[kortex]

Then you need central coordination, either a single central server containing the counter, or something like Snowflake where you have multiple counters, each assigned orthogonal blocks ahead of time (that need to coordinate with a central server).

UUIDs/ULIDs/etc are fully distributed, you can have two clients assign an ID without coordinating with ~0% of collision.

[poly2it]

You could also split the u64 to have the first 24 bits be unique to the client, and the last 40 be unique to the produced character. This would still allow 1 TiB of data produced per user and session. The single mutex would be the user ID counter.