> There is even another consistency hazard right in this very example: "The count of item availability was supposed to be reduced by 2 because 2 items were ordered by different clients at a same time, but a race condition (the two transactions both individually reading the same old value before writing back the new value) led to the item count only reduced by 1. There is now an extra item 'available' that does not exist."
You don't need ACID to solve that kind of problem though; event sourcing and eventual consistency handles it fine. What you need ACID for is synchronous transactions; for example you want to check that both item A and item B are available, order both of them, and have it either both orders go through or neither of them, even though those are separate orders to separate tables (of course if you have a concept of an "order" as a first-class entity then you can trivially solve this - but again, in that case you don't need ACID at all).
> Care to elaborate how transactions are "inherently useless" in a web service here, and how the consistency issue happens "between client and server" communication?
So the example above - the whole point of ACID is to let you do two separate things at once, e.g. you want to open the page for item A and for item B, check that both are available, and then order them both on their respective pages. With an old-fashioned synchronous client-server system with a dedicated fat client you could do that, but on a web system obviously it's not possible (other than by creating some kind of "batched request" or "combined order" concept for doing both orders in the same request - but again, once you've done that you don't need ACID at all) because there's no way for the client to do two page loads in the same transaction.