To ensure the integrity of data during a transaction (A transaction is a unit of program that updates various data items, read more about it here), the database system maintains the following properties. These properties are widely known as ACID properties:

This property ensures that either all the operations of a transaction reflect in database or none. Let’s take an example of banking system to understand this: Suppose Account A has a balance of 400$ & B has 700$. Account A is transferring 100$ to Account B. This is a transaction that has two operations a) Debiting 100$ from A’s balance b) Creating 100$ to B’s balance. Let’s say first operation passed successfully while second failed, in this case A’s balance would be 300$ while B would be having 700$ instead of 800$. This is unacceptable in a banking system. Either the transaction should fail without executing any of the operation or it should process both the operations. The Atomicity property ensures that.

To preserve the consistency of database, the execution of transaction should take place in isolation (that means no other transaction should run concurrently when there is a transaction already running). For example account A is having a balance of 400$ and it is transferring 100$ to account B & C both. So we have two transactions here. Let’s say these transactions run concurrently and both the transactions read 400$ balance, in that case the final balance of A would be 300$ instead of 200$. This is wrong. If the transaction were to run in isolation then the second transaction would have read the correct balance 300$ (before debiting 100$) once the first transaction went successful.

In a database system where more than one transaction are being executed simultaneously and in parallel, the property of isolation states that all the transactions will be carried out and executed as if it is the only transaction in the system. No transaction will affect the existence of any other transaction.

Once a transaction completes successfully, the changes it has made into the database should be permanent even if there is a system failure. The recovery-management component of database systems ensures the durability of transaction.