Advantages of using a Transaction
- Maintains a consistent and valid database after each transaction.
- Makes certain that updates to the database don’t affect its dependability or accuracy.
- Enables simultaneous use of numerous users without sacrificing data consistency.
Transaction Management
Transactions are a set of operations used to perform a logical set of work. It is the bundle of all the instructions of a logical operation. A transaction usually means that the data in the database has changed. One of the major uses of DBMS is to protect the user’s data from system failures. It is done by ensuring that all the data is restored to a consistent state when the computer is restarted after a crash. The transaction is any one execution of the user program in a DBMS. One of the important properties of the transaction is that it contains a finite number of steps. Executing the same program multiple times will generate multiple transactions.
Example: Consider the following example of transaction operations to be performed to withdraw cash from an ATM vestibule.
Steps for ATM Transaction
- Transaction Start.
- Insert your ATM card.
- Select a language for your transaction.
- Select the Savings Account option.
- Enter the amount you want to withdraw.
- Enter your secret pin.
- Wait for some time for processing.
- Collect your Cash.
- Transaction Completed.
A transaction can include the following basic database access operation.
- Read/Access data (R): Accessing the database item from disk (where the database stored data) to memory variable.
- Write/Change data (W): Write the data item from the memory variable to the disk.
- Commit: Commit is a transaction control language that is used to permanently save the changes done in a transaction
Example: Transfer of 50₹ from Account A to Account B. Initially A= 500₹, B= 800₹. This data is brought to RAM from Hard Disk.
R(A) -- 500 // Accessed from RAM.
A = A-50 // Deducting 50₹ from A.
W(A)--450 // Updated in RAM.
R(B) -- 800 // Accessed from RAM.
B=B+50 // 50₹ is added to B's Account.
W(B) --850 // Updated in RAM.
commit // The data in RAM is taken back to Hard Disk.
Stages of Transaction
Note: The updated value of Account A = 450₹ and Account B = 850₹.
All instructions before committing come under a partially committed state and are stored in RAM. When the commit is read the data is fully accepted and is stored on a Hard Disk.
If the transaction is failed anywhere before committing we have to go back and start from the beginning. We can’t continue from the same state. This is known as Roll Back.