Group of tasks

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)