The Data Access Protocol — DAP 2.0
This document defines the OPeNDAP Data Access Protocol (DAP), a data transmission protocol designed specifically for science data. The protocol relies on the widely used and stable HTTP and MIME standards, and provides data types to accommodate gridded data, relational data, and time series, as well as allowing users to define their own data types.
Recommendation to Endorse DAP 2.0 as a NASA Earth Science Data Systems Community Standard:
The Earth Science - Data Systems Working Group (ES-DSWG) Standards Process Group (SPG) recommends that DAP 2.0 (OPeNDAP is one implementation) be a community standard for transport of NASA science data. This specification was reviewed as ESE-RFC-004.
The SPG received considerable community feedback on the implementation and operational use of DAP in Earth science data systems and applications over the course of a two phase review. In January, the SPG recommended that RFC-004 be endorsed as a "draft standard" pending review of operational effectiveness. That recommendation was based on enthusiastic support represented by 19 direct responses to our request for comments on ease of implementation of DAP 2.0.
The SPG has now performed a review of responses to how the DAP performs in operational environments. This review consisted of a a general invitation to comment via email and one-to-one telephone interviews with representatives of organizations or projects that have implemented the specification. These sites represent a variety of operational experience, from sites with high volumes and large user communities, to smaller volume sites that serve more targeted communities. Experiences with DAP were generally highly positive. We reviewed operational experience with DAP at 15 sites.
Data Access Protocol (DAP), a data transmission protocol designed specifically for science data. The protocol relies on the widely used and stable HTTP and MIME standards, and provides data types to accommodate gridded data,relational data, and time series, as well as allowing users to define their own data types.
The DAP specification (ESE-RFC-004) and comments received by the Technical Working Group may be found at http://earthdata.nasa.gov/our-community/esdswg/standards-process-spg/rfc/esds-rfc-004
SWOT (Strengths, Weaknesses, Opportunities and Threats) Analysis:
- Enthusiastic Adoption: Many of the reviewers were quite enthusiastic about the prospect of wider use of the DAP.
- Clarity of Specification: In general, the specification was judged to be clear. Implementation experience was good. Many reviewers simply installed the reference server implementation called OPeNDAP. Many reviewers also used provided client applications.
- Significant use reported: Many reviewers reported that DAP is central to their organization's effective use of data or plans for interoperability.
- Cross platform/Cross Format Reviewers report that the have installed DAP on various platforms and that they use it with varied data sets.
- Completeness: Specification may not be complete for all applications (although it is a proper starting point). In particular several reviewers related reported difficulty in developing DAP clients or in modifying components in particular DAP implementations. These areas are beyond the scope of the specification.
- Fitness: There are some parts of the DAP that could be improved. These shortcomings sometimes resulted in decisions not to use DAP for particular purposes. All requests for enhancements to the specifications have been forwarded to the developers of DAP.
- Expanded availability of NASA data: According to reviewers, NASA data systems that use DAP would have enhanced potential for interoperability with (there may be others):
- Ocean.US Integrated Ocean Observing System (IOOS))
- Digital Library for Earth Systems Education
- US Joint Global Ocean Flux Study Data Management Office
- NOAA Operational Model Archive and Distribution System (NOMDAS) at the National Climatic Data Center"
- SouthEast Atlantic Coastal Ocean Observing System (SEACOOS) [part of IOOS]
- "the atmospheric science community, through the Unidata program at UCAR/NCAR"
- Good will: Additionally, several reviewers expressed that NASA use of DAP would demonstrate general openness to work with outside groups. Particular comments include:
-"this would demonstrate to the academic and research communities that NASA clearly wants them to be using their data.
- "It would be helpful if federal agencies(USGS, NWS, NOAA, NASA) made their data available via OPeNDAP"
- Technology Leadership: DAP is one of the technologies that is "in the mix" for interoperability in a number of efforts. There is strong interest in using DAP with other interoperability technologies that are currently in development or use by Earth science communities. These include most notably computation and storage grid technology components and in Open GIS Consortium (OGC) technologies. One reviewer stated:
"I think DAP 2.0 is an essential dataset transmission standard, and I believe it will evolve to become an even better dataset-transmission solution."
- Technical Obsolescence: There are other approaches to facilitate data transmission interoperability. Some of these may cause DAP to become obsolete. It is the Standard Process Group's opinion that this is a longer term threat and that future technical obsolescence is a threat common to almost all data systems practices. Current indications are that DAP is more likely to evolve as a particular science information system component within various systems interoperability approaches.
- Narrowed Focus: DAP is closely linked in many users' minds with NetCDF. This perception derives from historical association. There is a risk that, even though the specification is data format neutral, providers that use formats other than netCDF will be reluctant to implement DAP interoperability. We believe that endorsement by NASA will encourage these providers to participate.