Say the server has a counter. When you load the page, it's at 57, so it displays that you would be ordering #57. While you're looking at this, someone else loads the page - what number do you show them? If you show 57, then whoever orders first gets it and the other person gets a message "Sorry, not available. Want 58 instead?" but the same thing could then happen to them with #58, too – "Sorry, not available. Want 59 instead?"
So maybe instead you show the 2nd person counter+1, i.e. 58. And you show the 3rd person counter+2, i.e. 59. But what if #59 purchases but 57 and 58 don't? What do you show the NEXT person, 57 or 60?
I'm not saying it's intractable but it merits thought.
Next up pumice.rocks , limestone.rocks, basalt.rocks?
“All park resources are protected so that all visitors may enjoy them. It is against the law to remove any of the natural (petrified wood, other rocks, plants, animals) or cultural resources (pottery pieces, arrowheads, Route 66 debris), including picking flowers.”
Do you have a low salt version?
Is there an option for pasteurized or raw?
What about sustainable practices?
What about concerns related to gravitational change from such operations?
Is there a recycling program in place?
Do they come with an MSDS and waste disposal instructions?
What about a warranty? Isn’t that required by federal law?
Mayhe it is in here somewhere: https://yami-ichi.download/
Quick question... Have you considered letting customers come mine their own rocks?
Sequential assignment follows strict order of payment completion only. No race conditions, no held inventory, no time windows. You either complete the transaction and receive the next sequential number, or you don't.
The constraint is designed to eliminate the entire apparatus of purchase optimization, including queue management systems.