Transactions are a group of tasks that form a logical unit, such as withdrawing money from an account. They have four main properties: atomicity, consistency, isolation, and durability (ACID). A transaction goes through various states from active to committed or aborted. Read operations load data into memory from disk, while write operations transfer modified data back to disk.

1. TRANSACTIONS
“A group of tasks that form a single logical unit”

2. TRANSACTIONS
 Example:
 To withdraw 100 from an account – Operations:
 check the account balance
 If sufficient balance is present request for withdrawal.
 Get the money
 Calculate balance = Balance – 100
 Update account with new balance

3. Transactions - Process
 Read Operation(READ(X))
 To read a database object, it is first brought
into main memory (Local Buffer) from
disk(database), and then its value is copied into
a program variable.

4. Transactions - Process
 Read Operation(READ(X))

5. Transactions - Process
 Write Operation (WRITE(X))
 To write a database object, an in-memory
copy of the object is first modified and then
written to disk (i.e) Transfers the data item x
from the local buffer to the transaction that
executed the write back to the data base.

6. Transactions - Process
 Write Operation (WRITE(X))

7. ACID Properties
 Atomicity:
 each transaction must be considered as a single unit
and must be completed fully or not completed at all.
 Database should be in a state either before the
transaction execution or after the transaction
execution.

8. ACID Properties
 Consistency:
 The database must remain in consistent state after
performing any transaction.

9. ACID Properties
 Isolation:
 For every pair of transactions, one transaction
should start execution only when the other finished
execution.

10. ACID Properties
 Durability
 Once a transaction completes successfully, the
changes it has made into the database should be
permanent even if there is a system failure.
 If the system failure happens after customer getting
the money then the system should be strong enough to
update database with his new balance, after system
recovers

11. Transaction - States

12. Transaction - States
 Active:
 insertion, deletion or updation of record is done here.
 But data is not saved to database
 Partially committed:
 when a transaction executed its final operation
 Failed:
 if any of the checks made by the database recovery
system fails. A failed transaction can no longer
proceed further.

13. Transaction - States
 Aborted:
 If a transaction is failed to execute, then the database
recovery system will make sure that the database is in its
previous consistent state.
 If not, it brings the database to consistent state by aborting
or rolling back the transaction.
 Committed:
 If a transaction executes all its operations successfully.

14. Transaction - Example
T1 T2
Read (A) Assume A=100
A=A+50 A=150
Write (A)
Read (A) A=150
A=A+100 A=250
Rollback
A=100 (restore back to original value
which is before Transaction T1)
Write (A)