Transaction in DBMS

Last Updated : 14 Jan, 2025

In a Database Management System (DBMS), a transaction is a sequence of operations performed as a single logical unit of work. These operations may involve reading, writing, updating, or deleting data in the database. A transaction is considered complete only if all its operations are successfully executed, Otherwise the transaction must be rolled back, and the database should remain unchanged.

In this article, we will discuss what a transaction means, various operations of transactions, transaction states and properties of transactions in DBMS.

What does a Transaction mean in DBMS?

A transaction refers to a sequence of one or more operations (such as read, write, update, or delete) performed on the database as a single logical unit of work. A transaction ensures that either all the operations are successfully executed (committed) or none of them take effect (rolled back). Transactions are designed to maintain the integrity, consistency and reliability of the database, even in the case of system failures or concurrent access.

All types of database access operation which are held between the beginning and end transaction statements are considered as a single logical transaction. During the transaction the database is inconsistent. Only once the database is committed the state is changed from one consistent state to another.

Facts about Database Transactions

A transaction is a program unit whose execution may or may not change the contents of a database.
The transaction is executed as a single unit.
If the database operations do not update the database but only retrieve data, this type of transaction is called a read-only transaction.
A successful transaction can change the database from one CONSISTENT STATE to another.
DBMS transactions must be atomic, consistent, isolated and durable.
If the database were in an inconsistent state before a transaction, it would remain in the inconsistent state after the transaction.

Operations of Transaction

A user can make different types of requests to access and modify the contents of a database. So, we have different types of operations relating to a transaction. They are discussed as follows:

i) Read(X)

A read operation is used to read the value of X from the database and store it in a buffer in the main memory for further actions such as displaying that value.
Such an operation is performed when a user wishes just to see any content of the database and not make any changes to it. For example, when a user wants to check his/her account's balance, a read operation would be performed on user's account balance from the database.

ii) Write(X)

A write operation is used to write the value to the database from the buffer in the main memory. For a write operation to be performed, first a read operation is performed to bring its value in buffer, and then some changes are made to it, e.g. some set of arithmetic operations are performed on it according to the user's request, then to store the modified value back in the database, a write operation is performed.
For example, when a user requests to withdraw some money from his account, his account balance is fetched from the database using a read operation, then the amount to be deducted from the account is subtracted from this value, and then the obtained value is stored back in the database using a write operation.

iii) Commit

This operation in transactions is used to maintain integrity in the database. Due to some failure of power, hardware, or software, etc., a transaction might get interrupted before all its operations are completed. This may cause ambiguity in the database, i.e. it might get inconsistent before and after the transaction.
To ensure that further operations of any other transaction are performed only after work of the current transaction is done, a commit operation is performed to the changes made by a transaction permanently to the database.

iv) Rollback

This operation is performed to bring the database to the last saved state when any transaction is interrupted in between due to any power, hardware or software failure.
In simple words, it can be said that a rollback operation does undo the operations of transactions that were performed before its interruption to achieve a safe state of the database and avoid any kind of ambiguity or inconsistency.

Transaction Schedules

When multiple transaction requests are made at the same time, we need to decide their order of execution. Thus, a transaction schedule can be defined as a chronological order of execution of multiple transactions.

There are broadly two types of transaction schedules discussed as follows:

i) Serial Schedule

In this kind of schedule, when multiple transactions are to be executed, they are executed serially, i.e. at one time only one transaction is executed while others wait for the execution of the current transaction to be completed. This ensures consistency in the database as transactions do not execute simultaneously.
But, it increases the waiting time of the transactions in the queue, which in turn lowers the throughput of the system, i.e. number of transactions executed per time.
To improve the throughput of the system, another kind of schedule are used which has some more strict rules which help the database to remain consistent even when transactions execute simultaneously.

ii) Non-Serial Schedule

To reduce the waiting time of transactions in the waiting queue and improve the system efficiency, we use non-serial schedules which allow multiple transactions to start before a transaction is completely executed. This may sometimes result in inconsistency and errors in database operation.
So, these errors are handled with specific algorithms to maintain the consistency of the database and improve CPU throughput as well.
Non-serial schedules are also sometimes referred to as parallel schedules, as transactions execute in parallel in these kinds of schedules.

Serializable

Serializability in DBMS is the property of a non-serial schedule that determines whether it would maintain the database consistency or not.
The non-serial schedule which ensures that the database would be consistent after the transactions are executed in the order determined by that schedule is said to be Serializable Schedules.
The serial schedules always maintain database consistency as a transaction starts only when the execution of the other transaction has been completed under it.
Thus, serial schedules are always serializable.
A transaction is a series of operations, so various states occur in its completion journey.

To read about Transaction States Refer Here.

Properties of Transaction

As transactions deal with accessing and modifying the contents of the database, they must have some basic properties which help maintain the consistency and integrity of the database before and after the transaction. Transactions follow 4 properties, namely, Atomicity, Consistency, Isolation, and Durability.
Generally, these are referred to as ACID properties of transactions in DBMS. ACID is the acronym used for transaction properties. A brief description of each property of the transaction is as follows.

i) Atomicity

This property ensures that either all operations of a transaction are executed or it is aborted. In any case, a transaction can never be completed partially.
Each transaction is treated as a single unit (like an atom). Atomicity is achieved through commit and rollback operations, i.e. changes are made to the database only if all operations related to a transaction are completed, and if it gets interrupted, any changes made are rolled back using rollback operation to bring the database to its last saved state.

ii) Consistency

This property of a transaction keeps the database consistent before and after a transaction is completed.
Execution of any transaction must ensure that after its execution, the database is either in its prior stable state or a new stable state.
In other words, the result of a transaction should be the transformation of a database from one consistent state to another consistent state.
Consistency, here means, that the changes made in the database are a result of logical operations only which the user desired to perform and there is not any ambiguity.

iii) Isolation

This property states that two transactions must not interfere with each other, i.e. if some data is used by a transaction for its execution, then any other transaction can not concurrently access that data until the first transaction has completed.
It ensures that the integrity of the database is maintained and we don't get any ambiguous values. Thus, any two transactions are isolated from each other.

iv) Durability

This property ensures that the changes made to the database after a transaction is completely executed, are durable.
It indicates that permanent changes are made by the successful execution of a transaction.
In the event of any system failures or crashes, the consistent state achieved after the completion of a transaction remains intact. The recovery subsystem of DBMS is responsible for enforcing this property.

Please refer Transaction States in DBMS for different states of a transaction.

Conclusion

Transactions in DBMS are pivotal in maintaining the integrity and consistency of the database through a series of well-defined operations and states. From the initial execution of operations to handling errors and ensuring consistency, transactions must adhere to the ACID properties—Atomicity, Consistency, Isolation, and Durability. These properties guarantee that transactions are processed reliably and that the database remains stable even in the face of failures or concurrent operations.

FAQs on Transaction in DBMS

What is meant by a transaction in DBMS?

In DBMS, a transaction is a set of logical operations performed to access and modify the contents of the database as per the user's request.

What is meant by ACID properties in transactions?

ACID is an acronym used for the properties of transaction in DBMS. It is used to denote the properties of transactions, i.e. Atomicity, Consistency, Isolation and Durability.

Which operation of transactions ensures the durability property?

In DBMS, the durability of a transaction, i.e. the changes made by it are saved to the database permanently, is ensured by the 'Commit' operation. A transaction is completed only if data is saved using 'Commit' operation. And then, the changes remain durable, i.e. in case of any system failures, the last saved state of the database can be recovered through database recovery subsystem in DBMS.

What is meant by schedules of transactions in DBMS?

When multiple transaction requests are made at the same time, we need to decide the order of execution of these transactions. This chronological order of execution of transactions is called as a schedule of transactions in DBMS. It is mainly of two types, i.e. Serial Schedules and Non Serial Schedules.

What do you mean by serializability in DBMS?

Serializability is the property of a schedule of transactions in DBMS which determines whether the database would be in consistent state or not if the transactions are executed following the given schedule.

Transaction States in DBMS

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