Database Consistency, Durability, Backup and Disaster Crash Recovery
Ensuring Database Consistency, Durability, Backup, and Disaster Crash Recovery
Durability means that once a transaction, i.e. a set of update operations on the data, is committed, it cannot be abrogated. In the centralized databases systems, checkpoint and log are normally used to recover the state of the database in case of a system failure (e.g. the contents of main memory disappear due to a power loss and the content of a broken disk becoming illegible) [22]. Checkpoint is the point of synchronization between the database and transaction log file when all buffers are force-written to secondary storage [7]. For this kind of failure, the database can be reconstructed only if:
To protect the database against media failures an up-to-date backup of the database, i.e. a copy of the database separate from the database itself, is used [22]. A backup of the database and its log can be periodically copied onto offline storage media [7]. In case of database corruption or device failure, the database can be restored from one of the backup copies, typically the most recent one [3]. In this case, the recovery is carried out using the backup and the log – see [15], for more details.
A database has a Disaster Crash when the main memory and the log, or a part of the log, are lost. Therefore, to recover the database, an old, maybe obsolete, backup of the database is used. Data which was not part of the backup will be lost. In case of a disaster crash, the system cannot guarantee the durability property. However, in a centralized database, recovery from a backup provides a database which has a consistent state.
