Puzzled with ACID rule

[sripri]

Recently I came across a term called as ACID rule of thumb for transactions. But there was no details placed in the book about this. Can anyone say me what is this term? Has it got anything to do with database?

[jamesravid]

The ACID model is one of the most important concepts of database theory. It sets forward four goals that every database management system must strive to achieve: Atomicity, Consistency, Isolation and Durability. No database that fails to meet any of these four goals can be considered reliable.

Let’s take a moment to examine each one of these characteristics in detail:

Atomicity states that database modifications must follow an [B]all or nothing[/B” rule. Each transaction is said to be atomic. If 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.

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 requires that multiple transactions occurring at the same time not impact each other’s execution. For example, if Joe issues a transaction against a database at the same time that Mary issues a different transaction, both transactions should operate on the database in an isolated manner. The database should either perform Joe’s entire transaction before executing Mary’s or vice-versa. This prevents Joe’s transaction from reading intermediate data produced as a side effect of part of Mary’s transaction that will not eventually be committed to the database.

Durability ensures that any transaction committed to the database will not be lost.

--James