Introduction to database management systems

A transaction is a unit of program execution that accesses and possibly updates various data items. A transaction must see a consistent database. During transaction execution database may be inconsistent. When transaction is committed, database must be consistent.

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The ACID model is one of oldest and most important concepts of database theory.   

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

• Consistency states that only valid data will be written to database. If, for some reason, a transaction is executed that violates databases consistency rules, entire transaction will be rolled back and database will be restored to a state consistent with those rules.

• Isolation the transaction runs in isolation, it appears to be only action that system is carrying out at one time. If there are two transactions that are both performing same function and are running at same time, transaction isolation will ensure that each transaction thinks it has exclusive use of system.
• Durability ensures that any transaction committed to database will not be lost. Durability is ensured through use of database backups and transaction logs that restoration of committed transactions in spite of any subsequent software or hardware failures.