Azure SQL Database Transactions

Database Transactions in Azure SQL Database

Transactions are a fundamental concept in relational databases, ensuring data integrity and consistency. In Azure SQL Database, transactions are handled using the standard SQL transaction control language. A transaction is a logical unit of work that consists of one or more database operations. These operations are treated as a single, indivisible unit: either all operations within the transaction are completed successfully, or none of them are.

ACID Properties

Transactions in Azure SQL Database adhere to the ACID properties:

Atomicity: Ensures that all operations within a transaction are performed completely or not at all. If any operation fails, the entire transaction is rolled back, leaving the database in its original state.
Consistency: Guarantees that a transaction will bring the database from one valid state to another. It ensures that any data written to the database must be valid according to all defined rules, including constraints, cascades, triggers, and any combination thereof.
Isolation: Defines how and when the changes made by one concurrent transaction become visible to other concurrent transactions. Azure SQL Database supports various isolation levels to manage this concurrency.
Durability: Ensures that once a transaction has been committed, it will remain committed even in the event of a system failure, such as power outage or crash.

Transaction Control Language (TCL)

You control transactions using the following Transact-SQL statements:

BEGIN TRANSACTION: Marks the beginning of a transaction.
COMMIT TRANSACTION: Ends a transaction successfully, making all its changes permanent.
ROLLBACK TRANSACTION: Ends a transaction by undoing all its changes since the beginning of the transaction.
SAVE TRANSACTION: Creates a save point within a transaction to which you can later roll back.

Example of a Transaction

                
                    BEGIN TRANSACTION;

                    UPDATE Accounts
                    SET Balance = Balance - 100
                    WHERE AccountId = 101;

                    UPDATE Accounts
                    SET Balance = Balance + 100
                    WHERE AccountId = 202;

                    // Check for sufficient funds before committing
                    IF EXISTS (
                        SELECT 1
                        FROM Accounts
                        WHERE AccountId = 101 AND Balance < 0
                    )
                    BEGIN
                        ROLLBACK TRANSACTION;
                        PRINT 'Transaction rolled back due to insufficient funds.';
                    END
                    ELSE
                    BEGIN
                        COMMIT TRANSACTION;
                        PRINT 'Transaction committed successfully.';
                    END;
                
            

Isolation Levels

Azure SQL Database supports the following standard SQL Server isolation levels:

READ UNCOMMITTED: The lowest isolation level. Transactions can read rows that have been modified by other transactions but not yet committed.
READ COMMITTED: Transactions only see data that has been committed. This is the default isolation level in SQL Server and Azure SQL Database.
REPEATABLE READ: Guarantees that if a transaction reads a row, subsequent reads of that same row will return the same data.
SERIALIZABLE: The highest isolation level. Transactions are completely isolated from one another. Each transaction executes as if it were the only transaction running on the system.

You can set the isolation level for a specific transaction using the SET TRANSACTION ISOLATION LEVEL statement.

Performance Considerations

While higher isolation levels provide stronger data consistency guarantees, they can also lead to increased locking and contention, potentially impacting performance. It's crucial to choose an isolation level that balances your application's data integrity requirements with its performance needs.

Implicit Transactions

When implicit transactions are enabled, each individual DML statement is treated as its own transaction. This behavior can be changed using the SET IMPLICIT_TRANSACTIONS ON/OFF command. By default, implicit transactions are OFF, and you must explicitly use BEGIN TRANSACTION.

Best Practices

Keep transactions as short as possible to minimize blocking.
Perform only necessary operations within a transaction.
Use appropriate isolation levels.
Handle errors and rollbacks gracefully.
Avoid long-running transactions that hold locks for extended periods.