CONCURRENCY CONTROL 18.1 Serial and Serializable Schedule By: Nitin Mathur Id: 110 CS: 257

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.

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

Transaction Manager Scheduler Buffer Read & Writes Read / Write Requests

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?

Basic Example Schedule T1 READ (A,t) t := t+100 WRITE (A,t) READ (B,t) t := t+100 WRITE (B,t) T2 READ (A,s) s := s*2 WRITE (A,s) READ (B,s) s := s*2 WRITE (B,s) A=B=50 To be consistent the final state should be A=B

Serial Schedule T1T2AB50 READ (A,t) t := t+100 WRITE (A,t)150 READ (B,t) t := t+100 WRITE (B,t)150 READ (A,s) s := s*2 WRITE (A,s)300 READ (B,s) s := s*2 WRITE (B,s)300 (T1,T2) A := 2*(A+100)

Does the order really matter? T1T2AB50 READ (A,s) s := s*2 WRITE (A,s)100 READ (B,s) s := s*2 WRITE (B,s)100 READ (A,t) t := t+100 WRITE (A,t)200 READ (B,t) t := t+100 WRITE (B,t)200 (T2,T1) The final state of a database is not independent of the order of transaction.

Serializable Schedule T1T2AB50 READ (A,t) t := t+100 WRITE (A,t)150 READ (A,s) s := s*2 WRITE (A,s)300 READ (B,t) t := t+100 WRITE (B,t)150 READ (B,s) s := s*2 WRITE (B,s)300 Serializable but not Serial Schedule

Non-Serializable Schedule T1T2AB50 READ (A,t) t := t+100 WRITE (A,t)150 READ (A,s) s := s*2 WRITE (A,s)300 READ (B,s) s := s*2 WRITE (B,s)100 READ (B,t) t := t+100 WRITE (B,t)200 A := 2*(A+100) B := 2*B + 100

A Serializable Schedule with details T1T2AB50 READ (A,t) t := t+100 WRITE (A,t)150 READ (A,s) s := s*1 WRITE (A,s)150 READ (B,s) s := s*1 WRITE (B,s)50 READ (B,t) t := t+100 WRITE (B,t)150 A := 1*(A+100) B := 1*B + 100

Notations for Transaction 1.Action : An expression of the form r i (X) or w i (X) meaning that transaction T i reads or writes, respectively, the database X. 2.Transaction : A transaction T i is a sequence of actions with subscript. 3.Schedule : A schedule S of a transactions T is a sequence of actions, in which for each transaction T i in T, the action of T i appear in the definition of T i itself.

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

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