Digital Archive Directions (DADs) Workshop

(A part of the ISO Archiving Workshop Series)


Position Paper

Digital Archive Directions (DADs) Workshop

DATE: June 22-26, 1998

HOST: The National Archives and Records Administration
Archives II
8601 Adelphi Road
College Park, MD 20740-6001



1. Identification of Proposed Topic [Required]

1.1 Title

The Planetary Data System Object Access Library

1.2 Contributor(s)

J. Steven Hughes
MS 171-264
4800 Oak Grove Dr.
Pasadena, CA 91109
818.393.3405 (fax)

1.3 Description of Proposed Project

The Object Access Library (OAL), developed by the Planetary Data System (PDS) in collaboration with the Laboratory for Space Physics at the University of Colorado, Boulder, CO, is the standard software library available for accessing the five terabyte PDS archive. The library has two primary components, the OAL proper which allows access to the science data and the Label Library Light (L3) which allows access to the meta-data in the PDS labels. This software is written in ANSI C, has been ported to over seven hardware platforms, and is distributed as source code.

The OAL uses descriptive information contained in the PDS label to manipulate the science data at three levels. At the stream layer, the OAL handles physical record differences such as those associated with stream record delimiters. At the structure layer the OAL has the ability to read and interpret data that is represented in formats not compatible with the host machine. For example, when running the OAL on a machine architecture that uses IEEE_REAL, data stored as VAX_REAL would be converted. Finally in the object layer, the OAL allows the science data to be accessed using object-based function calls.

The flexible architecture of the OAL has allowed the inclusion of standard routines for processing specialized representations. For example, the decompression routines for JPG, GIF, and other standard compression schemes have been included. PDS labels have also been created to describe non-PDS data formats such as FITS and VICR labeled images. Using these "detached" PDS labels to describe non-PDS formatted data, standard OAL function calls can still be used to access the data.

The L3 parses PDS labels creating a parse tree and provides access to the information using standard function calls. It is distributed with the OAL but is often used stand alone for accessing PDS label information.

As a standard library providing access to a diversity of data types and representations, the OAL is a successful example of a support service that will be required for any science digital archive. While the OAL is specific to the PDS standards architecture, it is an important example of a flexible architecture with a standard object-based interface and an ability to handle diverse data types. The architecture of the OAL could easily be extended beyond the PDS. This is illustrated by the current work to develop JAVA wrappers to enable JAVA applet access to the archive and the continued inclusion of libraries for non-PDS standard formats.

1.4 Justification

The Object Access Library provides users with a programmatic interface to the Planetary Data System Archive. It allows an object-based API for the search and retrieval of archived meta-data and science data and handles diverse data types and representations.

1.5 Definitions of Concepts and Special Terms

The PDS uses ODL/PVL to both describe a meta-data model and to capture meta-data. These languages have the ability to group attributes to describe objects in the archive. The OAL represents the process (methods) associated with the objects.

1.6 Expected Relationship with OAIS Reference Model


The PDS Object Access Library represents a standard protocol for the search and retrieval of meta-data and science data from a digital archive.



2. Scope of Proposed Standard [Desired]

2.1 Recommended Scope of Standard

2.2 Existing Practice in Area of Proposed Standard

2.3 Expected Stability of Proposed Standard with Respect to Current and Potential Technological Advances

Wider Views

Overview of the DADs Workshop
Overview of US Effort
Overview of International Effort


A service of NOST at NSSDC. Access statistics for this web are available. Comments and suggestion are always welcome.

Author: J. Steven Hughes ( +1.818.354.9338
Curator: John Garrett ( +1.301.286.3575
Responsible Official: Code 633.2 / Don Sawyer ( +1.301.286.2748
Last Revised: May 11, 1998, Steve Hughes (May 26, 1998, John Garrett)