[Esip-preserve] On Earth Science Data File Uniqueness

[Curt Tilmes]

On 02/09/11 13:27, Lynnes, Christopher S. (GSFC-6102) wrote:
>> If two granules have the same object identifier, you are talking about
>> the same object (two copies of the same object).  If you are talking
>> about the same object.  If you want to verify the fixity of the
>> content, the UUID won't give you that.  You still need to use SHA-1 or
>> MD5 or whatever to verify integrity/fixity.
> 
> This speaks directly to the practical usefulness.  If I have two
> objects that are "the same" by UUID, but have different contents,
> then they aren't really the same from a practical standpoint.

Yeah, we'd call that case "broken".  You've had a failure of your
fixity.

> For any of the operations I am going to apply to them (e.g., science
> processing, analysis), I need to treat them as different, and the
> UUID comparison has not helped in the least.  On the other hand, if
> the contents are identical, but the UUIDs are different, I can still
> proceed with most of the operations I would want to do under the
> assumption that they are the same.

Absolutely.  (We actually take advantage of this property and store
only one copy of the content, even if that content is associated with
two distinct objects.)

> Only in a few rare cases (e.g., formal attribution) is this risky.
> And even those cases presume that I also have a thorough provenance
> to go along with it.)

Those are the cases where we want distinct object identifiers, even if
the content matches.  We want to associate properties with the object,
so we associate them with the identifier for the object.

If the content identifier is the only identifier for the object I
have, then I can't distinguish them.  In such a model, they aren't
really two different objects, they are the same object.  I've lost my
ability to separately associate any properties with one or the other.

Curt