I read an article a few days ago in the latest issue of DMReview titled, "Global Keys: A Unified Key Mapping Architecture".
I visited the authors blog at http://www.globalkeysdesign.com/global_keys/blog_index.html and found it to be a good start of a very important discussion about creating a centralized mapping of keys for multiple data repositories that contain equivalent data (or identities).
Check out my post
New World, New Rules: Securing the Future State
5 months ago