Properties of Transaction

As transactions deal with accessing and modifying the contents of the database, they must have some basic properties which help maintain the consistency and integrity of the database before and after the transaction. Transactions follow 4 properties, namely, Atomicity, Consistency, Isolation, and Durability. Generally, these are referred to as ACID properties of transactions in DBMS. ACID is the acronym used for transaction properties. A brief description of each property of the transaction is as follows.

i) Atomicity

This property ensures that either all operations of a transaction are executed or it is aborted. In any case, a transaction can never be completed partially. Each transaction is treated as a single unit (like an atom). Atomicity is achieved through commit and rollback operations, i.e. changes are made to the database only if all operations related to a transaction are completed, and if it gets interrupted, any changes made are rolled back using rollback operation to bring the database to its last saved state.

ii) Consistency

This property of a transaction keeps the database consistent before and after a transaction is completed. Execution of any transaction must ensure that after its execution, the database is either in its prior stable state or a new stable state. In other words, the result of a transaction should be the transformation of a database from one consistent state to another consistent state. Consistency, here means, that the changes made in the database are a result of logical operations only which the user desired to perform and there is not any ambiguity.

iii) Isolation

This property states that two transactions must not interfere with each other, i.e. if some data is used by a transaction for its execution, then any other transaction can not concurrently access that data until the first transaction has completed. It ensures that the integrity of the database is maintained and we don’t get any ambiguous values. Thus, any two transactions are isolated from each other. This property is enforced by the concurrency control subsystem of DBMS.

iv) Durability

This property ensures that the changes made to the database after a transaction is completely executed, are durable. It indicates that permanent changes are made by the successful execution of a transaction. In the event of any system failures or crashes, the consistent state achieved after the completion of a transaction remains intact. The recovery subsystem of DBMS is responsible for enforcing this property.

When the data of users is stored in a database, that data needs to be accessed and modified from time to time. This task should be performed with a specified set of rules and in a systematic way to maintain the consistency and integrity of the data present in a database. In DBMS, this task is called a transaction. It is similar to a bank transaction, where the user requests to withdraw some amount of money from his account. Subsequently, several operations take place such as fetching the user’s balance from the database, subtracting the desired amount from it, and updating the user’s account balance. This series of operations can be called a transaction. Transactions are very common in DBMS. In this article, we will discuss what a transaction means, various operations of transactions, transaction states, and properties of transactions in DBMS.