Atomicity Consistency Isolation Durability (ACID) Properties in DBMS
A transaction is a single logical unit of work which accesses and possibly modifies the contents of a database. Transactions access data using read and write operations. A transaction in a database system must maintain Atomicity, Consistency, Isolation, and Durability − commonly known as ACID properties − in order to ensure accuracy, completeness, and data integrity.
- Atomicity − This property states that a transaction must be treated as an atomic unit, that is, either all of its operations are executed or none. There must be no state in a database where a transaction is left partially completed. States should be defined either before the execution of the transaction or after the execution/abortion/failure of the transaction.
It involves following two operations.
—Abort: If a transaction aborts, changes made to database are not visible.
—Commit: If a transaction commits, changes made are visible.
Atomicity is also known as the ‘All or nothing rule’.
- Consistency − The database must remain in a consistent state after any transaction. This means that integrity constraints must be maintained so that the database is consistent before and after the transaction. It refers to correctness of a database.
- Durability − This property ensures that once the transaction has completed execution, the updates and modifications to the database are stored in and written to disk and they persist even if system failure occurs. These updates now become permanent and are stored in a non-volatile memory. The effects of the transaction, thus, are never lost.
- Isolation − Ensures that transactions are securely and independently processed at the same time without interference, but it does not ensure the order of transactions. No transaction will affect the existence of any other transaction.
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