11 PM
  Subject: Re: [CF-metadata] [netcdfgroup] [Hdf-forum] Detecting netCDF versus 
HDF5 -- PROPOSED SOLUTIONS --REQUEST FOR COMMENTS


  1) I completely agree with the idea of adding system metadata that indicates 
the library version(s) that wrote the file.


  2) the way shared dimensions are implemented by netcdf4 is a unique signature 
that would likely identify (100 - epsilon) % of real data files in the wild. 
One could add such detection to the netcdf4 and/or hdf5 libraries, and/or write 
a utility program to detect.


  there are 2 variants:



  2.1) one could write a netcdf4 file without shared dimensions, though im 
pretty sure no one does. but you could argue then that its fine to just treat 
it as an hdf5 file and read through hdf5 library


  2.2) one could write a netcdf4 file with hdf5 library, if you knew what you 
are doing. i have heard of this happening. but then you could argue that its 
really a netcdf4 file and you should use netcdf library to read.


  i have written various blogs on the unidata site about why netcdf4 != hdf5, 
and what the unique signature for shared dimensions looks like, in case you 
want details.


  On Thu, Apr 21, 2016 at 4:18 PM, Pedro Vicente 
<pedro.vicente@xxxxxxxxxxxxxxxxxx> wrote:

      If you have hdf5 files that should be readable, then I will undertake to
      look at them and see what the problem is.



    ok, thank you


      WRT to old files:  We could produce a utility that would redef the file 
and insert the
           _NCProperties attribute. This would allow someone to wholesale
           mark old files.



    Excellent idea , Dennis

    ----------------------
    Pedro Vicente
    pedro.vicente@xxxxxxxxxxxxxxxxxx
    https://twitter.com/_pedro__vicente
    http://www.space-research.org/


    ----- Original Message ----- From: <dmh@xxxxxxxx>
    To: "Pedro Vicente" <pedro.vicente@xxxxxxxxxxxxxxxxxx>; 
<cf-metadata@xxxxxxxxxxxx>; "Discussion forum for the NeXus data format" 
<nexus@xxxxxxxxxxxxxxx>; <netcdfgroup@xxxxxxxxxxxxxxxx>
    Sent: Thursday, April 21, 2016 5:02 PM
    Subject: Re: [netcdfgroup] [Hdf-forum] Detecting netCDF versus HDF5 --  
PROPOSED SOLUTIONS --REQUEST FOR COMMENTS



      If you have hdf5 files that should be readable, then I will undertake to
      look at them and see what the problem is.
      WRT to old files:  We could produce a utility that would redef the file 
and insert the
           _NCProperties attribute. This would allow someone to wholesale
           mark old files.
      =Dennis Heimbigner
        Unidata


      On 4/21/2016 2:17 PM, Pedro Vicente wrote:

        Dennis


                I am in the process of adding a global attribute in the root 
group

          that captures both the netcdf library version and the hdf5 library 
version
          whenever a netcdf file is created. The current  form is
          _NCProperties="version=...|netcdflibversion=...|hdflibversion=..."



        ok, good to know, thank you



              > 1. I am open to suggestions about changing the format or adding
              info > to it.



        I personally don't care, anything that uniquely identifies a netCDF 
file (HDF5 based) as such will work



              2. Of course this attribute will not exist in files written using 
older

          versions of the netcdf library, but at least the process will have 
begun.


        yes



          3. This technically does not address the original issue because there 
exist
               hdf5 files  not written by netcdf that are still compatible with 
and can be
               read by netcdf. Not sure this case is important or not.


        there will always be HDF5 files  not written by netcdf that netCDF will 
read as we are now.

        this is not really the issue, but you just made a further issue :-)

        the issue is that I would like an application that reads a netCDF (HDF5 
based) file to decide to use the netCDF or HDF5 API.
        your attribute writing will do , for future files.
        for older nertCDF files there may be  a way to detect the current 
attributes and data structures to see if we can make it "identify itself"
        as netCDF. A bit of debugging will confirm that, since Dimension Scales 
are used, that would be an (imperfect maybe) way to do it

        regarding the "further issue " above

        you could go one step further and for any HDF5 files  not written by 
netcdf , you could make netCDF reject the file reading,
        because it's not "netCDF compliant".
        Since having netCDF read pure HDF5 files is not a problem (at least for 
me), I don't know if you would want to do this, just an idea.
        In my mind taking complexity and ambiguities of problems is always a 
good thing


        ah, I forgot one thing, related to this


        In the past I have found several pure HDF5 files that netCDF failed in 
reading.
        Since netCDF is HDF5 binary compatible, one would expect that all HDF5 
files will be read by netCDF.
        Except if you specifically wrote something in the code that makes it to 
fail if some condition is not met,
        This was a while ago, I'll try to find those cases and I'll send a bug 
report to the bug report email

        ----------------------
        Pedro Vicente
        pedro.vicente@xxxxxxxxxxxxxxxxxx
        https://twitter.com/_pedro__vicente
        http://www.space-research.org/

        ----- Original Message ----- From: <dmh@xxxxxxxx>
        To: "Pedro Vicente" <pedro.vicente@xxxxxxxxxxxxxxxxxx>; "HDF Users 
Discussion List" <hdf-forum@xxxxxxxxxxxxxxxxxx>; <cf-metadata@xxxxxxxxxxxx>; 
"Discussion forum for the NeXus data format" <nexus@xxxxxxxxxxxxxxx>; 
<netcdfgroup@xxxxxxxxxxxxxxxx>
        Cc: "John Shalf" <jshalf@xxxxxxx>; <Richard.E.Ullman@xxxxxxxx>; 
"Marinelli, Daniel J. (GSFC-5810)" <daniel.j.marinelli@xxxxxxxx>; "Miller, Mark 
C." <miller86@xxxxxxxx>
        Sent: Thursday, April 21, 2016 2:30 PM
        Subject: Re: [netcdfgroup] [Hdf-forum] Detecting netCDF versus HDF5 --  
PROPOSED SOLUTIONS --REQUEST FOR COMMENTS



          I am in the process of adding a global attribute in the root group
          that captures both the netcdf library version and the hdf5 library 
version
          whenever a netcdf file is created. The current  form is
          _NCProperties="version=...|netcdflibversion=...|hdflibversion=..."
          Where version is the version of the _NCProperties attribute and the 
others
          are e.g. 1.8.18 or 4.4.1-rc1.
          Issues:
          1. I am open to suggestions about changing the format or adding info 
to it.
          2. Of course this attribute will not exist in files written using 
older versions
              of the netcdf library, but at least the process will have begun.
          3. This technically does not address the original issue because there 
exist
               hdf5 files  not written by netcdf that are still compatible with 
and can be
               read by netcdf. Not sure this case is important or not.
          =Dennis Heimbigner
             Unidata


          On 4/21/2016 9:33 AM, Pedro Vicente wrote:

            DETECTING HDF5 VERSUS NETCDF GENERATED FILES
            REQUEST FOR COMMENTS
            AUTHOR: Pedro Vicente

            AUDIENCE:
            1) HDF, netcdf developers,
            Ed Hartnett
            Kent Yang
            2) HDF, netcdf users, that replied to this thread
            Miller, Mark C.
            John Shalf
            3 ) netcdf tools developers
            Mary Haley  , NCL
            4) HDF, netcdf managers and sponsors
            David Pearah  , CEO HDF Group
            Ward Fisher, UCAR
            Marinelli, Daniel J. , Richard Ullmman, Christopher Lynnes, NASA
            5)
            [CF-metadata] list
            After this thread started 2 months ago, there was an annoucement on 
the [CF-metadata] mail list
            about
            "a meeting to discuss current and future netCDF-CF efforts and 
directions.
            The meeting will be held on 24-26 May 2016 in Boulder, CO, USA at 
the UCAR Center Green facility."
            This would be a good topic to put on the agenda, maybe?
            THE PROBLEM:
            Currently it is impossible to detect if an HDF5 file was generated 
by the HDF5 API or by the netCDF API.
            See previous email about the reasons why.
            WHY THIS MATTERS:
            Software applications that need to handle both netCDF and HDF5 
files cannot decide which API to use.
            This includes popular visualization tools like IDL, Matlab, NCL, 
HDF Explorer.
            SOLUTIONS PROPOSED: 2
            SOLUTION 1: Add a flag to HDF5 source
            The hdf5 format specification, listed here
            https://www.hdfgroup.org/HDF5/doc/H5.format.html
            describes a sequence of bytes in the file layout that have special 
meaning for the HDF5 API. It is common practice, when designing a data format,
            so leave some fields "reserved for future use".
            This solution makes use of one of these empty  "reserved for future 
use" spaces to save a byte (for example) that describes an enumerator
            of "HDF5 compatible formats".
            An "HDF5 compatible format" is a data format that uses the HDF5 API 
at a lower level (usually hidden from the user of the upper API),
            and providing its own API.
            This category can still be divide in 2 formats:
            1) A "pure HDF5 compatible format". Example, NeXus
            http://www.nexusformat.org/
            NeXus just writes some metadata (attributes) on top of the HDF5 
API, that has some special meaning for the NeXus community
            2) A "non pure HDF5 compatible format". Example, netCDF
            Here, the format adds some extra feature besides HDF5. In the case 
of netCDF, these are shared dimensions between variables.
            This sub-division between 1) and 2) is irrelevant for the problem 
and solution in question
            The solution consists of writing a different enumerator value on 
the "reserved for future use" space. For example
            Value decimal 0 (current value): This file was generated by the 
HDF5 API (meaning the HDF5 only API)
            Value decimal 1: This file was generated by the netCDF API (using 
HDF5)
            Value decimal 2: This file was generated by <put here another HDF5 
based format>
            and so on
            The advantage of this solution is that this process involves 2 
parties: the HDF Group and the other format's organization.
            This allows the HDF Group to "keep track" of new HDF5 based 
formats. It allows to make the other format "HDF5 certified" .
            SOLUTION 2: Add some metadata to the other API on top of HDF5
            This is what Nexus uses.
            A Nexus file on creation writes several attributes on the root 
group, like "NeXus_version" and other numeric data.
            This is done using the public HDF5 API calls.
            The solution for netCDF consists of the same approach, just write 
some specific attributes, and a special netCDF API to write/read them.
            This solutions just requires the work of one party (the netCDF 
group)
            END OF RFC
            In reply to people that commented in the thread
            @John Shalf
            >>Perhaps NetCDF (and other higher-level APIs that are built on top 
of
            HDF5) should include an attribute attached
            >>to the root group that identifies the name and version of the API
            that created the file?  (adopt this as a convention)
            yes, that's one way to do it, Solution 2 above
            @Mark Miller
            >>>Hmmm. Is there any big reason NOT to try to read a netCDF 
produced
            HDF5 file with the native HDF5 library if someone so chooses?
            It's possible to read a netCDF file using HDF5, yes.
            There are 2 things that you will miss doing this:
            1) the ability to inquire about shared netCDF dimensions.
            2) the ability to read remotely with openDAP.
            Reading with HDF5 also exposes metadata that is supposed to be 
private to netCDF. See below
            >>>> And, attempting  to read an HDF5 file produced by Silo using 
just
            the HDF5 library (e.g. w/o Silo) is a major pain.
            This I don't understand. Why not read the Silo file with the Silo 
API?
            That's the all purpose of this issue, each higher level API on top 
of HDF5 should be able to detect "itself".
            I am not familiar with Silo, but if Silo cannot do this, then you 
have the same design flaw that netCDF has.

            >>> In a cursory look over the libsrc4 sources in netCDF distro, I 
see
            a few things that might give a hint a file was created with netCDF. 
. .
            >>>> First, in NC_CLASSIC_MODEL, an attribute gets attached to the
            root group named "_nc3_strict". So, the existence of an attribute 
on the root group by that name would suggest the HDF5 file was generated by 
netCDF.
            I think this is done only by the "old" netCDF3 format.
            >>>>> Also, I tested a simple case of nc_open, nc_def_dim, etc.
            nc_close to see what it produced.
            >>>> It appears to produce datasets for each 'dimension' defined 
with
            two attributes named "CLASS" and "NAME".
            This is because netCDF uses the HDF5 Dimension Scales API 
internally to keep track of shared dimensions. These are internal attributes
            of Dimension Scales. This approach would not work because an HDF5 
only file with Dimension Scales would have the same attributes.

            >>>> I like John's suggestion here.
            >>>>>But, any code you add to any applications now will work *only*
            for files that were produced post-adoption of this convention.
            yes. there are 2 actions to take here.
            1) fix the issue for the future
            2) try to retroactively have some workaround that makes possible 
now to differentiate a HDF5/netCDF files made before the adopted convention
            see below

            >>>> In VisIt, we support >140 format readers. Over 20 of those are
            different variants of HDF5 files (H5part, Xdmf, Pixie, Silo, 
Samrai, netCDF, Flash, Enzo, Chombo, etc., etc.)
            >>>>When opening a file, how does VisIt figure out which plugin to
            use? In particular, how do we avoid one poorly written reader 
plugin (which may be the wrong one for a given file) from preventing the 
correct one from being found. Its kinda a hard problem.

            Yes, that's the problem we are trying to solve. I have to say, that 
is quick a list of HDF5 based formats there.
            >>>> Some of our discussion is captured here. . .
            http://www.visitusers.org/index.php?title=Database_Format_Detection
            I"ll check it out, thank you for the suggestions
            @Ed Hartnett
            >>>I must admit that when putting netCDF-4 together I never 
considered
            that someone might want to tell the difference between a "native" 
HDF5 file and a netCDF-4/HDF5 file.
            >>>>>Well, you can't think of everything.
            This is a major design flaw.
            If you are in the business of designing data file formats, one of 
the things you have to do is how to make it possible to identify it from the 
other formats.

            >>> I agree that it is not possible to canonically tell the
            difference. The netCDF-4 API does use some special attributes to 
track named dimensions,
            >>>>and to tell whether classic mode should be enforced. But it can
            easily produce files without any named dimensions, etc.
            >>>So I don't think there is any easy way to tell.
            I remember you wrote that code together with Kent Yang from the HDF 
Group.
            At the time I was with the HDF Group but unfortunately I did follow 
closely what you were doing.
            I don't remember any design document being circulated that explains 
the internals of the "how to" make the netCDF (classic) model of shared 
dimensions
            use the hierarchical group model of HDF5.
            I know this was done using the HDF5 Dimension Scales (that I 
wrote), but is there any design document that explains it?
            Maybe just some internal email exchange between you and Kent Yang?
            Kent, how are you?
            Do you remember having any design document that explains this?
            Maybe something like a unique private attribute that is written 
somewhere in the netCDF file?

            @Mary Haley, NCL
            NCL is a widely used tool that handles both netCDF and HDF5
            Mary, how are you?
            How does NCL deal with the case of reading both pure HDF5 files and 
netCDF files that use HDF5?
            Would you be interested in joining a community based effort to deal 
with this, in case this is an issue for you?

            @David Pearah  , CEO HDF Group
            I volunteer to participate in the effort of this RFC together with 
the HDF Group (and netCDF Group).
            Maybe we could make a "task force" between HDF Group, netCDF Group 
and any volunteer (such as tools developers that happen to be in these mail 
lists)?
            The "task force" would have 2 tasks:
            1) make a HDF5 based convention for the future and
            2) try to retroactively salvage the current design issue of netCDF
            My phone is 217-898-9356, you are welcome to call in anytime.
            ----------------------
            Pedro Vicente
            pedro.vicente@xxxxxxxxxxxxxxxxxx 
<mailto:pedro.vicente@xxxxxxxxxxxxxxxxxx>
            https://twitter.com/_pedro__vicente
            http://www.space-research.org/

                ----- Original Message -----
                *From:* Miller, Mark C. <mailto:miller86@xxxxxxxx>
                *To:* HDF Users Discussion List 
<mailto:hdf-forum@xxxxxxxxxxxxxxxxxx>
                *Cc:* netcdfgroup@xxxxxxxxxxxxxxxx
                <mailto:netcdfgroup@xxxxxxxxxxxxxxxx> ; Ward Fisher
                <mailto:wfisher@xxxxxxxx>
                *Sent:* Wednesday, March 02, 2016 7:07 PM
                *Subject:* Re: [Hdf-forum] Detecting netCDF versus HDF5

                I like John's suggestion here.

                But, any code you add to any applications now will work *only* 
for
                files that were produced post-adoption of this convention.

                There are probably a bazillion files out there at this point 
that
                don't follow that convention and you probably still want your
                applications to be able to read them.

                In VisIt, we support >140 format readers. Over 20 of those are
                different variants of HDF5 files (H5part, Xdmf, Pixie, Silo,
                Samrai, netCDF, Flash, Enzo, Chombo, etc., etc.) When opening a
                file, how does VisIt figure out which plugin to use? In
                particular, how do we avoid one poorly written reader plugin
                (which may be the wrong one for a given file) from preventing 
the
                correct one from being found. Its kinda a hard problem.

                Some of our discussion is captured here. . .

            http://www.visitusers.org/index.php?title=Database_Format_Detection

                Mark


                From: Hdf-forum <hdf-forum-bounces@xxxxxxxxxxxxxxxxxx
                <mailto:hdf-forum-bounces@xxxxxxxxxxxxxxxxxx>> on behalf of John
                Shalf <jshalf@xxxxxxx <mailto:jshalf@xxxxxxx>>
                Reply-To: HDF Users Discussion List 
<hdf-forum@xxxxxxxxxxxxxxxxxx
                <mailto:hdf-forum@xxxxxxxxxxxxxxxxxx>>
                Date: Wednesday, March 2, 2016 1:02 PM
                To: HDF Users Discussion List <hdf-forum@xxxxxxxxxxxxxxxxxx
                <mailto:hdf-forum@xxxxxxxxxxxxxxxxxx>>
                Cc: "netcdfgroup@xxxxxxxxxxxxxxxx
                <mailto:netcdfgroup@xxxxxxxxxxxxxxxx>"
                <netcdfgroup@xxxxxxxxxxxxxxxx
                <mailto:netcdfgroup@xxxxxxxxxxxxxxxx>>, Ward Fisher
                <wfisher@xxxxxxxx <mailto:wfisher@xxxxxxxx>>
                Subject: Re: [Hdf-forum] Detecting netCDF versus HDF5

                    Perhaps NetCDF (and other higher-level APIs that are built 
on
                    top of HDF5) should include an attribute attached to the 
root
                    group that identifies the name and version of the API that
                    created the file?  (adopt this as a convention)

                    -john

                        On Mar 2, 2016, at 12:55 PM, Pedro Vicente
                        <pedro.vicente@xxxxxxxxxxxxxxxxxx
            <mailto:pedro.vicente@xxxxxxxxxxxxxxxxxx>> wrote:
                        Hi Ward
                        As you know, Data Explorer is going to be a general
                        purpose data reader for many formats, including HDF5 and
                        netCDF.
                        Here
                        http://www.space-research.org/
                        Regarding the handling of both HDF5 and netCDF, it seems
                        there is a potential issue, which is, how to tell if any
                        HDF5 file was saved by the HDF5 API or by the netCDF 
API?
                        It seems to me that this is not possible. Is this 
correct?
                        netCDF uses an internal function NC_check_file_type to
                        examine the first few bytes of a file, and for example 
for
                        any HDF5 file the test is
                        /* Look at the magic number */
                           /* Ignore the first byte for HDF */
                           if(magic[1] == 'H' && magic[2] == 'D' && magic[3] == 
'F') {
                             *filetype = FT_HDF;
                             *version = 5;
                        The problem is that this test works for any HDF5 file 
and
                        for any netCDF file, which makes it impossible to tell
                        which is which.
                        Which makes it impossible for any general purpose data
                        reader to decide to use the netCDF API or the HDF5 API.
                        I have a possible solution for this , but before going 
any
                        further, I would just like to confirm that
                        1)      Is indeed not possible
                        2)      See if you have a solid workaround for this,
                        excluding the dumb ones, for example deciding on a
                        extension .nc or .h5, or traversing the HDF5 file to see
                        if it's non netCDF conforming one. Yes, to further
                        complicate things, it is possible that the above test 
says
                        OK for a HDF5 file, but then the read by the netCDF API
                        fails because the file is a HDF5 non netCDF conformant
                        Thanks
                        ----------------------
                        Pedro Vicente
                        pedro.vicente@xxxxxxxxxxxxxxxxxx
                        <mailto:pedro.vicente@xxxxxxxxxxxxxxxxxx>
                        http://www.space-research.org/
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