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1 File Servers

The DODS and OPenDAP projects have used the OPeNDAP FreeForm ND Data Handler to present a catalog of data files to the world as a single dataset. In many ways this was

a very successful system, providing catalogs for multi-granule datasets that could be searched by date and time. However, the OPeNDAP project has decided (winter 2006) to adopt the THREDDS xml-based catalog system developed at Unidata, Inc. The remainder of this chapter describes the 'file servers' that can be built using the FreeForm data handler. Even though we feel it's best to adopt the THREDDS catalogs, there are good reasons to keep existing catalog servers running and to build new catalogs as a stop-gap measure to

support existing client software.

Normally, in the OPeNDAP argot, a "dataset" is contained in a single file on a disk. However, this paradigm is often broken by large datasets that may contain many thousands or tens of thousands of data files. The OPeNDAP file server is a way to make these dicrete datasets appear to be a single large dataset.

The OPeNDAP file server is an OPeNDAP server that returns a URL or set of URLs in response to a query containing selection variables. For example, a dataset organized by date and geographic location might provide a file server that allowed you to query the dataset with a range of dates and longitudes. This fileserver would return a list of one or more URLs corresponding to files within that dataset that fell within the given range.

1.1 The Problem

Consider the following (imaginary) list of files: ...

These appear to be a set of netCDF files, arranged by date\footnote{A serial date, with a year and the day of the year, expressed in an ordinal number from 1 to 365 or 366.}

If you want data from the first week of January, 1998, it is fairly clear which files to request. However, the OPeNDAP server provides no way to request data from more than one file, so your request would have to be split into 7 different requests, from to This could be represented as a set of seven DODS URLs:


But what if you then uncover another similar dataset whose data you want to compare to the first? Or what if you want to expand the inquiry to cover the entire year? Keeping track of this many URLs will quickly become burdensome.

What's more, another similar dataset could be arranged in two different directories, 1997 and 1998, each with files:

and so on. Now you have to keep track of two large sets of URLs, in two different forms. But you could also imagine files called:



That is, the number of possible sensible arrangements may not, in fact, be infinite, but it may seem that way to a scientist who is simply trying to find data.

1.2 The OPeNDAP File Server Solution

To create a system that allows data providers to assert a degree of uniformity over wildly variable dataset organizations, OPeNDAP provides for the installation of an OPeNDAP \new{file server}. The file server is a server that provides access to a special dataset, containing associations between the names of files within a dataset and some "selectable" data values.

1.2.1 Selectable Data

The concept of \new{selectable data} requires some explanation. This is used to indicate the data variables you might ordinarily use to narrow your search for data in the first pass at a dataset.

For geophysical data, the selectable data is often the time and location of the data, since typical searches for data often begin by specifying a part of the globe that bears examining, or a date of some event. For other types of data, other data variables will seem more appropriate. Model data, for example, which has no real location or time, might be arranged by the parameters that varied between runs.

A comprehensive definition of selectable data has so far eluded the OPeNDAP group, but there are some guidelines, albeit fairly vague ones:

  • The selectable data is generally not recorded within each

data file. However, the selectable data may often include a range summarizing some of the data within each file.

  • The selectable data should help a user decide whether a

particular data file in a dataset is useful. A temperature range might not be as useful as a time range, since data searches more often start with time. (Both would presumably be still more useful, but there is a trade-off between the utility of the file server and the time spent maintaining it.)

1.2.2 What It Looks Like

Consider again the set of data files shown in ( fs,problem). We could associate each one of these files with a date, and this would provide the rudiments of a file server if we then serve that data with an OPeNDAP server such as the OPeNDAP FreeForm ND Data Handler .

1997/360 http://opendap/dap/data/
1997/361 http://opendap/dap/data/
1997/362 http://opendap/dap/data/
1997/363 http://opendap/dap/data/
1997/364 http://opendap/dap/data/
1997/365 http://opendap/dap/data/
1998/001 http://opendap/dap/data/
1998/002 http://opendap/dap/data/
1998/003 http://opendap/dap/data/
1998/004 http://opendap/dap/data/
1998/005 http://opendap/dap/data/
1998/006 http://opendap/dap/data/
1998/007 http://opendap/dap/data/
1998/008 http://opendap/dap/data/
1998/009 http://opendap/dap/data/
1998/010 http://opendap/dap/data/

This list represents a set of DAP URLs, each identified by a date, given as a year and a serial day. The files appear to be netCDF format files, served by an OPeNDAP netCDF server, but that is not important for this discussion.

To use the OPeNDAP FreeForm ND Data Handler for your file server, you could use a format description file with an input section like this:

ASCII_input_data "File Server Example Input"
year 1 4 short 0
serial_day 6 8 short 0
DODS_Url 10 46 char 0