2.2 The OPeNDAP Services

Up to now, we have treated the OPeNDAP server as if it has only one service: providing data to clients who ask for it. It is true that this is the most important service a server provides. However, it is also true that the server provides several other services besides that. In fact, fulfilling a request for data actually requires three separate requests from the client, using three different services of the OPeNDAP server.

The services requested from an OPeNDAP server are specified in a suffix appended to the URL described in figure 2.1. Depending on the suffix supplied, the server will provide one of these services:

Data Attribute

This service returns the entire data attribute structure for the given dataset. This is a text file describing the attributes of each data quantity in that dataset. (See Section 6.4.2 for more information about data attributes.) This service is activated when the server receives a URL ending with .das.  

Data Descriptor

This service returns the entire data descriptor structure for the given dataset. This is a text file describing the structure of the variables in the dataset. (See Section 6.4.1 for more information about data descriptors.) This service is activated when the server receives a URL ending with .dds.  

OPeNDAP Data

This service returns the actual data requested by a given URL. This is not a text file, but is encoded as a Multipurpose Internet Mail Extensions (MIME) document. This service is activated when the server receives a URL ending with .dods  

ASCII Data

This service returns an ASCII representation of the requested data. This can make the data available to a wide variety of browser programs. This service is activated when the server receives a URL ending with .asc or .ascii.  

WWW Interface

When the server receives a URL ending in .html, it produces an HTML form containing information from the dataset that you can use to construct a sensible URL with which to request OPeNDAP data. The WWW Interface is also triggered when the OPeNDAP server receives a URL that references a directory instead of a file.  

Information

This service returns information about the server and dataset, in human-readable HTML form. The returned document may include information about both the data server itself (e.g. server functions implemented), and the dataset referenced in the URL. The server administrator determines what information is returned in response to such a request. This service is activated when the server receives a URL ending with .info. See Section 5.2.3 for more information about how to configure the information service.  

Version

This service returns the version information for the OPeNDAP server software running on the server. This service is triggered by a URL ending with .ver.  

Help

This service returns some help text in response to an improperly specified URL. This service is triggered by a URL ending in any suffix that is not recognized by the OPeNDAP server.  

NOTE: A request for data from an OPeNDAP client will generally make three different service requests, for data attributes, data descriptors, and for data. The prepackaged OPeNDAP clients do this for you, so you may not be aware that three requests are made for each URL. That is, an OPeNDAP client may accept an OPeNDAP URL specifying some data, such as the one shown in figure 2.1. In this case, the OPeNDAP client library (such as nc-dods) will accept the input URL, and append the different suffixes to that URL, making three distinct requests to the OPeNDAP server.

• 2.2.1 WWW Interface