Saturday, January 18, 2014


In Database, ACID (atomicity, consistency, isolation, durability) is a set of properties that guarantee that database transactions are processed reliably. In the context of databases, a single logical operation on the data is called a transaction.

An example of a transaction is a transfer of funds from one bank account to another, even though it might consist of multiple individual operations (such as debiting one account and crediting another).


Atomicity refers to the ability of the DBMS to guarantee that either all of the tasks of a transaction are performed or none of them are.

For example, the transfer of funds from one account to another can be completed or it can fail for a multitude of reasons, but atomicity guarantees that one account won’t be debited if the other is not credited.

Atomicity states that database modifications must follow an “all or nothing” rule. Each transaction is said to be “atomic” if when one part of the transaction fails, the entire transaction fails. It is critical that the database management system maintain the atomic nature of transactions in spite of any DBMS, operating system or hardware failure.


The consistency property ensures that the database remains in a consistent state before the start of the transaction and after the transaction is over (whether successful or not).

Consistency states that only valid data will be written to the database. If, for some reason, a transaction is executed that violates the database’s consistency rules, the entire transaction will be rolled back and the database will be restored to a state consistent with those rules. On the other hand, if a transaction successfully executes, it will take the database from one state that is consistent with the rules to another state that is also consistent with the rules.


Isolation refers to the requirement that other operations cannot access or see the data in an intermediate state during a transaction. This constraint is required to maintain the performance as well as the consistency between transactions in a DBMS. Thus, each transaction is unaware of other transactions executing concurrently in the system.


Durability refers to the guarantee that once the user has been notified of success, the transaction will persist, and not be undone. This means it will survive system failure, and that the database system has checked the integrity constraints and won’t need to abort the transaction.

Many databases implement durability by writing all transactions into a transaction log that can be played back to recreate the system state right before a failure. A transaction can only be deemed committed after it is safely in the log.

Durability does not imply a permanent state of the database. Another transaction may overwrite any changes made by the current transaction without hindering durability.

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