Relational databases ensure transactional consistency primarily through the use of the ACID properties: Atomicity, Consistency, Isolation, and Durability. These principles guide how transactions are processed to maintain a reliable state in the database. Atomicity guarantees that a transaction will either complete fully or not at all. For instance, in a banking scenario where money is transferred from one account to another, both the debit from one account and the credit to another must succeed together. If there's a failure during the process, the database rolls back any changes, ensuring that no partial transaction is recorded.
Consistency ensures that any transaction brings the database from one valid state to another while adhering to all defined rules, such as constraints and triggers. For example, if a database maintains a rule that no account can go below a zero balance, the database will reject any transaction that violates this rule. This check prevents any inconsistency in the database, so once transactions are completed successfully, the data remains accurate and reliable.
Isolation is critical for concurrent transactions. When multiple users attempt to modify data simultaneously, isolation ensures that each transaction is processed independently. For instance, if two users are trying to update the same record, isolation prevents one user's changes from interfering with the other's until both transactions are complete. This can be implemented through locking mechanisms, which can range from row-level locks to table-level locks. Lastly, Durability guarantees that once a transaction has been committed, it will remain so, even in the event of system failures. This is achieved through the database maintaining logs that help restore the database to a consistent state after a crash. Together, these ACID properties form a robust framework for ensuring transactional consistency in relational databases.