ACID is an acronym that stands for Atomicity, Consistency, Isolation, and Durability. These are a set of properties that ensure reliable and consistent transactions in relational databases. Transactions are sequences of one or more database operations, such as reading, inserting, updating, or deleting data, that are executed as a single unit of work.
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Atomicity: Atomicity guarantees that either all the operations within a transaction are completed successfully, or none of them are executed at all. If any operation within the transaction fails, the entire transaction is rolled back, and the database is reverted to the state it was in before the transaction started. This ensures that the database remains in a consistent state even in the case of errors or system failures.
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Consistency: Consistency ensures that the database transitions from one consistent state to another after every transaction. This means that the database starts in a consistent state, and once the transaction has been completed, all the data integrity rules, constraints, and business rules are preserved, maintaining the overall consistency of the database. If a transaction would violate any of these rules, it is aborted and rolled back.
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Isolation: Isolation ensures that each transaction is isolated from other concurrent transactions, meaning that the intermediate states of one transaction are not visible to others. This property provides the illusion that each transaction is executed serially, even if they are running concurrently. Isolation levels can be adjusted based on the application's requirements, balancing the trade-offs between performance and the potential for concurrency-related issues, such as dirty reads, non-repeatable reads, and phantom reads.
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Durability: Durability guarantees that once a transaction has been committed, its changes to the database are permanent, even in the case of system failures, crashes, or power outages. This property is usually achieved by writing transaction logs to non-volatile storage, which can be used to recover the database to a consistent state after a failure.
ACID properties are essential for ensuring the reliability, consistency, and robustness of relational databases, particularly in systems where data integrity and accuracy are critical. These properties are typically enforced by the database management system (DBMS) to maintain a high level of data quality and prevent anomalies or inconsistencies.