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Supports billions of identifiers

Don't know; limited to the practical capacity of the underlying database.

Don't know – mention of 1996 tests resolving 50 resolutions/sec with a database of 500,000 PURLS in Long Introduction to PURLs

Robust architecture

Yes

PURLz is just a simple HTTP server using MySQL

Robust implementation

Yes

No - alpha stage

ability to request metadata about the identifier

Yes

Yes

lightweight understanding of identifier equivalence

Yes

Cloning an existing PURL and chaining a PURL

should be easily discoverable by Google

Yes;  the resolvers can't be crawled without providing a dump

Yes;  the resolvers can't be crawled without providing a dump

integrate well with the "web architecture"

Yes

Yes

vitality checking

? 
?

not built in

Need to avoid unbounded generation of surrogate persistent identifiers

?

?

PID corresponding to every Cornell NetID

?

?