These are the essential requirements that justify a Transaction Processing System.
Atomicity (In full or Not at all)
This means that either a transaction is completed and recorded in full, or not completed hence not recorded at all. TPS ensures that transactions take place in their entirety.
Example: While withdrawing cash from an ATM machine, two things can happen. First, the transaction may not be complete due to wrong pin, insufficient funds or wrong account selection. Here, the transaction is not complete hence it is not recorded in the system.
Or, the transaction will be complete. The ATM will dispense the correct amount of cash, and the proper account of the user is debited with the amount withdrawn and thus, properly recorded by the system.
The system consistently follows the rules which are related to a transaction. All of these rules are checked properly by the system before the transaction is completed.
Examples include: Rules like Daily withdrawal limit, Actual account Balance, Correct PIN, etc should be followed to complete a transaction at an ATM machine.
Isolation (Simultaneously & Immediately)
If a transaction is completed, all the relevant entries related to the transaction are automatically processed.
Eg: When the ATM dispenses cash, the relevant user account is simultaneously and immediately debited.
Durability (once done can't be undone)
A TPS system is such that once a transaction is complete, it can not be undone. A user can not undo the transaction after the ATM has dispensed the cash.
Durability also means that once the transaction is completed, a log is generated by the system so that in the event of system crashes, all the entries made by the system can be retrieved from the log.