Patent ID: 7640314

Claim:
A network system comprising a plurality of devices connected to a network each of which is assigned an ordinal rank in the system in which a device of a highest ordinal rank functions as a master device while devices of the other ordinal ranks function as general devices and shared data to be shared by the plurality of devices is supplied from the master device to the general devices, wherein: each of the devices includes master device units that operate when the device functions as the master device and general device units that operate when the device functions as a general device, and the master device units include: a shared data storing unit that stores the shared data on the device operating as the master device; a shared data updating unit that updates the shared data stored in the shared data storing unit; a first on-update backup data supplying unit that supplies the shared data before update to a device operating as the general device with the highest ordinal rank as backup data when the shared data is updated by the shared data updating unit; an on-restoration backup data acquiring unit that acquires the backup data from a device other than the device operating as the master device when a restoration instruction is given; and a shared data restoring unit that restores the shared data to that before update by storing the backup data acquired by the on-restoration backup data acquiring unit in the shared data storing unit as the shared data, and the general device units include: a backup data storing unit that stores the backup data on a selected device operating as a general device; a backup data updating unit that updates the backup data by acquiring backup data when the backup data is supplied from a device other than the selected device and storing the acquired backup data in the backup data storing unit; a second on-update backup data supplying unit that supplies the backup data before update to a device with the ordinal rank immediately below the ordinal rank of the selected device as backup data when the backup data is updated by the backup data updating unit; and an on-restoration backup data supplying unit that supplies the backup data in response to a request from the device operating as the master device.