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ACID is an acronym that stands for Atomicity, Consistency, Isolation, and Durability. These are the four key properties that define the reliability and integrity of transactions in database systems. They ensure that all operations are processed correctly, consistently, and remain durable, providing a strong foundation for data integrity and trustworthiness in critical applications.

Here is a summary of each component of ACID:

• Atomicity - Atomicity ensures that each transaction is treated as a single, indivisible unit of work. This means that either all operations within the transaction are completed successfully, or none of them are. There is no partial completion.
  • Atomicity prevents the database from being left in an inconsistent state. 
  • Example: in a banking system, if a transaction involves transferring money from one account to another, atomicity ensures that both the debit and credit operations occur together. If either operation fails, the transaction is rolled back, leaving the system as if the transaction never happened.

• Consistency - Consistency ensures that a transaction brings the database from one valid state to another, maintaining database rules and constraints. Before and after the transaction, all data must conform to the defined rules, such as unique keys, foreign keys, and other integrity constraints.
  • Consistency guarantees that the database remains in a valid state even in the presence of failures. 
  • Example: in an e-commerce system, consistency ensures that product inventories are correctly updated and do not allow negative stock levels or violate business rules.

• Isolation - Isolation ensures that concurrently executing transactions do not interfere with each other. Each transaction should appear as though it is the only transaction in the system, even when multiple transactions are happening simultaneously.
  • Isolation prevents issues such as dirty reads, non-repeatable reads, and phantom reads, which can lead to incorrect or inconsistent data. 
  • Example: in a multi-user system, isolation ensures that one user's transaction does not affect another user's transaction in unpredictable ways.

• Durability - Durability guarantees that once a transaction has been committed, it will remain so, even in the event of a system crash or power failure. The changes made by the transaction are permanently recorded in the database.
  • Durability ensures that committed data is never lost. 
  • Example: after a purchase transaction is completed and confirmed to the user, durability ensures that the purchase record remains in the database despite any subsequent failures.