What is Transaction & Transaction log also Explain Transaction Properties or ACID Properties ?

Transaction: A transaction is a logical unit of work that must be entirely completed or entirely aborted no intermediate states are accepted.

To understand the transaction: Suppose you sell a product to a customer the transaction consists following properties. v  You must write a new customer in invoice v  You must update the account transactions. v  You must update the customer balance.

In data base terms, a transaction is any action that reads from and / or writes to a database . Ex : v  Simple select statement generates a list of table contents. v  Insert statement to add rows to one or more tables.

Transaction properties :   each individual transaction must satisfy the atomicity, consistency, isolation , durability. These properties are called acid test.

Atomicity : perform all properties or cancel all properties.

all operations of a transaction be completed. If not the transaction is aborted.

if the transaction T1 has 4 SQL request if any of the request fail cancel the first executed request also.

Consistency : Before the transaction & after the transaction the result must be same.

Database is in always consistent state.

Isolation :one transaction should not effect to another transaction (or)

Data used during the execution of a transaction can not be used by a second transaction until the first one is executed.

Durability : once the transaction changes are done that means committed , they can not be undone or lost even if any system failures.

Transaction log : Transaction log is used to keep track of all transactions that update the data base.

The information stored in this log is used by the DBMS for recovery requirements triggered by a rollback statement.

Transaction log stores: v  A record for the beginning of the transaction v  For each transaction in notes v  What type of operation being performed (update, delete, insert) v  The name of the objects affected by the transaction. v  Before & after values for the fields being updated. v  The ending of the transaction. v  Each and every transactions beginning, ending, time & processing (update, insert, delete) details are noted in a file either manually or automatically. The file is called transaction log.

Transaction log is used to recovery the data base & provide precedence relationships among transactions.