Re: [h5md-user] [h5md-commit] [SCM] UNNAMED PROJECT branch, master, updated. c234149781bfa9f5dc81aa139038e7dc6689b64d

[Pierre de Buyl]

Hi,

On Wed, May 29, 2013 at 03:01:57PM +0200, Felix Höfling wrote:
> Am 29.05.2013, 13:48 Uhr, schrieb Pierre de Buyl
> <address@hidden>:
> 
> >I would like to make the "particles" attribute optional. It is
> >useless in many
> >cases.
> 
> We should try to minimise the number of optional items and thus the
> variants a data group can have. Otherwise evaluating the file
> becomes really difficult since one has to check every time whether a
> certain piece of information is present or not. The decision whether
> a certain piece of data is present or not should happen at the level
> of the data groups.

HDF5 is quite powerful with respect to this kind of check. We should not make
optional something that has to be present anytime, but we should not refrain
from making optional data optional.

> Concerning the "particles" attribute: it does not hurt to store the
> information (just a few bytes), but one is in real trouble if one
> needs it and it is missing. And I think it is good practice to store
> some statistical information along with averaged data.

My concern is that this attribute is not always needed and may even be
misleading. Whether in experiments or simulations, not every observable is
associated to a number of particles. A few examples:
  - A quantity associated with a thermostat, such as a program for the
    temperature. 
  - State of a PRNG. This is in my opinion a very valid usecase. If a code uses
    H5MD, it should be able to store a maximum amount of data in the H5MD file.
    As the state of a PRNG depends of the time and can be used for
    checkpointing, it is very well suited for the observables group.
  - Data about simulated or measured actuators or probes.

Sticking a mandatory "particles" attribute makes no sense for those usecases. As
the need for it is frequent I still think that it should be a reserved and
optional attribute name.

Pierre