Stepwise Migration and Data Storage

Rome, as has often been observed, was not built in a day (and it is still under construction!), and neither can a system redesign and reconfiguration take place overnight. Therefore, the step-by-step changeover to a new system is an important topic. It should be simple to create interfaces to existing systems, a requirement that results in something even more important: In the ideal case it should make no difference to a particular application where data is stored. It can be located in a database belonging to the new system, in a database belonging to an old system, or indeed in an external database that does not belong to the system at all.