Redundancy is Very Very Very Good
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