Exclusive lock, Database Management System

Assignment Help:

Exclusive lock

  • It is asked for by a transaction on a data item that it wants to update.
  • No other transaction can put either a shared lock or an exclusive lock on a data item that is locked in an exclusive mode.

Let us explain the above two modes with the help of an instance. We will once again consider the transactions T1 and T2 but in addition a transaction T11 that search the total of accounts Y and Z.

Schedule

T1

T2

T11

S_Lock X

 

S_Lock X

 

S_Lock Y

 

S_Lock Y

 

Read X

 

Read X

 

S_Lock Y

 

 

S_Lock Y

S_Lock Z

 

 

S_Lock Z

 

 

 

Read Y

 

 

 

Read Z

X_Lock X

X_Lock X. The exclusive lock request on X is denied as T2 holds the Read lock. The transaction T1 Waits.

Read Y

 

Read Y

 

Display X+Y

 

Display X+Y

 

Unlock X

 

Unlock X

 

X_Lock Y

X_Lock Y. The previous exclusive lock request on X is granted as X is unlocked. But the new exclusive lock request on Y is not granted as Y is locked by T2 and T11 in read mode. Thus T1 waits till both T2 and T11 will release the read lock on Y.

Display Y+Z

 

 

Display Y+Z

Unlock Y

 

Unlock Y

 

Unlock Y

 

 

Unlock Y

Unlock Z

 

 

Unlock Z

Read X

Read X

 

 

Subtract 100

Subtract 100

 

 

Write X

Write X

 

 

Read Y

Read Y

 

 

Add 100

Add 100

 

 

Write Y

Write Y

 

 

Unlock X

Unlock X

 

 

Unlock Y

Unlock Y

 

 

                                                               Figure: Example of Locking in multiple-modes

Therefore, the locking as above results in a serialisable schedule. Now the query is can we release locks a bit early and have still no concurrency related problem?Yes, we can do it if we lock using two-phase locking protocol.


Related Discussions:- Exclusive lock

Abap/4 statements to access internal tables without header, What should be ...

What should be declared explicitly in the corresponding ABAP/4 Statements to access internal tables without header lines & why? Work Area. This is needed as the Work Area is

Triggers and Stored Procedures, 10A.53 Code an Oracle Database trigger to e...

10A.53 Code an Oracle Database trigger to enforce the constraint that an employee can never change his or her department. 10A.54 Code an Oracle Database trigger to allow the delet

Data definition language, Data Definition Language It organises the dat...

Data Definition Language It organises the data in the form of columns and rows. But what does the data field column of table accumulate? How do you describe it using SQL? Th

NORRMALIZATION, Home based assignment Consider an ORDER table with the foll...

Home based assignment Consider an ORDER table with the following attributes: • Order Number (ONo),Customer Number (CNo),Customer Name (CNa),Item Number (INo),Unit Price (UP), Quant

Explain recovery process after system failure, Explain recovery process aft...

Explain recovery process after system failure using checkpoint. Ans: Checkpoint scheme or method is an extra component of the log based recovery system. This scheme is used to

Describe the capabilities offered via mysql client apis, Describe the capab...

Describe the capabilities offered via MySQL client APIs? . • Checking for errors. . • Connecting to the MySQL server, selecting a database, disconnecting from the server

Prepare an erd for the scenario, Question: (a) Prepare an ERD for the s...

Question: (a) Prepare an ERD for the scenario given below. (b) Convert the ER diagram produced in (a) above into its physical design. You have to choose an appropriate Pri

What is a transaction, What is a Transaction? A transaction is an even...

What is a Transaction? A transaction is an event which happens on the database. Generally a transaction reads a value from the database or writes a value to the database. If y

Inheritance, what is relationship inheritance and attribute inheritance

what is relationship inheritance and attribute inheritance

Determine Which is not a consequence of concurrent operation, Determine Whi...

Determine Which is not a consequence of concurrent operations? Ans: Update anomaly.

Write Your Message!

Captcha
Free Assignment Quote

Assured A++ Grade

Get guaranteed satisfaction & time on delivery in every assignment order you paid with us! We ensure premium quality solution document along with free turntin report!

All rights reserved! Copyrights ©2019-2020 ExpertsMind IT Educational Pvt Ltd