Types of locks, Database Management System

Types of Locks

There are two necessary types of locks:

  • Binary lock: This locking mechanism has two states for to a data item: locked or unlocked
  • Multiple-mode locks: In this locking type every data item can be in 3 states read locked or shared locked, write locked or exclusive locked or unlocked.

Let us first take an instance for binary locking and describes how it solves the concurrency related troubles. Let us reconsider the transactions T1 and T2 for this purpose; though we will add to needed binary locks to them.

Schedule

T1

T2

Lock X

Lock X

 

Read X

Read X

 

Subtract 100

Subtract 100

 

Write X

Write X

 

Unlock X

Unlock X

 

Lock X

 

Lock X

Lock Y

 

Lock Y

Read X

 

Read X

Read Y

 

Read Y

Display X+Y

 

Display X+Y

Unlock X

 

Unlock X

Unlock Y

 

Unlock Y

Lock Y

Lock Y

 

Read Y

Read Y

 

Add 100

Add 100

 

Write Y

Write Y

 

Unlock Y

Unlock Y

 

 

 

 

                                                         Figure: An incorrect locking implementation

Does the locking as completed above solve the problem of concurrent transactions? No the similar problems still remains. Try working with the old value. Therefore, locking should be completed with some logic in order to make sure that locking results in no concurrency related problem. One such answer is given below:

Schedule

T1

T2

Lock X

Lock X

 

Lock Y

Lock Y

 

Read X

Read X

 

Subtract 100

Subtract 100

 

Write X

Write X

 

Lock X (issued by

T2)

Lock X: denied as T1 holds the lock.

The transaction T2 Waits and T1 continues.

Read Y

Read Y

 

Add 100

Add 100

 

Write Y

Write Y

 

Unlock X

Unlock X

 

 

The lock request of T2 on X can now

be granted it can resumes by locking X

Unlock Y

Unlock Y

 

Lock Y

 

Lock Y

Read X

 

Read X

Read Y

 

Read Y

Display X+Y

 

Display X+Y

Unlock X

 

Unlock X

Unlock Y

 

Unlock Y

                                                              Figure: A correct but restrictive locking implementation

Therefore, the locking as above when you get all the locks at the starting of the transaction and release them at the end ensures that transactions are implemented with no concurrency related problems. Though, such a scheme limits the concurrency. We will talk about a two-phase locking method in the next subsection that gives sufficient concurrency. Though, let us first discuss multiple mode locks.

Posted Date: 3/12/2013 2:36:18 AM | Location : United States







Related Discussions:- Types of locks, Assignment Help, Ask Question on Types of locks, Get Answer, Expert's Help, Types of locks Discussions

Write discussion on Types of locks
Your posts are moderated
Related Questions
1. Explain different addressing modes of 8086 Explanation of all addressing modes 2. Write a sequence of instructions to exchange two register contents using stack

You are managing the following database system: • DBMS: Oracle 11g • OS: Sun Solaris • System architecture: Web-based application using ASP.Net • Purpose of Server: HR managem

Events4Fun is an event management company in Europe with branches in South America as well. The company is well-known for its efficiency, good-quality services, and affordable char

Define database management system? Database management system (DBMS) is a set of interrelated data and a combination of programs to access those data.


Design an algorithm to implement operations  Each and every operation specified in the functional model could be formulated as an algorithm. The algorithm indicates how operat

Define a job scheduling strategy that will meet business requirement of reporting availability by 6am CST for the following cubes? Show the job scheduling dependencies in a pictori

Define According to the levels of abstraction, the schema at the intermediate level is called  Ans: As per to the levels of abstraction, the schema at the intermediate level is

Define shadow paging. An alternative to log-based crash recovery method is shadow paging. This technique requires fewer disk accesses than do the log-based methods.

i need help for assignment ..