AIS Using NcML

Introduction

This and the Aggregation Using NcML page go hand-in-hand. The essential idea is to use NcML as a syntax to describe both aggregations of data sets (e.g., HDF4 files) and ancillary information that should be added to a data set. The motivation for using NcML is to not invent a new syntax and instead build on an accepted one, maybe adding new features where we need them.

Straw man Design

this is a Straw man design, as the title suggests, so that there's something to bounce the use cases off of.

We will build a new handler, similar to the HTML Form interface or ASCII Data handlers that uses DDS/DAS/DDX and/or DataDDS/DataDDX objects from other handlers along with information in NcML files, to return a new response. This handler will combine information in a NcML file with information in another DAP object to return a new, modified, DAP object.

Simple example: Suppose you wanted to add the string attribute "color" with value "red" to a variable "temperature" in some data set. In NcML, this would look like:

<netcdf xmlns:nc="..."
        location="file:/.../data.nc">
    <variable name="red">
        <attribute name="color" type="string" value="red"/>
    </variable>
</netcdf>

And lets say this is stored in new_data.ncml

When the BES is asked to return the DAS for /.../new_data.ncml:

1. The NcML handler would open this file, parse it and see that the it contains new information for the data file file:/.../data.nc.
2. It will use the BES to get the DAS for file:/.../data.nc
3. It will then apply the changes in the new_data.ncml file
   1. first parse the variable element and find the named variable in the DAS initially returned by the BES
   2. See that the attribute color is to be added (or overwritten, using NcML's rules for applying this stuff)
4. Return the resulting DAS

This means that the the NcML file effectively defines a new data set. The data.nc data set is still available as before.

We will scrap the existing AIS code in libdap - it's just too far from anything we want.

Definitions

Data set

Anything that can be referenced by a DAP URL and will return the DAP responses when requested.

NcML

Syntax for ancillary data (attributes and variables) and aggregations used by the TDS