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6 The OPeNDAP FreeForm ND Data Handler
The OPeNDAP FreeForm ND Data Handler is a OPeNDAP server add-on that uses FreeForm ND to convert and serve data
in formats that are not directly supported by DODS servers. Bringing
FreeForm ND's data conversion capacity into the OPeNDAP world widens data
access for DAP2 clients, since any format that can be described in
FreeForm ND can now be served by the OPeNDAP data server.
Like all DAP2 servers, the OPeNDAP FreeForm ND Data Handler responds to client requests for data
by returning either data values or information about the data. It
differs from other DAP2 servers because it invokes FreeForm ND to read the
data from disk before serving it to the client.
The following sequence of steps illustrates how the OPeNDAP FreeForm ND Data Handler works:
- A DAP2 client sends a request for data to a OPeNDAP FreeForm ND Data Handler. The request
must include the name of the file that contains the data, and may
include a constraint expression to sample the data.
- The OPeNDAP FreeForm ND Data Handler looks in its path for two files: a data file with the
name sent by the client, and a format definition file to use with
the data file. The format definition file contains a description of
the data format, constructed according to the FreeForm ND syntax.
- The server uses both files in invoking the FreeForm ND engine. The
FreeForm ND engine reads the data file and the format file, using the
instructions in the latter to convert the former into data which is
then passed back to the OPeNDAP FreeForm ND Data Handler.
- On receiving the converted data, the OPeNDAP FreeForm ND Data Handler converts the data
into the DAP2 transmission format. The conversion may involve some
adjustment of data types; these are listed in
Section 6.2. The server also applies any constraint
expressions the client sent along with the URL.
- The server then constructs DDS and DAS files based on the format
of the converted data. If the server has access to DDS and DAS
files that describe the data, it applies those definition before
sending them back to the client.
- Finally, the server sends the DDS, DAS, and converted data back
to the client.
For information about how to write a FreeForm ND data description, refer to
the Chapter 3 for sequence data and
Chapter 4 for array data.
For an introduction to DAP2 and to the OPeNDAP project, please refer
to The OPeNDAP User Guide.
Tom Sgouros and James Gallagher, 2006-02-12