Concept of concurrency in dbms

-
                          CONCURRENCY
  • Concurrency is the ability of a database to allow multiple users to affect multiple transactions. 
  • This is one of the main properties that separates a database from other forms of data storage like spreadsheets.
  • Data concurrency means that many users can access data at same time.

Advantages of concurrency

The good is to serve many users and provides better throughput by sharing resources.
  • Reduced waiting time response time or turn around time.
  • Increased throughput or resource utilization
  • If we run only one transaction at a time than the acid property is sufficient but it is possible that when multiple transactions are executed concurrently than database may become inconsistent.
  • Overlapping with the input-output activity with CPU also makes the response time better.
  • But interleaving of instruction between transaction may also lead to many problems due to which concurrency control is required.

Problems due to concurrency

There are many which may occur due to concurrency,
1) Dirty read problem
If a transaction reads an uncommitted temporary value written by some other transaction than it is called dirty read problem. In this one transaction read a data item updated by another uncommitted transaction that may be future be aborted or failed. In such cases, the read value disappears from the database upon abort this is turned on dirty read the reading transaction end with incorrect results.
Example
    T1      T2
            R(A)
            W(A)
    R(A)
The values of item x which is read by T2 is called dirty read data because this data can be created by a transactions that has not been committed yet.
2) Loss update problem/ write - write problem
This problem occur when two transactions access the same data item and have their operations interleaved in a way that makes the value of some database items incorrect.
If there are two write operations of the different transaction on some data values and in between them there are no read operations then the second write over the first .consider the schedule below,
Example
    T1      T2
    R(A)
    W(A)
            W(A)
Here is a blind write that means write without a read. Here the changes made by transaction T1 are lost which is updated by a transaction T2.
3) Unrepeatable and phantom read problem
When a transaction cannot repeat the read instructions because the variable is deleted by some other transaction then this problem is called phantom read problem. In this problem at different instances of time a transaction read gives different values it is because data item might have been updated by another transaction.
This causes a problem while execution of some aggregate by a transaction and due to changes in the values of the data item by another transaction it leads to incorrect results. When a transaction read values of data item twice and another transaction's updates data item in between then the results of two read operations will differ.
Example
    T1          T2
    R(A)                
                R(A)
    Delete(A)                         
                R(A)
4) Incorrect summary problem
When one of the transactions is checking on aggregate summary function while other transactions are updating then this problem is called incorrect summary problem. The aggregate functions may calculate some values before they updated and others after they are updated.

Thank you viewers.
This is published by soumy sinha. .
If you have doubt and want to ask same related questions then pls tell us comment box.
Share with your friends and subscribe our page.

Comments

Post a Comment

Popular posts from this blog

Introduction of Digital computer

-
Image
INTRODUCTION Digital computer concept:- 1. Digital computers are designed to store, process and communicate information in digital form. 2. Digital computer is any of a class of digital devices capable of solving problems by processing information in discrete form. 3. It is used for processing data represented by discrete, localized physica signals, as the presence or electic current. 4.The main three components of digital computer:- >input:- The user gives a set of input data through input devices. >Processing :- The input data is processed with the help of CPU by well defined and finite sequence of step. >Output : - Some data available from the processing step are output to the user. TYPE OF DIGITAL COMPUTER :- 1.Microcomputer/Desktop computer 2.Minicomputer 3.Mainframe computer 4.Supercomputers Microcomputer :- 1.Microcomputer is also called a personal computer or desktop computer. 2.It includes a microprocessor, memory,...
Read more

INTRODUCTION OF DBMS

-
What is Database :- The database is a collection of inter-related data which is used to retrieve, insert and delete the data efficiently. It is also used to organize the data in the form of a table, schema, views, and reports, etc. For example: The college Database organizes the data about the admin, staff, students and faculty etc. Using the database, you can easily retrieve, insert, and delete the information. DBMS (Database Management System):- Database management system is a software which is used to manage the database. For example: MySQL, Oracle, etc are a very popular commercial database which is used in different applications.DBMS provides an interface to perform various operations like database creation, storing data in it, updating data, creating a table in the database and a lot more.It provides protection and security to the database. In the case of multiple users, it also maintains data consistency. DBMS allows users the following tasks:- Data Definitio...
Read more

Introduction to cache memory

-
Image
Cache Memory in Computer Organization Cache Memory  is a special very high-speed memory. It is used to speed up and synchronizing with high-speed CPU. Cache memory is costlier than main memory or disk memory but economical than CPU registers. Cache memory is an extremely fast memory type that acts as a buffer between RAM and the CPU. It holds frequently requested data and instructions so that they are immediately available to the CPU when needed. Cache memory is used to reduce the average time to access data from the Main memory. The cache is a smaller and faster memory which stores copies of the data from frequently used main memory locations. There are various different independent caches in a CPU, which stored instruction and data. Levels of memory: Level 1 or Register – It is a type of memory in which data is stored and accepted that are immediately stored in CPU. Most commonly used register is accumulator, Program counter, address register etc. Level 2 or...
Read more