Redundancy
To protect against computer crashes, disk failures, loss of power, etc. duplicate the hardware, disks, power sources, etc.
The duplicate systems can compare answers as a means of detecting errors
RAID systems are arrays of disks that contain “hot spares” and special data encodings to recover from disk failures
By keeping a snapshot of the database and a record of all of the transactions, it is possible in case of catastrophic disaster to reconstruct the database by applying all of the transactions to the old database