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Part III. Appendix 2: Data TransportAnnex C: OPeNDAP/NVODSFor several years, a data exchange network, the Distributed Oceanographic Data System (DODS),has connected scores of oceanographic data servers and an even greater number of clients. Becausethe data access protocol underlying this network is discipline-neutral and has begun to be adoptedby groups in other disciplines, a nonprofit corporation called the Open source Project for a NetworkData Access Protocol (OPeNDAP) has been formed to maintain and evolve the data accessprotocol. The overlying features that are specific to the exchange of oceanographic data have beenencapsulated in the National Virtual Oceanographic Data System (NVODS). Together, OPeNDAP/NVODS is for all practical purposes equivalent to DODS.In the following, “OPeNDAP” refers to OPeNDAP’s data access protocol itself.THE OPENDAP DATA MODELOPeNDAP’s task is to ensure that all data in numeric form can be interchanged between arbitrarydata repositories and users. It does not assume the task of exchanging, for example, multimedia(images, video, audio, etc.). OPeNDAP was designed based upon faith that all numeric data storageformats could be replicated and all client application data needs could be met by using a three-partinformation transfer from server to client:• the DDS, which would express the structure of the numeric data, that is, syntactic metadata,• the DAS, which would contain all relevant semantic metadata, configured according to the structureexpressed in the DDS, and• the DataDDS, which would contain the numeric data itself in the linear form generated by adepth-first traversal of the structure expressed in the DDS.Thus, the success of OPeNDAP depended upon the adequacy of the DDS to express any arbitrarydata storage scheme.Acting on the supposition that it was unlikely to encounter a data storage format that could notbe expressed in a modern programming language, and since C, FORTRAN, and Lisp data declarationsshould be able to express all the likely possibilities for data storage format components, theOPeNDAP DDS provides 13 data structures (eight simple types and five compound types) analogousto the ones in these languages:• SIMPLE TYPES• 8-bit unsigned integers; characters• 16-bit signed integers• 16-bit unsigned integers• 32-bit signed integers• 32-bit unsigned integers185