Editing Hierarchical Data Format (section)

==HDF5==
The HDF5 format is designed to address some of the limitations of the HDF4 library, and to address current and anticipated requirements of modern systems and applications. In 2002 it won an [[R&D 100 Award]].<ref>[http://www.rdmag.com/Awards/RD-100-Awards/2002/09/Flexible-Data-Management/ R&D 100 Awards Archives] {{webarchive|url=https://web.archive.org/web/20110104062241/http://www.rdmag.com/Awards/RD-100-Awards/2002/09/Flexible-Data-Management/ |date=2011-01-04 }}</ref>

HDF5 simplifies the file structure to include only two major types of object:
[[Image:HDF-Structure-Example.gif|thumb|right|HDF Structure Example]]
*Datasets, which are typed multidimensional arrays
*Groups, which are container structures that can hold datasets and other groups

This results in a truly hierarchical, filesystem-like data format.{{clarify|date=November 2018}}{{citation needed|date=November 2018}} In fact, resources in an HDF5 file can be accessed using the [[POSIX]]-like syntax ''/path/to/resource''. Metadata is stored in the form of user-defined, named attributes attached to groups and datasets. More complex storage APIs representing images and tables can then be built up using datasets, groups and attributes.

In addition to these advances in the file format, HDF5 includes an improved type system, and dataspace objects which represent selections over dataset regions. The API is also object-oriented with respect to datasets, groups, attributes, types, dataspaces and property lists.

The latest version of [[NetCDF]], version 4, is based on HDF5.

Because it uses [[B-trees]] to index table objects, HDF5 works well for [[time series]] data such as stock price series, network monitoring data, and 3D meteorological data. The bulk of the data goes into straightforward arrays (the table objects) that can be accessed much more quickly than the rows of an [[SQL]] database, but B-tree access is available for non-array data. The HDF5 data storage mechanism can be simpler and faster than an SQL [[star schema]].

{{example needed|date=November 2018}}

=== Feedback ===

Criticism of HDF5 follows from its monolithic design and lengthy specification.
*HDF5 does not enforce the use of [[UTF-8]], so client applications may be expecting ASCII in most places. 
*Dataset data cannot be freed in a file without generating a file copy using an external tool (h5repack).<ref>{{cite web|last1=Rossant|first1=Cyrille|title=Moving away from HDF5|url=http://cyrille.rossant.net/moving-away-hdf5/|website=cyrille.rossant.net|access-date=21 April 2016}}</ref>