Comment by Lammy

> I thought this is technically impossible, and it will never happen

I always hated this meme/mindset, because if you dig in to the history of them you'll see that their original purpose was to collide. They were labels to identify messages in Apollo's distributed computing architecture. UID and later UUIDs were a reversible way to mark an intersection point between two dimensions.

Any two nodes in a distributed system would generate the same UID/UUID for the same two inputs, and a recipient of an identified message could reverse the identifier back into the original components. They were designed as labels for ephemeral messages so the two dimensions were time and hardware ID (originally Apollo serial number, later 802.3 hwaddress etc).

I think a lot of the confusion can be traced to the very earliest AEGIS implementation where the Apollo engineers started using “canned” (their term, i.e. static or well-known) UIDs to identify filesystems. Over time the popular usage of UUID fully shifted from ephemeral identifiers where duplicates were intentional toward canned identifiers where duplicates were unwanted and the two dimensions were random-and-also-random.