3 Different Types of Read Write Conflict in DBMS [Explained with Example]

What is the Read-Write conflict in the database? Why does it occur in the DBMS (Database Management System)?

Data is crucial for any project. Text data required for the project can be saved in a file or in any reliable database. Days are gone when we were using the text files system to save the data.

In a database management system (DBMS), data is saved in the database. There are many advantages of using a DBMS system over a file system.

In this article, we will see all the different types of read-write conflict in DBMS.

How does Data Conflict Occur in DBMS?

Data is saved into the database. To fetch and write data from the database, you need to perform read-and-write operations on the data.

There are multiple entities (multiple users, multiple APPs, multiple websites…) performing these operations on the same data simultaneously.

Just as an instance, there is a centralized banking database where all the information about the users and the bank-related information is saved.

There might be multiple user logins to their bank account and access the same bank information at the same time. They perform various operations like reading, updating or deleting the data.

So basically there are two types of operations that can be performed on the database.

1. Read operation:

Read the data value from the database. It is a safe operation, as it does not update any information in the database.

2. Write operation:

It writes data into the database and saves it for further use. After writing data, data has to be committed (Commit Operation) to making updated information available for further operations. This operation is more prone to vulnerability as it involves modifying database information.

For a data update operation, it involves both reading and writing operations.

What is Transaction in Database?

The set of these multiple operations/instructions (to accomplish a particular task) are performed on the data. This set is called a Transaction.

Every entity accessing data holds different transactions and all the transactions execute simultaneously.

The operations involved in the successful Transaction:

• Read the data
• Modify read data values
• Write the data back
• Commit the transaction

Performing these operations by the different users on the same data at the same time may create data conflict. As like, accessing data by one transaction before committing another transaction does not hold well and gives data conflict.

There are different types of data conflict that happens during reading and writing operations called “Read-Write conflict”.

Different Types of Read-Write Conflict in DBMS:

As I mentioned it earlier, the read operation is safe as it does not modify any information. So, there is no Read-Read (RR) conflict in the database.

Any number of transactions are free to read the same data (without conflict) anytime as long as there is no write operation.

So, there are three types of conflict in the database transaction.

• Write-read (WR) conflict
• Read-write (RW) conflict
• Write-write (WW) conflict

Let’s see one-by-one with the example.

What is a Write-Read (WR) conflict?

This conflict occurs when a transaction reads the data which is written by the other transaction before committing.

In the following diagram, I am using the following notions

A, B - two different data objects from database

T1, T2 - two different transactions

R(A) - reading data A

W(A) - Writing data A

Com. - committing transaction

Here, the transaction T2 is reading the data which is written by the T1 before T2 commits. It is also called a Dirty Read.

It violates the ACID property of the data consistency rule.

Expected behavior:

The new transaction (says T2) should not read the value of any other transaction (say T1) that already has written the data and has not committed.

What is a Read-Write (RW) conflict?

Transaction T2 is Writing data which is previously read by transaction T1.

Here if you look at the diagram above, data read by transaction T1 before and after T2 commits is different.

Example:

Suppose Alice and Bob want to book a flight for their vacation. Alice opened the airline’s website to check the availability and cost of the ticket. There is only one ticket available. Alice finds it expensive and looks for other airline fares if she gets any offers.

Meanwhile, Bob logged in to the same airline portal and booked the ticket. Now, there is no more flight ticket available.

Alice does not find any good offers on other airline portals, she comes back to the previous airline portal. And tried to book a flight ticket even though it was not available. Its Conflict, Read-Write (WR) conflict.

What is Write-Write (WW) conflict?

Here Transaction T2 is writing data that is already written by other Transaction T1. T2 overwrites the data written by T1. It is also called a blind write operation.

Data written by T1 has vanished. So it is a data update loss.

Example:

Alice and Bob share a common Google sheet online. Both read the file. Alice updates the document and forgets to save the file. On another end, Bob updates the Google sheet and saves the file.

The content updated by Alice is overwritten by Bob. The data updated by Alice is lost. It is called a Write-Write (WW) conflict.

This is all about different types of read-write conflicts in DBMS. If you have any doubts or any thought about this article, write a comment below. I will reply to you back as soon as possible.

Aniruddha Chaudhari

I am a Python enthusiast who loves Linux and Vim. I hold a Master of Computer Science degree from NIT Trichy and have 10 years of experience in the IT industry, focusing on the Software Development Lifecycle from Requirements Gathering, Design, Development to Deployment. I have worked at IBM, Ericsson, and NetApp, and I share my knowledge on CSEstack.org.