The fileTypeId is used by ncWMS to decide on what algorithm to use to extract
data. Compressed data (e.g. NetCDF4) and data read over OPeNDAP, have very
different performance characteristics to uncompressed, local data (e.g.
See the code here:
So I guess the fileTypeId of your virtual dataset should match the underlying
file type. If this isn't easy, then from the WMS point of view you can put any
old string as the fileTypeId and the WMS will be conservative and won't assume
that data-reading is "cheap".
This is an example of the adage "all abstractions are leaky"... performance
concerns are notorious for messing up nice clean abstractions.
Date: Wed, 27 Apr 2011 17:09:13 -0500
From: Roland Schweitzer <Roland.Schweitzer@xxxxxxxx>
Subject: Re: [thredds] WMS and F-TDS virtual datasets [was: Re:
aggregation / arithmetic between variables]
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Jon and Ethan,
Help me understand the best way forward with these FileTypeId in the IOSP.
On 04/15/2011 03:37 PM, Ethan Davis wrote:
> Hi Roland,
>> What should these [getFileTypeId] values be, Ethan? Is there an
>> official enumeration I can reference for know values for these?
>> I just grabbed them off the Web page:
>> It makes sense to me to use netCDF since it is the intent of the IOSP
>> to act like netCDF OPeNDAP in every case.
> The ID values should uniquely identify the "file type". The web page
> enumerates the values we know. We encourage everyone that implements
> an IOSP to select an ID not on the list and let us know so we can
> update the list.
> So, I think rather than use "netCDF" you should decide on an ID unique
> to your IOSP. Or, if all the datasets behind one of your virtual
> datasets are always going to be the same type, you could use the type
> of the backing datasets. (That is what the CDM Aggregation class does,
> it uses the "file type" of the aggregations "typical dataset").
Jon, what does the ncWMS do with the FileTypeId? The best decision
from my point of view for what to return seems to depend on how the value is
being used by clients. I thought the point of the the CDM was everything looks
like netCDF. If folks are making optimizations based on the FileTypeId then
for some cases like F-TDS it seems like they might miss out if the FileTypeId
was something other than netCDF.
> Let us know if you decide on a new unique ID and we'll add it to the list .
End of thredds Digest, Vol 27, Issue 34