Transaction in DBMS-

 

“Transaction is a set of operations which are all logically related.”

OR

“Transaction is a single logical unit of work formed by a set of operations.”

 

Operations in Transaction-

 

The main operations in a transaction are-

1. Read Operation
2. Write Operation

 

1. Read Operation-

 

• Read operation reads the data from the database and then stores it in the buffer in main memory.
• For example- Read(A) instruction will read the value of A from the database and will store it in the buffer in main memory.

 

2. Write Operation-

 

• Write operation writes the updated data value back to the database from the buffer.
• For example- Write(A) will write the updated value of A from the buffer to the database.

 

Transaction States-

 

A transaction goes through many different states throughout its life cycle.

These states are called as transaction states.

Transaction states are as follows-

1. Active state
2. Partially committed state
3. Committed state
4. Failed state
5. Aborted state
6. Terminated state

 

 

1. Active State-

 

• This is the first state in the life cycle of a transaction.
• A transaction is called in an active state as long as its instructions are getting executed.
• All the changes made by the transaction now are stored in the buffer in main memory.

 

2. Partially Committed State-

 

• After the last instruction of transaction has executed, it enters into a partially committed state.
• After entering this state, the transaction is considered to be partially committed.
• It is not considered fully committed because all the changes made by the transaction are still stored in the buffer in main memory.

 

3. Committed State-

 

• After all the changes made by the transaction have been successfully stored into the database, it enters into a committed state.
• Now, the transaction is considered to be fully committed.

 

NOTE-

 

• After a transaction has entered the committed state, it is not possible to roll back the transaction.
• In other words, it is not possible to undo the changes that has been made by the transaction.
• This is because the system is updated into a new consistent state.
• The only way to undo the changes is by carrying out another transaction called as compensating transaction that performs the reverse operations.

 

4. Failed State-

 

• When a transaction is getting executed in the active state or partially committed state and some failure occurs due to which it becomes impossible to continue the execution, it enters into a failed state.

 

5. Aborted State-

 

• After the transaction has failed and entered into a failed state, all the changes made by it have to be undone.
• To undo the changes made by the transaction, it becomes necessary to roll back the transaction.
• After the transaction has rolled back completely, it enters into an aborted state.

 

6. Terminated State-

 

• This is the last state in the life cycle of a transaction.
• After entering the committed state or aborted state, the transaction finally enters into a terminated state where its life cycle finally comes to an end.

 

Next Article- ACID Properties of Transaction

 

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