What is ACID Property in Databases?
The ACID properties are a set of rules that ensure reliable and consistent transaction processing in a database. They define the behavior of a database system when handling transactions, ensuring the integrity and correctness of data even in cases of system crashes, power failures, or concurrent access by multiple users.
ACID Properties Overview
- Atomicity: Ensures that each transaction is treated as a single, indivisible unit. If any part of the transaction fails, the entire transaction is rolled back, and no changes are applied to the database. Example: In a bank transfer, if money is deducted from one account but not added to the other due to an error, the entire operation is undone.
BEGIN TRANSACTION;
UPDATE Accounts SET Balance = Balance - 100 WHERE AccountID = 1;
UPDATE Accounts SET Balance = Balance + 100 WHERE AccountID = 2;
ROLLBACK; -- If any update fails, revert all changes.
- Consistency: Ensures that a transaction transforms the database from one valid state to another while maintaining all defined rules, such as constraints, triggers, and relationships. Example: If a transaction violates a foreign key constraint, the database prevents the operation.
INSERT INTO Orders (OrderID, CustomerID) VALUES (101, 9999);
-- Fails if CustomerID 9999 does not exist.
- Isolation: Ensures that transactions are executed independently without interfering with each other. The intermediate state of a transaction is invisible to other transactions. This property prevents issues like dirty reads, non-repeatable reads, and phantom reads. Example: While one transaction updates a record, another cannot read the uncommitted changes.
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
-
Durability:
Guarantees that once a transaction is committed, its changes are permanent and survive system failures. The database ensures committed data is safely stored, usually by writing it to persistent storage.
Example:
After a
COMMIT, data is saved even if the system crashes.
COMMIT; -- Data is now permanently saved.
ACID in Practice
Atomicity Example:
BEGIN TRANSACTION;
DELETE FROM Inventory WHERE ProductID = 10;
INSERT INTO Archive (ProductID, ProductName) VALUES (10, 'ProductX');
IF @@ERROR > 0
ROLLBACK;
ELSE
COMMIT;
- If deleting from
Inventoryfails, the insertion intoArchivewill also be undone.
Consistency Example:
INSERT INTO Orders (OrderID, CustomerID, OrderDate)
VALUES (101, 5, '2024-12-18');
-- Ensures foreign key and date constraints are respected.
Isolation Levels:
Common isolation levels in SQL are:
- Read Uncommitted: Allows dirty reads.
- Read Committed: Prevents dirty reads.
- Repeatable Read: Ensures the same data is read multiple times within a transaction.
- Serializable: Strictest level, ensuring complete isolation.
Durability Example:
BEGIN TRANSACTION;
UPDATE Accounts SET Balance = Balance - 500 WHERE AccountID = 101;
COMMIT; -- Guarantees changes persist even in case of a crash.
Why ACID Matters
- Data Integrity: Ensures the database remains accurate and reliable.
- Concurrency Control: Prevents conflicts between simultaneous transactions.
- Error Recovery: Protects data from corruption due to unexpected failures.
- Reliability: Builds trust in systems requiring high data consistency, such as banking and e-commerce platforms.
Challenges with ACID
- May lead to performance overhead due to strict adherence to rules.
- Increased complexity in distributed systems where transactions span multiple databases.
Databases like MySQL, PostgreSQL, and Oracle implement ACID properties to ensure data reliability and correctness, making them vital for robust application development.
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