Error recovery

1. Define a policy how the system is restored to a "normal" state for each internal error or exception.
2. Define a policy how the system is restored to a "normal" state for each possible type of input error from the user or wrong data from external systems.
3. Define a policy for handling data corruption in the database.
4. Define a policy for handling database unavailability: how new data can still be entered into the system and merged later into the old database after it retorted.
5. Define a policy for exchanging data between systems.