Schedules in dbms

Schedules

Schedules are sequences that indicate the chronological order in which instructions of concurrent transactions are executed.

A schedule must preserve the order in which the instructions appear in each individual transaction.

Types of schedule:

1. Serial schedule
2. Non serial schedule

1. Example of serial schedule
Transaction2 starts after completion of transaction1.

Transaction1

Transaction2

Read(X);

 

X=X-1000;

 

Write(X);

 

Read(Y);

 

Y=Y-1000;

 

Write(Y);

 

 

Read(X);

 

X=X+1000;

 

Write(X)


2. Example of non serial schedule
Transaction2 start while transaction1 was in process.

Transaction1

Transaction2

Read(X);

 

X=X-1000;

 

 

Read(X);

 

X=X+1000;

Write(X);

 

Read(Y);

 

 

Write(X);

Y=Y-1000;

 

Write(Y)

 


Schedule recoverability:

Sometimes a transaction may be incomplete due to a software issue, system crash or hardware failure.
Than failed transaction nedd to be rollback.

A transaction needed to be saved at each point.

In SQL we use commit to save the transactions tate.

This example uses commit at proper point, which maked it recoverable schedule. In case of fail it can be roll back to commit point.

Transaction1

Transaction1

Buffer

Transaction2

Transaction2

Buffer

Database

 

 

 

 

X = 1000

Read(X);

X = 10000

 

 

X = 10000

X=X-1000;

X = 9000

 

 

X = 10000

Write(X);

X = 9000

 

 

X = 9000

Commit;

 

 

 

 

 

 

Read(X);

X = 9000

X = 9000

 

 

X=X+1000;

X = 10000

X = 9000

 

 

Write(X);

X = 10000

X = 10000

 

 

Commit;

 

 



Check out this video lecture, for more information on schedule

More topics from DBMS to read:

References:

  1. Korth, Silbertz,Sudarshan, “Fundamental of Database System”, McGraw Hill
  2. Atul Kahate , “ Introduction to Database Management System”, Pearson Educations

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  2. Louden, “Programming Languages: Principles & Practices” , Cengage Learning 
  3. Tucker, “Programming Languages: Principles and paradigms “, Tata McGraw –Hill. 
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