1. Extending the NetCDF Supported Data Formats using a Dispatch Layer
Dr. Dennis Heimbigner
Unidata netCDF Workshop
October 26, 2012

2. Overview Extensible netCDF Architecture Dispatch Table Selection Rules
Adding a New Dispatch Table
Questions

3. Extensible netCDF Architecture
The idea is to add a layer of indirection between the client and the code that implements the API
Client
Program
nc_open() …
nc_get_var()
nc_close()
netCDF API
Wrapper
Procedures
nc_open()
nc_get_var() …
nc_close
netCDF-4
Dispatch Table
NC4_open()
NC4_get_var() …
NC4_close

4. Extensible netCDF Architecture (cont.)
Actual semantics of a netCDF API call will be determined by the choice of dispatch table
Currently support:
netCDF-3 (classic)
netCDF-4 (enhanced)
DAP => netCDF-3
CDM-Remote => netcdf-4
Proposed additions: EOS, DAP4
Allow for user-community defined extensions
Note that HDF-5 is currently implementing something similar.

5. Dispatch Table Selection Rules
Key problem: when the client calls nc_open() or nc_create(), which dispatch table do we use?
The decision can be based on the following pieces of information:
Path – file path, url, or something else
URL markers – e.g. <url>#cdmremote
Mode argument – a specific flag (e.g. NC_NETCDF4)
Contents of existing file (when using nc_open)
NC_set_dispatch_override() – to force a choice
Environment variables (not currently used)
.rc file (not currently used)

6. Adding a New Dispatch Table
Briefly, the steps are as follows.
Build a library that implements the netcdf-3 or netcdf-4 API in terms of your Data format
Implement a dispatch table that maps the netcdf API to the equivalents in your library
All dispatch table entries must be supported, even if only to return an error.
Modify the nc_create and nc_open wrappers to determine when to use your dispatch table

7. Questions?

8. Dispatch Architecture
Path
Mode flags
Path Markers
Existing File (open)
Client
Program
nc_open() …
nc_get_var()
nc_close()
netCDF API
Wrapper
Procedures
nc_open()
nc_get_var() …
nc_close
Dispatch
Table
For e.g.
Netcdf-4