In an Oracle database, a transaction is a logically related series of database operations that are performed as if they were a single unit of work. Transactions are used to ensure data consistency and integrity, and to enable recovery in the event of system failures or errors. In a multi-user environment, transactions isolate concurrent access to data and enforce data locking and serialization.
The ACID properties are a set of characteristics that ensure database transactions are executed reliably. ACID stands for Atomicity, Consistency, Isolation, and Durability:
1. Atomicity: A transaction is atomic, meaning it is indivisible and all-or-nothing. Either all the operations in a transaction are completed successfully and the transaction is committed, or none of the operations are completed and the transaction is rolled back.
2. Consistency: A transaction ensures that the database moves from one consistent state to another. The data is transformed according to rules that ensure that it remains consistent, even if a transaction fails.
3. Isolation: Transactions execute in isolation from each other, which means that concurrent transactions cannot interfere with each other. They are isolated from each other using locking and serialization techniques.
4. Durability: Once a transaction has been committed, the changes made to the database are permanent and survive system crashes or power failures.
For example, suppose a user performs a transaction to transfer money from one bank account to another. The transaction would involve two database operations: debiting one account and crediting another account. If either operation fails, the whole transaction is rolled back, and both accounts remain unchanged. When the transaction is committed, the changes to both accounts are durable and can survive system failures. Moreover, the transaction keeps the data consistent and does not interfere with other transactions that may also modify the same accounts.