Transaction Management in Relational Databases
Transactions are fundamental to the reliability and integrity of relational databases. A transaction is a single unit of work that consists of one or more database operations. These operations are treated as a single, indivisible whole: either all operations within the transaction are completed successfully, or none of them are. This ensures data consistency even in the face of errors, system failures, or concurrent access.
What is a Transaction?
In a database context, a transaction is a sequence of operations performed as a single logical unit of work. Key characteristics of transactions are often referred to by the acronym ACID:
- Atomicity: Ensures that all operations within a transaction are completed successfully. If any operation fails, the entire transaction is rolled back, and the database remains in its state before the transaction began.
- Consistency: Guarantees that a transaction brings the database from one valid state to another. It enforces all rules and constraints defined for the database.
- Isolation: Ensures that concurrent transactions do not interfere with each other. Each transaction appears to execute in isolation, as if it were the only transaction running.
- Durability: Guarantees that once a transaction has been committed, its changes are permanent and will survive any subsequent system failures, such as power outages or crashes.
Basic Transaction Commands
Most relational database management systems (RDBMS) provide standard SQL commands to control transaction behavior:
Starting a Transaction
Transactions typically begin implicitly when the first data modification statement is executed. However, explicit control can be achieved with:
START TRANSACTION;
or in some systems:
BEGIN TRANSACTION;
Committing a Transaction
When all operations within a transaction are successful and you want to make the changes permanent, you commit the transaction:
COMMIT;
Rolling Back a Transaction
If an error occurs, or if you decide not to proceed with the changes, you can undo all operations performed since the transaction began:
ROLLBACK;
Example Scenario: Transferring Funds
Consider a common scenario: transferring money from one bank account to another. This involves two primary operations:
- Deducting money from the source account.
- Adding money to the destination account.
These operations must be atomic. If the deduction succeeds but the addition fails, the money is lost. If the addition succeeds but the deduction fails, money is created out of thin air. A transaction ensures this:
START TRANSACTION;
-- Deduct money from source account
UPDATE Accounts
SET Balance = Balance - 100
WHERE AccountID = 123;
-- Add money to destination account
UPDATE Accounts
SET Balance = Balance + 100
WHERE AccountID = 456;
-- If both operations are successful, commit the transaction
COMMIT;
If at any point during the execution of the `UPDATE` statements an error occurs (e.g., insufficient funds in the source account, or the destination account doesn't exist), the database will automatically roll back the transaction (or if the `COMMIT` is not reached, the `ROLLBACK` can be explicitly called). This leaves the database in its consistent state before the transaction began.
Isolation Levels
The `ISOLATION LEVEL` clause allows you to specify how transaction isolation is enforced. Different levels offer trade-offs between consistency and performance. Common isolation levels include:
- READ UNCOMMITTED: Transactions can read uncommitted data from other transactions (lowest isolation, highest concurrency, lowest consistency).
- READ COMMITTED: Transactions can only read data that has been committed.
- REPEATABLE READ: Guarantees that if a transaction reads a row multiple times, it will see the same data each time.
- SERIALIZABLE: The highest level of isolation, where transactions are executed serially, as if they were run one after another.
You can set the isolation level for the current session:
SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;