ACID is a set of properties that guarantee that database transactions are processed reliably. ACID stands for Atomicity, Consistency, Isolation, and Durability.
Atomicity
Atomicity ensures that each transaction is treated as a single “unit,” which either completes in its entirety or does not happen at all. This means if any part of the transaction fails, the database will revert any changes made during the transaction back to its previous state, as if the transaction never happened. This prevents partial updates to the database that could cause data inconsistencies.
Consistency
Any data written to the database must be valid according to all defined rules, including constraints and cascade updates or deletions. Consistency checks for logical integrity of the data before and after a transaction.
Isolation
Isolation ensures that concurrent execution of transactions results in a system state that would be obtained if transactions were executed serially, i.e., one after the other. This means the transactions are isolated from each other, hiding the effects of intermediate transactions from other concurrently executed transactions. Until a transaction is committed, its changes are invisible to other transactions. This property prevents transactions from interfering with each other and ensures that concurrent transactions result in the same state that would be obtained if these transactions were executed sequentially.
Durability
Durability guarantees that once a transaction has been committed, it will remain so, even in the event of power loss, crashes, or errors. This means that the database logs the changes of the transaction, ensuring that committed transactions are never lost. The system ensures recovery from hardware or software failures, protecting users from data losses.
In practical terms, these principles guide how databases manage operations, ensuring data integrity and consistency even in the face of errors, crashes, or power failures.