How to prevent race condition in online hotel booking

Question

I am writing a hotel booking software using PHP and MySQL. I am pretty much done with it but I ran into a race condition problem. For example there is only one standard room

Answer 1

One solution is to add two columns to a table in the database. One column is the session ID or user ID or whatever of the user that is being offered the room. The second column is a timestamp indicating when that offer will expire.

Then, in your app, only show rooms that have an expired timestamp in the hold column. (Set the initial timestamp to 0 so that it starts out expired.) When a room is selected, check the column again. If there's an unexpired timestamp there, the user gets a "sorry, you were too slow" message. Otherwise, put a timestamp there for 15 minutes into the future or whatever, and proceed.

You see this on travel sites and ticket-purchasing sites a lot where it says something like "We're holding these seats for you for another 14 minutes. Please complete the transaction by then or it will be released blah blah blah."

Answer 2

I would also go with the pending state. You could save the state together with the session id of this user and have a cronjob that deletes all pending states that have expired session ids associated to them.