Database Transactions

Why Transactions are Crucial

• Atomicity: Ensures that a transaction is treated as a single, indivisible unit. If any part of the transaction fails, the entire transaction is rolled back, leaving the database in its original state.
• Consistency: Guarantees that a transaction brings the database from one valid state to another. It prevents invalid data from being written.
• Isolation: Ensures that concurrent transactions do not interfere with each other. Each transaction appears to run as if it were the only transaction executing.
• Durability: Guarantees that once a transaction has been committed, its changes are permanent and will survive system failures (e.g., power outages, crashes).

These four properties are often referred to by the acronym ACID.

Starting a Transaction

A transaction typically begins with a command to explicitly start it. The syntax varies depending on the database system.

-- Example SQL: START TRANSACTION;
-- Example SQL: BEGIN TRANSACTION;

Committing a Transaction

If all operations within the transaction are successful and the desired state is reached, the transaction is committed. This makes the changes permanent.

-- Example SQL: COMMIT;

Rolling Back a Transaction

If an error occurs, or if the transaction needs to be undone for any reason, it can be rolled back. This discards all changes made since the transaction began.

-- Example SQL: ROLLBACK;

ACID Properties Explained

Let's delve deeper into the ACID properties:

Atomicity

Imagine transferring money between two bank accounts. This involves debiting one account and crediting another. Atomicity ensures that either both operations complete successfully, or neither does. If the credit fails, the debit is rolled back, preventing money loss.

Consistency

A transaction must preserve database invariants. For example, if a table has a constraint that the sum of account balances must always be positive, a transaction that would violate this would be rejected.

Isolation

Databases provide different isolation levels (e.g., Read Uncommitted, Read Committed, Repeatable Read, Serializable) to control how transactions interact. The goal is to prevent issues like dirty reads, non-repeatable reads, and phantom reads.

Durability

Once a transaction is committed, the database manager ensures that the changes are written to non-volatile storage (like disk) and will survive system restarts. This is often achieved through mechanisms like transaction logs.

Example Scenario: Online Order Processing

1. Start Transaction: A customer places an order.
2. Operation 1: Decrement the stock count for the ordered items.
3. Operation 2: Create a new order record in the `orders` table.
4. Operation 3: Update the customer's order history.
5. Check for Errors: If stock is insufficient, or if any database operation fails, Rollback Transaction.
6. Commit Transaction: If all operations are successful, Commit Transaction to make the order permanent.

Without transactions, an incomplete order (e.g., stock decremented but order not recorded) could lead to significant business problems.

Transaction Management in Practice

Most programming languages and database connectors provide APIs for managing transactions. You'll typically see methods like:

• beginTransaction()
• commit()
• rollback()

Error handling is paramount. Always wrap your transactional code in try-catch blocks to ensure proper rollback in case of exceptions.