Tuesday, March 6, 2012


ACID, which stands for Atomicity, Consistency, Isolation, and Durability, has become the gold standard to defi ne the highest level of transactional integrity in database systems. As the acronym suggests it implies the following:

» Atomicity — Either a transactional operation fully succeeds or completely fails. Nothing that is inconsistent between the two states is acceptable. The canonical example that illustrates this property is transferring funds from one account, say A, to another, say B. If $100 needs to be transferred from A to B, $100 needs to be debited from (taken out of) A and credited to (deposited into) B. This could logically mean the operation involves two steps: debit from A and credit to B. Atomicity implies that if for some reason, debit from A occurs successfully and then the operation fails, the entire operation is rolled back and the operation is not left in an inconsistent state (where the money has been debited from A but not credited to B).

» Consistency — Consistency implies that data is never persisted if it violates a predefined constraint or rule. For example, if a particular field states that it should hold only integer values, then a fl oat value is not accepted or is rounded to the nearest integer and then saved. Consistency is often confused with atomicity. Also, its implication in the context of RDBMS often relates to unique constraints, data type validations, and referential integrity. In a larger application scenario, consistency could include more complex rules imposed on the data but in such cases the task of maintaining consistency is mostly left to the application.

» Isolation — Isolation gets relevant where data is accessed concurrently. If two independent processes or threads manipulate the same data set, it’s possible that they could step on each other’s toes. Depending on the requirement, the two processes or threads could be isolated from each other. As an example, consider two processes, X and Y, modifying the value of a field V, which holds an initial value V0. Say X reads the value V0 and wants to update the value to V1 but before it completes the update Y reads the value V0 and updates it to V2. Now when X wants to write the value V1 it fi nds that the original value has been updated. In an uncontrolled situation, X would overwrite the new value that Y has written, which may not be desirable. Isolation assures that
such discrepancies are avoided. The different levels and strategies of isolation are explained later in a following section.

» Durability — Durability implies that once a transactional operation is confirmed, it is assured. The case where durability is questioned is when a client program has received confirmation that a transactional operation has succeeded but then a system failure prevents the data from being persisted to the store. An RDBMS often maintains a transaction log. A transaction is confirmed only after it’s written to the transaction log. If a system fails between the confirmation and the data persistence, the transaction log is synchronized with the persistent store to bring it to a consistent state.

The ACID guarantee is well recognized and expected in RDBMSs. Often, application frameworks and languages that work with RDBMS attempt to extend the ACID promise to the entire application. This works fine in cases where the entire stack, that is, the database and the application, resides on a single server or node but it starts getting stretched the moment the stack constituents are distributed out to multiple nodes.

Source of Information : NoSQL
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