1. Accessing Remote Datasets through the netCDF interface.
Dr. Dennis Heimbigner
Unidata netCDF Workshop
October 28, 2010

2. Overview What is OPeNDAP and DAP? Goal: Access to Remote Data
The netCDF-DAP Integration
Client-Server Architecture
Specifying a DAP Data Source
General URL Format
Debugging
Build and Install NETCDF+DAP
Demo
Questions

3. What is OPeNAP and DAP?
DAP is a widely supported protocol and standard data format for accessing remote data
Standard is maintained by the OPenDAP organization(
DAP was expressly designed to serve as intermediate format for accessing a wide variety of data sources
The terms “DAP” and “OPeNDAP” will be used interchangeably in this talk

4. Goal: Access to Remote Data
Unidata has merged OPeNDAP and netCDF
This allows transparent remote access to DAP Data Servers through the netCDF API
Remote access allows the netCDF library to retrieve data that is stored on other machines in the internet.
Note! Only the minimal amount of needed data will be accessed
=> DAP access is faster than e.g. FTP

5. The netCDF-DAP Integration
Unidata has integrated the DAP protocol into the standard netCDF distribution.
It replaces the OPeNDAP provided libnc-dap library
It closely mimics the original libnc-dap translation
It is implemented completely in C,
versus libnc-dap, which is C++
=> You can use it with FORTRAN

6. Client-Server Architecture
DAP data access is analogous to accessing a web page through a web browser
Web Browser
URL
Request
Web
Server
Internet
Explorer
HTML (Web Page)
Response
DAP Client
(DAP) URL
Request
netCDF
Library
DAP
Server
Application
Code
(e,g. ncdump
DAP
Library
DAP Formatted
Data Response

7. Specifying a DAP Data Source
A DAP data source is specified using a URL that refers to the DAP server containing that data
Used in place of a file name in e.g. ncdump
Example:
Beware: Shell may require quotes around URL
“ dods/dts/Drifters”

8. Debugging When things go wrong it is usually a problem with the server
E.g. bad url, the server is down, or access rights
Example: ncdump "
First symptoms:
“NetCDF: Malformed or inaccessible DAP DDS”
=> Server sent back non-dap response
Step 1: Try again with the “-h” flag
ncdump -h
Ok => problem is the data, not the meta-data

9. Debugging (cont.) Step 2: Get more information
ncdump -h “[log][show=fetch]
[log] causes extra information to be printed
[show=fetch] shows which files are being accessed from the server
Step 3: Access the url in your web browser
Requires changing the url a bit.
[log][show=fetch]
=>

10. Debugging (cont.)
Step 5: Contact the site you are trying to access and send them the output from, especially, step 3.
Step 4: If the problem is not obvious at that point, then collect the information from the previous steps and send it to Unidata support.
BTW: the problem in this case is actually an access rights problem.
Other typical problems:
Asking for too much data
Asking for a variable that does not exist (usually a typo)

11. Building netCDF with DAP Support
DAP support is enabled by default
Use –disable-dap to turn it off
DAP requires the curl library (libcurl)
If your ./configure fails because it cannot find libcurl, then you must specify
--with-curl=<path to curl installation>
Similar to the way you specify HDF5
To test thoroughly, add the ./configure flag
--enable-dap-remote-tests

12. DEMO http://test.opendap.org:8080/dods/dts http://test.opendap.org

13. Questions?

14. Demos Sites http://test.opendap.org:8080/dods/dts/Drifters