Hi Steve,
Thank you for your continued engagement and responses.
Looks like CF hasn't the appetite for group hierarchies anytime soon.
I'll report the lessons learned in this fruitful discussion to our
NASA WG next week. My "concluding" (for now) remarks are interleaved.
Best,
Charlie
Le 25/09/2013 12:33, Steve Hankin a ?crit :>
> On 9/24/2013 9:45 PM, Charlie Zender wrote:
>> It is not my place to determine whether there is a consensus, or how
>> close we are, but it's clear to me there is no consensus yet. Bryan
>> Lawrence, Steve Hankin, Jonathan Gregory, Karl Taylor, and Philip
>> Cameron-Smith are not "on board". I hope they will speak-up and say if
>> they concur that maintaining the status quo (flat files) is best
>> (period), or whether they do wish to extend CF to hierarchies
>> (starting now), or the additional information they would need to decide.
>
> Hi Charlie et. al.,
>
> Since you have asked .... I have heard two points that seemed to
> bolster Bryan's pov that the multi-model use case is "great but not
> compelling". (See a more positive spin at the end.)
>
> 1. file size. Model outputs today are typically too large for even a
> single variable from a single model to be packaged in a single
> file. Addressing a model ensemble multiplies the size barrier by
> the ensemble size, N. Thus the use of groups to package a model
> ensemble applies only for the cases where user is interested in
> quite a small subset of the model domain, or perhaps in
> pre-processed, data-reduced versions of the models. A gut-estimate
> is that single file solutions, like netCDF4 groups addresses 25% or
> less of the stated use case. We could argue over that number, but
> it seems likely to remain on the low side of 50%. (Issues of
> THREDDS-aggregating files bearing groups also deserve to be
> discussed and understood. What works? what doesn't?)
Your remarks seem most applicable to "enterprise datasets" like CMIP5.
CMIP5 is fairly unique. It garners the most press, and is well-known.
There are numerous models of smaller scale output size that few have
heard of whose outputs strive for and benefit from CF-compliance.
I am unfamiliar with THREDDS support for netCDF4 features.
The HDF Group supports an HDF5 handler for Hyrax 1.8.8:
http://hdfeos.org/software/hdf5_handler/doc/install.php
Someone more knowledgable please say whether/how-well TDS integrates
group capabilities.
> 2. The problems of the "suitcase packing" metaphor were invoked time
> and again, further narrowing the applicability of the use case. The
> sweet spot that was identified is the case of a single user desiring
> a particular subset from a single data provider. Essentially a
> multi-model ensemble encoded using netCDF4 groups would offer a
> standardized "shopping basket" with advantages that will be enjoyed
> by some high powered analysis users.
My impression of suitcases is perhaps more graduated than others'.
Straightforward suitcases work on local machines.
Anyone can do it with free software right now:
http://nco.sf.net/nco.html#ncecat
http://nco.sf.net/nco.html#ncdismember
Let us not confound the issue of whether it works with whether
people know how to do it. Much resistance to software change stems
from abhorrence of reading the ****ing manual. There is little
difference between running unzip and ncdismember on a file other than
people are familiar with the former not the latter.
Unless there are "external metadata" involved, it is trivial (as in
already demonstrated, at least to my satisfaction) to pack and unpack
a netCDF4 group suitcase with a collection of flat netCDF3 files.
ncecat and ncdismember do this without loss of information.
So simple suitcases work on local machines.
Servers can leverage that, or re-implement, as they see fit.
In either case, it's more a logistical than a conceptual barrier.
More complex information like "ensemble axes" require conventions.
Conventions could enable use-cases well-beyond simple suitcases.
In the meantime, it would be helpful if CF reserved "ensemble" and
"group" in attribute names for future use pertaining to netCDF4 groups,
rather than some other kind of "group" based on flat files.
Doing so might help reduce conflicts with informal ensemble conventions
that NCO will test.
> For this narrower use case I couldn't help asking myself how the
> cost/benefit found through the use of netCDF4 groups compares with
> the cost/benefit of simply zip-packaging the individual CF model
> files. There is almost no cost to this alternative. Tools to pack
> and unpack zip files are universal, have UIs embedded into common
> OSes, and offer APIs that permit ensemble analysis to be done on the
> zip file as a unit at similar programming effort to the use of
> netCDF4 groups. Comprehension and acceptance of the zip
> alternative on the part of user communities would likely be
> instantaneous -- hardly even a point to generate discussion. Zip
> files do not address more specialized use cases, like a desire to
> view the ensemble as a 2-level hierarchy of models each providing
> multiple scenarios, but the "suitcase" metaphor discussions have
> pointed out the diminishing returns that accrue as the packing
> strategy is made more complex.
Yes, zipped files are a trusty standby. No argument there.
Users avoid installing and learning new software. Until they don't :)
My sense is that users will goad the providers to the "tipping point".
Once users see enough well-documented examples of suitcases, they will
want and ask data providers to give them suitcases, especially once
they realize many suitcases need not ever be unpacked.
> The tipping point for me is not whether a particular group of users
> would find value in a particular enhancement. It is whether the overall
> cost/benefit considerations -- the expanded complexity, the need to
> enhance applications, the loss of interoperabilty etc. versus the
> breadth of users and the benefits they will enjoy -- clearly motivate a
> change. My personal vote is that thus far the arguments fall well
> short of this tipping point. But maybe there are other use cases to be
> explored. Perhaps in aggregate they may tip the cost/benefit analysis.
> What about the "group of satellite swaths" scenario? -- a feature
> collection use case. AFAIK CF remains weak at addressing this need thus
> far. (If we pursue this line of discussion we should add the
> 'cf_satellite' list onto the thread. That community may have new work
> on this topic to discuss.)
Yes, CF has little "market share" for storage of satellite swaths,
granules, or so-called object/region references, all HDF-EOS mainstays.
Some well-documented solutions exist:
http://wiki.esipfed.org/index.php/NetCDF-CF_File_Examples_for_Satellite_Swath_Data
Our NASA WG will try to identify such netCDF-API options for storing
multiple satellite granules that are a compromise of convenience
(multiple granules/groups per file) and CF-compliance.
Our users deserve no less.
cz
> - Steve
--
Charlie Zender, Earth System Sci. & Computer Sci.
University of California, Irvine 949-891-2429 )'(
Received on Sun Sep 29 2013 - 21:46:15 BST
