permanent URIs for media

 "permanence" is a desirable characteristic for many things: data,
names, URIs, etc.

However, it's difficult to think how to "add" permanence. 

 

We are doing task A, and we have a pretty good way of doing it, but it
needs 

characteristic X, how do we add X?

 

I think "permanence" fits in with "security", "reliability",
"performance", "usable":

characteristics that are best approached from the negative: remove
things that

are "not X".  To make something secure, you discover all of the
vulnerabilities

and mitigate them. To make something fast, you discover the
bottlenecks and

optimize them.  To make something "permanent", you discover all of the
ways in

which might interrupt continuity and eliminate those.

 

Permanent IDs in XMP (eXtensible Metadata Platform)

 

http://www.adobe.com/devnet/xmp/ and in particular, page 19 of

http://www.adobe.com/devnet/xmp/pdfs/DynamicMediaXMPPartnerGuide.pdf

 

 

XMP uses GUID-based URIs for referencing content. It is assumed that
there is

a system/index which can be used to discover content if all you have
is the

GUID.

 

Each asset has at least two such identifiers: 

one which changes every time any edit is made by any tool (the
"instance ID"),

 and one of which does not change but refers to the entire version
history 

(the "document ID").  (There is a complex bit of infrastructure for
asserting

relationships between versions using History, Ingredients, and other
bits

of relationship metadata.)

 

The URI scheme "xmp.iid:" is a bit reflexive. 

 

   xmp.iid:b9db20421f30bb3fe10e5f90

 

is a URI which identifies the file that has the following
attribute/value

pair in its metadata:

 

         xmpMM:InstanceId = "xmp.iid:b9db20421f30bb3fe10e5f90"

 

(xmpMM is the XML namespace prefix used by the XMP media management
schema

 of metadata properties.)

 

  

Similarly

 

    xmp.did:b9db20421f30bb3fe10e5f90

 

is a URI which identifies any/all/the most recent file that has the
following

attribute/value pair in its metadata:

 

         xmpMM:DocumentId = "xmp.did:b9db20421f30bb3fe10e5f90"

 

xmp.iid and xmp.did URIs are used only for files that have XMP
actually 

stored in the file, not in sidecar files. Files that do not have
embedded

XMP still have InstanceID and DocumentID, but not in this form. Some

documents that do have embedded XMP might still have a DocumentID

that does not use xmp.did, because the document started out as a file

without XMP but with some other kind of unique document identifier.

 

These are URIs (that is, they uniformly identify resources), but they

are only useful as URLs (Uniform Resource Locators) if there is an
index

of files with XMP indexed by the IDs found within them. In the case

of xmp.did:, it might be necessary to examine modification dates

and history to determine which is the latest of multiple instances

with the same document ID.