In the preceding scenarios, no data was lost; however, there are several more complicated scenarios that can lead to permanent data loss. The reader is advised to become aware of three of these situations.
Scenario 5: Multiple master copies [tested]:
• S1 is active and has only master copies.
• S1 fails.
• S2 assumes S1's duties and creates its own master copies.
• S2 receives new data and updates its master copies.
• A power failure occurs.
• Power is restored.
• S1 recovers and reassumes its duties, using its obsolete master copies.
• S2 receives the obsolete data and overwrites its replicated data files.
• New data is permanently lost.
Scenario 6: Network split [tested]:
• A network split occurs.
• S1 is on one sub network.
• S2 is on the other sub network.
• S2 becomes the active server on its sub network and creates its own master copies.
• S2 receives new data and updates its master copies.
• All PCs are powered down so the network split can be repaired.
• Once the split is repaired, all PCs are powered up.
• S1 reassumes its duties over the entire network, using its master copies.
• S2 receives S1's data and overwrites its data files.
• New data is permanently lost.