2.1 How OPeNDAP Finds Data

Once linked to the OPeNDAP libraries, an OPeNDAP client created from an existing program will work exactly as before when run using local files. However, a user can also specify an OPeNDAP Uniform Resource Locator (URL) to indicate some data file on a remote host machine. When the program receives this URL, the OPeNDAP libraries will recognize it as remote data, and issue a network request for the data. If a user has also installed an OPeNDAP server on the local machine, then local data may be accessed either through their local filenames or their OPeNDAP URL.

A URL is simply a unique name for some Internet resource. The figure 2.1 shows the parts of a typical OPeNDAP URL.

>dncview http://dods.gso.uri.edu/cgi-bin/nph-nc/data/fnoc1.nc.das
   ^     ^      ^                        ^      ^    ^        ^
   |     |      |                        |      |    |        |
Program  |      |                        |      |    |        |
Protocol--      |                        |      |    |        |
Machine Name-----                        |      |    |        |
Server------------------------------------      |    |        |
Directory----------------------------------------    |        |
Filename----------------------------------------------        |
URL Suffix-----------------------------------------------------

The parts of the URL are:

protocol

The protocol of an Internet request may be thought of as the kind of conversation the client expects to have with the target machine. For example, a web browser like Netscape Navigator wants to find a server that can return hypertext documents, while an ftp client wants to find a server that can understand file transfer requests. A web browser equipped to display hypertext documents will specify http as the protocol for its conversation, and hope that the target machine has an httpd  daemon listening.

host

The host name in a URL is simply the Internet address of the host machine running whatever server can reply to the specified protocol.

server

A special feature of the httpd server process is that it may be configured to execute Common Gateway Interface (CGI) programs upon receipt of a properly specified URL. This is used, for example, by Internet search engines that ask a user to fill out a form. The CGI specification will be specific to the server in question, and the part of the URL that follows the CGI name is passed to the CGI upon invocation. This data may include a file name, but it may as easily be some arbitrary string of instructions. The OPeNDAP server is simply a set of CGI scripts executed on demand by the httpd server. Here, the OPeNDAP server is represented by a CGI script called nph-nc.

filename

If a CGI is not specified, the part of the URL after the host name is simply the name of a file that is to be returned to the inquiring browser. If a CGI is specified, the file is given to the program as its argument.

URL suffix

If you are issuing an OPeNDAP request from a non-OPeNDAP client, such as a web browser, you can specify the type of request by appending a suffix to the URL. Different suffixes demand different services from the server. The different services are listed in Section 2.2. If you are using OPeNDAP from an OPeNDAP client, or a client program adapted to use the OPeNDAP DAP library, you do not need to use a URL suffix. For example, to use OPeNDAP from Matlab, with the Matlab GUI or command-line clients, you do not need to use a suffix. To use OPeNDAP from a simple web browser like Netscape Navigator, you will need to use a suffix.

The URL in figure 2.1 shows a client request to the httpd server on the machine dods.gso.uri.edu, for a netCDF dataset (specified by the nph-nc in the cgi-bin directory) contained in a file called fnoc1.nc. Upon receiving this URL, the httpd server executes the specified OPeNDAP server module (nph-nc), which retrieves the file is in a directory called data relative to wherever the httpd server looks for its data⁶.

OPeNDAP URLs can get somewhat more complicated than this simple description. In particular, they can contain "constraint expressions" that limit a request to data satisfying a set of conditions, and they can contain requests to specific OPeNDAP services, besides the data delivery service suggested here. Constraint expressions are described in more detail in Section 4.1, while the array of services provided by OPeNDAP servers are described in Section 2.2.

• 2.1.1 Security