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[THREDDS #EMH-756335]: Questions regarding Dataset Query Capabilities Services



Hi Sargis,

Sargis Panosyan wrote: 
> I recently began looking at the DQC spec found at
> http://www.unidata.ucar.edu/projects/THREDDS/tech/dqc/DqcSpec.html
> while trying to find out if THREDDS supports any sort of dataset
> querying functionality. Digging deeper and trying to enable it on
> our test servers, I got an error message telling me I need to write
> a specific dqcHandler. Although searching on Google led me to some
> dqcHandler java files, I wasn't able to find any solid
> documentation on writing a dqcHandler, how it all fits together,
> and what I need to do in order to get DQC working with some of our
> data. So, here are some questions that I have:
> 
> 1) How does DQC work?
> 2) What is the granularity of the search: individual datasets,
> multiple datasets, a catalog, multiple catalogs? Is the search
> granularity something configurable or controllable through the
> handler?
> 3) Is there documentation on writing dqcHandlers?
> 4) What are the steps I need to take to make our data at PODAAC
> available through the THREDDS DQC service?

The DQC specification was never widely adopted and never really made it beyond 
an experimental stage. The specification and the software we developed around 
it have not been actively developed in several years. Our focus has been more 
on aggregation and on conceptual models of various data types and collection 
types. Along with this has been some work on querying dataset collections but 
at a level more specific to particular collection types rather than in a 
general way that DQC was trying to do.

> Also, does THREDDS currently support dynamic aggregation of datasets?
> To clarify, is there a way to aggregate datasets on-the-fly based on
> user-defined constraints received from web based forms for example?

I'm not sure if I'm understanding your question correctly, so I'll give two 
answers.

1) Yes, the THREDDS Data Server (TDS) can handle requests for subsets of a 
virtual dataset that behind the scenes is stored as a collection of datasets. 
The extraction of data from the appropriate datasets in the collection is done 
on-the-fly.

2) No, dataset aggregations must be specified in the TDS configuration files 
before they become available to users. But once available, users can constrain 
their request to indicate the desired subset of the aggregated dataset.

Here are two links on aggregation, the first somewhat general, the second more 
specific to use in the TDS:

http://www.unidata.ucar.edu/software/netcdf/ncml/v2.2/Aggregation.html
http://www.unidata.ucar.edu/projects/THREDDS/tech/tds4.2/tutorial/NcML.htm

> I would greatly appreciate any sort of clarification, documentation
> or examples that you can provide to help me exploit the capabilities
> of TDS.
> 
> Thank you very much for your time and effort in assisting me,

Hope that helps. Let us know if you have any further questions.

Ethan

> 
> Sargis
> 
> ------------------------------
> Sargis Panosyan
> Science Data Engineering and Archiving Group
> Instrument Software and Science Data Systems - Section 388
> 
> Jet Propulsion Laboratory


Ticket Details
===================
Ticket ID: EMH-756335
Department: Support THREDDS
Priority: Normal
Status: Closed


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