ServerDispatchOperations

Now that Hyrax has been released it has become clear that we need to publish a paper that describes the correct behavior of an OPENDAP server. This should include the details of:

• Dispatch - How the various request URL's get assessed and which responses are sent.
• Errors - Which errors should be associated with which dispatch paths.

URL Dispatch Operations

OPeNDAP servers provide an interface through which clients can access data held on there server. The server can provide sub-setting and, in some cases, additional preprocessing operations on the requested data. The protocol through which the OPeNDAP server s provide these data services is called the Data Access Protocol (DAP). The current version of the DAP is version 2, so The DAP is usually specified as DAP2.

In addition to DAP2 responses, most OPeNDAP servers provide a number of additional services such as:

• A navigation interface for browsing the data storage hierarchy.
• File access.
• Version reporting.
• Help pages and other documentation.
• THREDDS catalog views of the data system.

Dispatch is the process in which:

• An OPeNDAP server receives a request (that comes in the form of a URL).
• The server evaluates the URL to determine what type of response the URL indicates the server should provide.
• The server provides the requested response if possible, or returns the appropriate error (in the appropriate format) if the request cannot be fulfilled.

Terms

For the sake of clarity the following definitions will be used in this document:

URL

The URL

Constraint Expression (CE)

Everything following the ? character in the URL

Selection

Projection

The part of the CE prior to the first & character. This is where individual variables from a dataset can be requested, or if no variables are specified then all of the varaibles will be requested.

Don't count on the consistent use of these terms elsewhere in the OPeNDAP literature:

Special Requests

Hyrax and other servers may support some special requests. These may be a secure server administration interface, help information, server status, or system properties. There is no clearly defined set of special responses, and their implementation is left up to the various server developers.

Hyrax currently supports:

• help - The URL http://your.host:port/opendap/help will take you to an help web page.

DAP2 Service

DAP2 service requests are identified by analysis of the URL as follows:

If the URL