Transaction States,ACID properties and concurrency control.

1. DBMS Assignment
-Transaction states
- ACID properties
- Concurrency control
Presented by
Pranay Guha
MBA 3rd Semester
Major-Marketing
Techno India, Salt
Lake

2. Initiate
Transaction
Flowchart of Transaction States
Begin
Read, write
Active
End
Transaction
Partially
Committed
Commi
t Committed
Failed
Abort
Terminated
Abort

3. States for Transaction Execution
 Active State – A transaction enters into an active state immediately after it start
its execution where it can issue read or write operations.
 Partially Committed – A transaction goes into partially committed state after the
end of a transaction and at this point, some recovery protocol is needed.
 Committed State – Once the transaction is committed, it has concluded its
execution successfully and all of its changes are recorded to the database
permanently.
 Failed State – A transaction goes to the failed state if any one of the check fails
or if the transaction is aborted during its active state.
 Terminated State – This state corresponds to the transaction leaving the
system. The failed or aborted transactions may have restarted later either
automatically or after being resubmitted by the user.

4. ACID
Properties  Atomicity – The atomicity property ensures the completion of the transaction
execution. It is the responsibility of the recovery subsystem of the database
management system.
 Consistency – Transaction preserves database consistency. It is the responsibility
of the programmer who writes the database program or the DBMS modules who
enforce the integrity constraints.
 Isolation – If every transaction does not make its update visible to other
transactions until it is committed, it solves the temporary update problem and
eliminates cascading rollback. It defines different levels of isolation:-
1) Level 0.
2) Level 1.
3) Level 2.
4) Level 3.
 Durability – Once the transaction commits, its update survive even after the
subsequent system crash. It is the responsibility of the recovery subsystem of the
database management system.

5. Concurrency
Control
Isolation (+ Consistency) => Concurrency
Control
 Concurrency means allowing more than one transaction to run simultaneously on
the same database.
 When several transactions run concurrently database consistency can be destroyed.
 It is meant to coordinate simultaneous transactions while preserving data
integrity.
 It is about to control the multi-user access of database.
To illustrate the concept of concurrency control, consider two travellers who go to electronic
kiosks at the same time to purchase a train ticket to the same destination on the same train.
There's only one seat left in the coach, but without concurrency control, it's possible that
both travellers will end up purchasing a ticket for that one seat. However, with concurrency
control, the database wouldn't allow this to happen. Both travellers would still be able to
access the train seating database, but concurrency control would preserve data accuracy
and allow only one traveller to purchase the seat.
This example also illustrates the importance of addressing this issue in a multi-user
database.

6. Why concurrency control is necessary ?
 The Lost Update Problem
T1 T2
read_item(X);
X:= X+10;
write_item(X);
commit;
read_item(X);
X:= X+20;
write_item(X);
commit;
Lost
update
State of X
20
20
40
30
 Changes of T2 are lost.

7. Why concurrency control is necessary ? (Contd.)
 The Temporary Update/ Dirty Read Problem
T1 T2
read_item(X);
X:= X+10;
write_item(X);
X:=X+10;
write_item(X);
commit;
Dirty update
read_ item(X);
sum:= sum+X;
write_item(sum);
commit;
State of X sum
20
30
40
0
30
 T2 sees dirty data of T1.

8. Why concurrency control is necessary ? (Contd.)
 The Incorrect Summary Problem
T1 T2
read_item(X);
X:= X-10;
write_item(X);
commit;
read_item(Y);
Y:= Y+10;
write_item(Y);
commit;
read_item(A);
sum:= sum+A;
write_item(A);
commit;
read_item(X);
sum:= sum+X;
read_item(Y);
sum:= sum+Y;
State of X State of Y
Incorrect summary
30
20
10
10
20
Let A=100
sum
0
100
 T2 reads X after 10 is subtracted and reads Y before 10 is added, hence incorrect
summary.

9. Why concurrency control is necessary ? (Contd.)
 The Unrepeatable Read Problem
T1 T2
read_item(X);
read_item(X);
X:= X+10;
write_item(X);
commit;
read_item(X);
X:= X+20;
write_item(X);
commit;
State of X
20
20
40
40
Unrepeata
ble read
 T1 reads different values for X.