1. CONCURRENCY CONTROL 18.1 Serial and Serializable Schedule
By: Jiten Oswal
Id: 109
CS: 257

2. What Is Concurrency Control & Who controls it?
A process of assuming that the transactions preserve the consistency when executing simultaneously is called Concurrency Control.
This consistency is taken care by Scheduler.

3. Flow How Transaction is Executed
INPUT
READ
&
WRITE
Disk
Buffer
OUTPUT

4. Transaction Manager
Read / Write Requests
Scheduler
Read & Writes
Buffer

5. Correctness Principle
It’s a principle that states that a transaction starts in a correct database state and ends in a correct database state.
Does the system really follow the correctness principal all the time?

6. Basic Example ScheduleT1
READ (A,t)
t := t+100
WRITE (A,t)
READ (B,t)
WRITE (B,t) T2
READ (A,s)
s := s*2
WRITE (A,s)
READ (B,s)
WRITE (B,s)
A=B=50
To be consistent the final state should be A=B

7. Serial Schedule (T1,T2) A := 2*(A+100) T1 T2 A B 50 50 READ (A,t)
50 50
READ (A,t)
t := t+100
WRITE (A,t) 150
READ (B,t)
WRITE (B,t)
READ (A,s)
s := s*2
WRITE (A,s) 300
READ (B,s)
WRITE (B,s) 300
(T1,T2)
A := 2*(A+100)

8. Does the order really matter?
T1 T2 A B
50 50
READ (A,s)
s := s*2
WRITE (A,s) 100
READ (B,s)
WRITE (B,s) 100
READ (A,t)
t := t+100
WRITE (A,t) 200
READ (B,t)
WRITE (B,t)
(T2,T1)
The final state of a database is not independent of the order of transaction.

9. Serializable Schedule
T1 T2 A B
50 50
READ (A,t)
t := t+100
WRITE (A,t) 150
READ (A,s)
s := s*2
WRITE (A,s) 300
READ (B,t)
WRITE (B,t)
READ (B,s)
WRITE (B,s) 300
Serializable but not Serial Schedule

10. Non-Serializable Schedule
T1 T2 A B
50 50
READ (A,t)
t := t+100
WRITE (A,t) 150
READ (A,s)
s := s*2
WRITE (A,s) 300
READ (B,s)
WRITE (B,s) 100
READ (B,t)
WRITE (B,t)
A := 2*(A+100)
B := 2*B + 100

11. A Serializable Schedule with details
T1 T2 A B
50 50
READ (A,t)
t := t+100
WRITE (A,t) 150
READ (A,s)
s := s*1
WRITE (A,s) 150
READ (B,s)
WRITE (B,s) 50
READ (B,t)
WRITE (B,t)
A := 1*(A+100)
B := 1*B + 100

12. Notations for Transaction
Action : An expression of the form ri(X) or wi(X) meaning that transaction Ti reads or writes, respectively, the database X.
Transaction : A transaction Ti is a sequence of actions with subscript.
Schedule : A schedule S of a transactions T is a sequence of actions, in which for each transaction Ti in T, the action of Ti appear in the definition of Ti itself.

13. Notational Example T1 T2 READ (A,t) t := t+100 WRITE (A,t) READ (B,t)
WRITE (B,t) T2
READ (A,s)
s := s*2
WRITE (A,s)
READ (B,s)
WRITE (B,s)
Notation:
T1 : r1(A); w1(A); r1(B); w1(B)
T2 : r2(A); w2(A); r2(B); w2(B)

14. Thank you