ACID is an acronym that stands for Atomicity, Consistency, Isolation, and Durability. It is a set of properties or characteristics that guarantee reliable processing of database transactions in a relational database management system (RDBMS). These properties ensure that database transactions are processed reliably, even in the presence of system failures. Here’s what each component of ACID represents:
Atomicity:
- Atomicity ensures that a transaction is treated as a single, indivisible unit of work. Either all the changes made by the transaction are applied to the database, or none of them are. If any part of the transaction fails, the entire transaction is rolled back, and the database remains in a consistent state. Atomicity guarantees that a transaction is all or nothing.
Consistency:
- Consistency ensures that a database transitions from one consistent state to another consistent state after a transaction is completed. In other words, a transaction should bring the database from one valid state to another valid state, adhering to the defined integrity constraints, such as primary key constraints or foreign key constraints. If a transaction violates these constraints, it is rolled back.
Isolation:
- Isolation ensures that concurrent transactions do not interfere with each other. Each transaction appears to run in isolation, as if it were the only transaction being executed. This prevents issues like “dirty reads” (reading uncommitted changes), “non-repeatable reads” (reading different values of the same data in the same transaction), and “phantom reads” (seeing new rows in the same query). Isolation levels, such as Read Uncommitted, Read Committed, Repeatable Read, and Serializable, define the degree of isolation.
Durability:
- Durability guarantees that once a transaction is successfully committed, its changes are permanent and will survive any subsequent system failures, such as power outages or crashes. This is typically achieved by writing transaction changes to a durable storage medium (e.g., a hard disk) and maintaining a transaction log. Even in the event of a system failure, the database can be restored to a consistent state by replaying the transaction log.
ACID properties are essential for ensuring the reliability and integrity of data in database systems, particularly in scenarios where data consistency and accuracy are critical, such as financial systems, healthcare records, and e-commerce platforms. However, it’s important to note that enforcing strict ACID properties can sometimes lead to performance trade-offs in highly concurrent systems. In such cases, NoSQL databases may use a more relaxed consistency model (e.g., eventual consistency) to achieve better scalability while sacrificing some of the strict ACID guarantees.