Obviously, when ownership of some security is transferred from party A to party B, both parties' balances must be updated and recorded in an atomic transaction. Call this "an execution".
So in an exchange, what conditions dictate the moment a trade is actually executed? Specifically, are there a fixed number of "trading moments" during a day, or is it all "fluid"?
Or asked another way, do executions take place as soon as possible when buy/sell price conditions are met? Or are there a fixed number of execution timeslots in the course of a day in which buyer/seller matchups are sought?
Any resources appreciated. If the exchanges were open-source, I'd go read the code. ;-)