[Esip-preserve] On Earth Science Data File Uniqueness

[Lynnes, Christopher S. (GSFC-6102)]

On Feb 9, 2011, at 12:08 PM, Curt Tilmes wrote:

> On 02/09/11 11:50, Lynnes, Christopher S. (GSFC-6102) wrote:
>> I thought UUID was designed to answer only the question: are data
>> items A and B bitwise-identical?
> 
> Absolutely not.  You're thinking of digital signatures or hashes such
> as MD5 or SHA-1 which can be used to verify file content
> integrity/fixity.

I should have phrased that:  has someone asserted that data items A and B are bitwise identical, i.e., by assigning a UUID.  BTW, I thought our preferred method for assigning UUIDs was to derive them from the SHA-1, anyway?
> 
> UUID is just a way to make an identifier that is globally unique
> forever [1] and easily recognizable as a UUID.

OK, if it doesn't answer the question, are they identical, what question does the UUID answer???  Just having a unique identifier in and of itself is not intrinsically useful.

> 
>> If they have the same UUID, then the answer is yes.  If they have
>> different UUIDs, then the answer is that there is no evidence to say
>> that they are bitwise identical.
> 
> They can be assigned to the object arbitrarily without regard to
> content.

in that case, we have a misuse of UUIDs by the UUID creator.

> 
> For example, here is one: 0cdf7b24-f374-419e-8cce-9758432cfdfa
> It's totally unique in the world (go ahead, google it)
> 
> You could assign it to some chunk of data (some object) as you will.
> Of course, if two of us tried to assign it to different data, we'd
> have a problem, but we wouldn't do that.
> 
> Curt
> 
> [1] Ok, ok.  Practically globally unique forever, like really really
> likely to be globally unique forever.  Almost perfect.  Like it is
> "Hitchhiker's Guide to the Galaxy" improbable that you would get a
> conflict.
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--
Dr. Christopher Lynnes     NASA/GSFC, Code 610.2    phone: 301-614-5185