> No, any web service instance can receive requests at arbitrary times, and sometimes in the same millisecond zone. The probability is proportional to the number of concurrent users and requests.

Yes, but does it matter that you have out of order IDs within the same ms for concurrent requests? That's why I said batch. I only ever been an issue for me when I've chosen ULID as an ID for an event log (if the command produced more than one event, random bits will ruin the order)

> However, that "optimization" is a failure mode if you're not aware how ULID internals work.

That's not ULID internals, that's whatever library you're using. The rust implementation I've used, for example, will generate random bits unless you implicitly increment, and that requires `&mut`