Transaction

• Collection of operations that form a single logical unit of work.

• A transaction is a unit of program execution that accesses and possibly update various data items.

Example:

Bank balance $=₹ 100$ Initial State

Balance $=$ Balance $-10$ Transaction Bank balance $=₹ 90$ Final State

• A transaction is a logical unit of work that must be either entirely completed or aborted.

Transaction

• Transaction access data using two operations

• Read(x) - It transfer the data item x from database to a local buffer belonging to the transaction that executed the read operation.

• Write(x) - It transfer the data item x from local buffer of the transaction that executed the write back to the database.

• Example : Transaction to transfer 100 from account A to account B.

read(A) Let A = 300 (Initial State)

A = A -100 A = 300-100

write ( A) A = 200

read( B) Let B = 500 (Initial State)

B = B +100 B = 500 +100

write (B) B = 600

Transaction

• Two main issues -

1) Various types of failure like hardware failure and system crashes.

2) Concurrent execution of multiple transaction.