[Esip-discovery] Datacasting custom elements proposal

Bob Deen Bob.Deen at jpl.nasa.gov
Tue Mar 22 19:52:37 EDT 2011

Hi Chris et al...

Sigh.  This is why I hate jumping in to an ongoing thread...

So let's take a step back.  My understanding here is that we're trying 
to collectively decide the best way to describe custom metadata elements 
in a *casting feed.

Is that correct?  If not please stop me now and correct my 
misunderstanding.  Some of your comments made me wonder if I was off 
base in my assumptions.

Assuming it is correct, then please allow me to reframe the question. 
There are two separate but related topics:

1) How to represent the custom metadata itself.
2) How to represent the description of the custom metadata 
(metadata-about-metadata, or m-a-m).

For topic 1, there are two methods that seem viable:

1a) Use the metadata keyword as the actual XML element (or possibly 
attribute) name.
1b) Use name=xxx value=yyy attributes.

For topic 2, there are three potential locations for the m-a-m:

2a) In the XML schema.
2b) In the channel portion of the feed.
2c) In the item itself.

We can probably safely rule out 2c.  2a more or less requires 1a.

Is that an accurate restatement of the topic?

I won't go through your message point-by-point until I'm sure of the 
context here.  I just want to clarify one misunderstanding, or perhaps 
misstatement on my part regarding PDS:

>> They do not expect changes, and thus static schemas are not as hard to
>> deal with.
> That's not true. They expect changes all the time. New missions define new elements and extensions of the existing PDS data model.

I meant that changes do not occur quickly or often.  Of course each 
mission can be different.  But within a mission, once the metadata is 
defined, it pretty much does not change.  There may be updates or errata 
from time to time, but it's not constantly in flux.  I claim that's not 
the case with us; within one feed I would hope the metadata is augmented 
often.  Plus PDS might have a dozen new missions in a good year, whereas 
we should hope for thousands of feeds per year.  It also takes a good 
many man-months to properly define a mission for PDS; a feed shouldn't 
take more than a day.


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