What is a database transaction?

What is a database transaction?

A database transaction is a set of operations that are performed on a database as a single logical unit of work. A database transaction ensures that the data in the database remains consistent and reliable, even in the event of system failures or concurrent access by multiple users.

Why are database transactions important?

Database transactions are important for two main reasons: to provide reliable recovery from failures and to provide isolation between concurrent users.

Reliable recovery from failures

System failures are inevitable in any real-world scenario. For example, a power outage, a network error, or a hardware malfunction can cause a system to crash unexpectedly. When this happens, some operations on the database may be incomplete or inconsistent, leaving the database in an uncertain state. For example, suppose you are transferring money from one bank account to another. This transaction involves two operations: debiting the source account and crediting the destination account. If the system crashes after debiting the source account but before crediting the destination account, the money will be lost and the accounts will be out of balance.

To prevent such scenarios, database transactions provide a mechanism to ensure that either all or none of the operations in a transaction are executed. This property is called atomicity. If a transaction is atomic, it means that it is indivisible and irreducible. It cannot be split into smaller parts or interrupted by other transactions. If a transaction fails for any reason, it is rolled back to its original state, as if it never happened. If a transaction succeeds, it is committed to the database, making its changes permanent and visible to other transactions.

Isolation between concurrent users

Another challenge that databases face is how to handle multiple users accessing and modifying the same data at the same time. For example, suppose two customers are trying to book the last seat on a flight. If both transactions read the available seats at the same time, they will both see one seat left. If they both proceed to book the seat, they will both think they have succeeded, but only one of them will actually get the seat. This will result in an overbooking and an unhappy customer.

To avoid such conflicts, database transactions provide a mechanism to ensure that each transaction sees a consistent and isolated view of the data. This property is called isolation. If a transaction is isolated, it means that it does not interfere with or affect other transactions. It can only see the data that was committed before it started and the data that it has modified itself. It cannot see the data that is being modified by other concurrent transactions until they are committed.

How do database transactions work?

A database transaction typically goes through four stages: begin, execute, commit or rollback, and end.

Begin

The begin stage marks the start of a transaction. It establishes a connection to the database and allocates any resources needed for the transaction. It also sets the isolation level for the transaction, which determines how much data visibility and concurrency control the transaction has.

Execute

The execute stage performs the operations that make up the transaction. These operations can include reading, writing, updating, deleting, or querying data from the database. The execute stage also checks for any errors or violations of integrity constraints that may occur during the execution of the operations.

Commit or rollback

The commit or rollback stage decides whether to finalize or cancel the transaction based on its outcome. If all operations in the transaction are successful and no errors or violations are detected, the transaction is committed to the database. This means that all changes made by the transaction are made permanent and visible to other transactions. If any operation in the transaction fails or an error or violation is detected, the transaction is rolled back to its original state. This means that all changes made by the transaction are discarded and have no effect on the database.

End

The end stage marks the completion of a transaction. It releases any resources used by the transaction and closes the connection to the database.

What are some properties of database transactions?

Database transactions are often characterized by four properties: atomicity, consistency, isolation, and durability. These properties are collectively known as ACID¹.

Atomicity

Atomicity means that a transaction is either fully completed or not completed at all. There is no intermediate state where some operations are executed and some are not. If a transaction fails for any reason, all its changes are rolled back and have no effect on the database.

Consistency

Consistency means that a transaction preserves the integrity and validity of the data in the database. A transaction must follow all rules and constraints defined by the database schema and logic. For example, if a transaction transfers money from one account to another, it must ensure that both accounts have valid balances and that no money is created or destroyed.

Isolation

Isolation means that a transaction operates independently from other concurrent transactions. A transaction cannot see or affect data that is being modified by other transactions until they are committed. Each transaction has its own consistent view of the data.

Durability

Durability means that once a transaction is committed, its changes are permanent and persistent. Even if the system crashes or restarts, the changes made by the transaction will not be lost or corrupted.

What are some examples of database transactions?

Database transactions are widely used in various applications and domains that require data integrity and reliability. Some examples of database transactions are:

• Online banking: A transaction can transfer money from one account to another, check the balance of an account, or pay a bill.
• E-commerce: A transaction can process an order, update the inventory, or confirm a payment.
• Airline reservation: A transaction can book a flight, cancel a reservation, or check the availability of seats.
• Social media: A transaction can post a status, like a comment, or follow a user.

Conclusion

A database transaction is a set of operations that are performed on a database as a single logical unit of work. A database transaction ensures that the data in the database remains consistent and reliable, even in the event of system failures or concurrent access by multiple users. Database transactions have four properties: atomicity, consistency, isolation, and durability. Database transactions are widely used in various applications and domains that require data integrity and reliability.

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