CREATE PROCEDURE add_ticket -- parameters DECLARE free_seats int BEGIN TRANSACTION SELECT free_seats = COUNT(*) FROM tickets WHERE seat_is_not_taken IF free_seats <> 0 INSERT INTO tickets VALUES(...) -- some other statements END TRANSACTION
The problem is that two processes can read the amount of free tickets concurrently and both save a ticket, even if there are no free seats left. I need a way to block processes from reading the amount of free tickets while other processes running the add_ticket procedure have not yet inserted a new ticket. SET TRANSACTION ISOLATION LEVEL does not help in this situation, am I right?
That said, I think a more interesting option might be to employ SQL Service Broker. You could set up a conversation for each bus, and store the conversation handles in a table that can be referenced by readers before doing the RECEIVE. That way, the readers can filter appropriately. Drop a message into the queue for each seat on the bus. The readers can then simply RECEIVE the messages as needed (in the process, reserving seats on the bus). Service Broker will ensure that no message is received twice, meaning that you will no longer have any concurrency problems.
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