Properties of transactions

Properties of transactions

There are properties that all transactions should follow and possess.
The four basic are in combination termed as ACID properties.
ACID properties and its concepts of a transaction are put forwarded by Haerder and Reuter in the year 1983.
It is commonly known as ACID properties − in order to ensure accuracy, completeness, and data integrity.
The ACID has a full form and is as follows:

The 'all or nothing' property.
A transaction is an indivisible entity that is either performed in its entirety or will not get performed at all.
This is the responsibility or duty of the recovery subsystem of the DBMS to ensure atomicity.
Example:

Suppose Account A has a balance of 400 Rs. & B has 700 Rs.
Account A is transferring 100 Rs. to Account B.
This is a transaction that has two operations a) Debiting 100 Rs. from A’s balance b) Creating 100 Rs. to B’s balance.
Let’s say the first operation passed successfully while the second failed.
So, in this case, A’s balance would be 300 Rs. while B would be having 700 Rs. instead of 800 Rs.
This is unacceptable in a banking system.
Either the transaction should fail without executing any of the operations or it should process both the operations.
The Atomicity property ensures that.

A transaction must alter the database from one steady-state to another steady state.
This is the responsibility of both the DBMS and the application developers to make certain consistency.
The DBMS can ensure consistency by putting into effect all the constraints that have been mainly on the database schema such as integrity and enterprise constraints.
Example:

Account A is having a balance of 400 Rs. and it is transferring 100 Rs. to account B & C both.
So we have two transactions here.
Let’s say these transactions run concurrently and both the transactions read 400 Rs. balance.
In that case, the final balance of A would be 300 Rs. instead of 200 Rs.
This is wrong.
If the transaction were to run in isolation then the second transaction would have read the correct balance 300 Rs. (before debiting 100 Rs.)
Once the first transaction went successful.

Transactions that are executing independently of one another is the primary concept followed by isolation.
In other words, the frictional effects of incomplete transactions should not be visible or come into notice to other transactions going on simultaneously.
It is the responsibility of the concurrency control sub-system to ensure adapting the isolation.

The effects of an accomplished transaction are permanently recorded in the database and must not get lost or vanished due to subsequent failure.
So this becomes the responsibility of the recovery sub-system to ensure durability.