Crossref's been around for 15 years (and DOIs longer than that). The Crosstech blog has a lot of posts on the subject. You can start here http://crosstech.crossref.org/category/identifiers

Here's a short list of posts that will give some background:

http://crosstech.crossref.org/2015/06/rehashing-pids-without...

http://crosstech.crossref.org/2015/03/january-2015-doi-outag...

http://crosstech.crossref.org/2013/09/dois-unambiguously-and...

http://crosstech.crossref.org/2010/03/dois_and_linked_data_s...