Cool URIs don't change
Hot URIs change constantly
Warm URIs change sometimes, every once in a while
Frozen URIs are structurally immutable
Lady, EVOLVED 
@gaditb all URIs are structurally immutable…… they are strings………
@Lady Sure but "cool URIs don't change" is using "URI" as synechdoche for "the 'X identifies Y' relationship between the URI string and the content".
So similarly here. The relationship is immutable.
@gaditb mhm
in practice you can’t have a URL be both frozen and dereferencable but it’s really easy to have a URL which is frozen and not dereferencable!
@Lady @cpsdqs That said it is extremely possible this is not a useful framework -- it was entirely motivated by the pun of "Cool URIs don't change" + different "temperatures" of data storage by cloud providers, punningly reinterpreting the "cool" from its original meaning into one that didn't exist at the time (but amusingly can be made to work).
I wasn't thinking about whether it'd be a useful concept.
@gaditb it is a useful concept in library science! altho the term of art there is “persistent identifier”. see https://arks.org/
it would also be a useful concept on the fediverse if fedi developers ever bothered to read a book on information science
@Lady Response header X-Temperature: Cool
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My related hot take is DNS should be able to communicate similar information -- requests for "please limit request frequency to", commitments to "I'll aim to keep downtimes below X length, don't worry or assume I've broken down before then".
@gaditb amusingly i think the most practical application for knowing the “heat” of URLs is with webcrawlers so they know how often to recrawl