Transactions in relational databases are managed using the ACID principles, which stand for Atomicity, Consistency, Isolation, and Durability. These principles ensure that all operations within a transaction are completed successfully before the changes are committed to the database. If any part of a transaction fails, the entire transaction is rolled back, leaving the database in its original state. This is critical for maintaining the integrity of the data, especially in scenarios involving multiple operations that depend on each other.
For example, consider a banking application where a user transfers money from one account to another. This operation consists of at least two steps: debiting the money from one account and crediting it to another. Both operations must succeed; if the debit works but the credit fails, the system could end up with an incorrect balance, leading to inconsistencies. By encapsulating these steps within a single transaction, the database ensures that either both operations complete successfully, or neither does, thereby maintaining data integrity.
Moreover, the isolation aspect of transactions is crucial in multi-user environments where several transactions may occur simultaneously. Relational databases utilize locking mechanisms or versions of transactions to ensure that each transaction is executed in isolation. This means one transaction will not see the effects of another until it is committed. Developers can set different isolation levels (like Read Committed or Serializable) based on their application's requirements, balancing the trade-off between performance and the level of isolation needed to prevent issues like dirty reads or lost updates.