I am trying to understand at a conceptual level , how are databases synchronized when they are largely distributed. eg a rail/airline reservation system. Do each of the market leaders have their own proprietary solution to handle this scenario?
Are they all locally present and each transaction is checked against a master copy?
I have worked with fail over databases which are locally present, i.e. master-slave scenario where on takes over when the other fails; but how are database which require real time transaction ability synchronized?