The ACID principal is the foundation which has guided the implementation of transactions in relational databases. This acronym stands for the following:
The term atomicity applies to the ability to group multiple database updates into a single transaction that can be treated as an atomic update. The term atomic is derived from the Greek word for the smallest indivisible element. An atomic transaction is indivisible, since the failure of the one of the updates in the group is considered a failure for the whole group of updates. Either all of the updates will be completed or none be completed, and the transaction will be rolled back, leaving the database in the state it was in before the transaction started.
The term consistency applies to the various rules associated with the data in the transactions. These rules may apply to relationships between the tables in the update or to rules concerning the primary key for a table. These rules may be broken during the transaction, but the inconsistent data will not e visible to other users. Once the transaction is complete, the rules must be valid and the database must be in a consistent state.
A given transaction should appear as though it is running alone in the database. The work of other users must be coordinated with other transactions to maintain this isolated view. The work of other transactions is invisible to the user in a transaction.
The work of a committed transaction is guaranteed to persist and remain in the database even in the event of various system failures. A transaction is not considered durable until it commits. A system failure entails a database recovery, which includes a rollback procedure for all uncommitted transactions, ultimately leaving the database in a consistent state.
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