6.2 Data Access Protocol

The OPeNDAP Data Access Protocol (DAP) defines how an OPeNDAP client and an OPeNDAP server communicate with one another to pass data from the server to the client. The job of the functions in the OPeNDAP client library is to translate data from the DAP into the form expected by the data access API for which the OPeNDAP library is substituting. The job of an OPeNDAP server is to translate data stored on a disk in whatever format they happen to be stored in to the DAP for transmission to the client.

The DAP consists of four components:

1. An intermediate data representation for data sets. This is used to transport data from the remote source to the client. The data types that make up this representation may be thought of as the OPeNDAP data model.
2. A format for the ancillary data  needed to translate a data set into the intermediate representation, and to translate the intermediate representation into the target data model. The ancillary data in turn consists of two pieces:
   • A description of the shape and size of the various data types stored in some given data set. This is called the Data Description Structure (DDS).
   • Capsule descriptions of some of the properties of the data stored in some given data set. This is the Data Attribute Structure (DAS).
3. A procedure for retrieving data and ancillary data from remote platforms.
4. An API  consisting of OPeNDAP classes and data access calls designed to implement the protocol,

The intermediate data representation and the ancillary data formats are introduced in Section 6.3 and Section 6.4, below. The steps of the procedure are outlined in Section 5.1, and the OPeNDAP core software is described in the The DODS Toolkit Programmer's Guide.