TRANSACTION
A transaction can be defined as a group of tasks. A single task is the minimum processing unit which
cannot be divided further. Let’s take an example of a simple transaction. Suppose a bank employee
transfers Rs 500 from A's account to B's account. This very simple and small transaction involves
several low-level tasks.
A’s Account
Open_Account(A)
Old_Balance = A.balance
New_Balance = Old_Balance - 500
A.balance = New_Balance
Close_Account(A)
B’s Account
Open_Account(B)
Old_Balance = B.balance
New_Balance = Old_Balance + 500
B.balance = New_Balance
Close_Account(B)
ACID Properties
A transaction is a very small unit of a program and it may contain several low-level tasks. A transaction in a database
system must maintain Atomicity, Consistency, Isolation, and Durability — commonly known as ACID properties —in
order to ensure accuracy, completeness, and data integrity.
Atomicity: This property states that a transaction must be treated as an atomic unit, that is, either all of its operations
are executed or none. There must be no state in a database where a transaction is left partially completed. States should
be defined either before the execution of the transaction or after the execution/abortion/failure of the transaction.
Consistency: The database must remain in a consistent state after any transaction. No transaction should have any
adverse effect on the data residing in the database. If the database was in a consistent state before
the execution of a transaction, it must remain consistent after the execution of the transaction as well.
Durability: The database should be durable enough to hold all its latest updates even if the system fails or restarts. If a
transaction updates a chunk of data in a database and commits, then the database will hold the modified data. If a
transaction commits but the system fails before the data could be written on to the disk, then that data will be updated
once the system springs back into action.
Isolation: 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.
States of Transactions
A transaction in a database can be in one of the following states:
Active: In this state, the transaction is being executed. This is the initial state of every
transaction.
Partially Committed: When a transaction executes its final operation, it is said to be
in a partially committed state.
Failed: A transaction is said to be in a failed state if any of the checks made by the
database recovery system fails. A failed transaction can no longer proceed further.
Aborted: If any of the checks fails and the transaction has reached a failed state, then
the recovery manager rolls back all its write operations on the database to bring the database back to its original state
where it was prior to the execution of the transaction. Transactions in this state are called aborted. The database recovery
module can select one of the two operations after a transaction aborts:
o Re-start the transaction
o Kill the transaction
Committed: If a transaction executes all its operations successfully, it is said to be committed. All its effects are now
permanently established on the database system.
Security:
Features like multiple views offer security to some extent where users are unable to access data
of other users and departments. DBMS offers methods to impose constraints while entering data
into the database and retrieving the same at a later stage. DBMS offers many different levels of
security features, which enables multiple users to have different views with different features.
For example, a user in the Sales department cannot see the data that belongs to the Purchase
department. Additionally, it can also be managed how much data of the Sales department
should be displayed to the user. Since a DBMS is not saved on the disk as traditional file
systems, it is very hard for miscreants to break the code.
Remote backup provides a sense of security in case the primary location where the database is located gets destroyed.
Remote backup can be offline or real-time or online. In case it is offline, it is maintained manually.
Online backup systems are more real-time and lifesavers for database administrators and
investors. An online backup system is a mechanism where every bit of the real-time data is
backed up simultaneously at two distant places. One of them is directly connected to the
system and the other one is kept at a remote place as backup.
As soon as the primary database storage fails, the backup system senses the failure and
switches the user system to the remote storage. Sometimes this is so instant that the users
can’t even realize a failure.
