Atomicity Consistency Isolation Durability (ACID)

Techopedia explains Atomicity Consistency Isolation Durability (ACID)

A database guarantees the following four properties to ensure database reliability, as follows:

• Atomicity: A database follows the all or nothing rule, i.e., the database considers all transaction operations as one whole unit or atom. Thus, when a database processes a transaction, it is either fully completed or not executed at all.
• Consistency: Ensures that only valid data following all rules and constraints is written in the database. When a transaction results in invalid data, the database reverts to its previous state, which abides by all customary rules and constraints.
• Isolation: Ensures that transactions are securely and independently processed at the same time without interference, but it does not ensure the order of transactions. For example, user A withdraws $100 and user B withdraws $250 from user Z’s account, which has a balance of $1000. Since both A and B draw from Z’s account, one of the users is required to wait until the other user transaction is completed, avoiding inconsistent data. If B is required to wait, then B must wait until A’s transaction is completed, and Z’s account balance changes to $900. Now, B can withdraw $250 from this $900 balance.
• Durability: In the above example, user B may withdraw $100 only after user A’s transaction is completed and is updated in the database. If the system fails before A’s transaction is logged in the database, A cannot withdraw any money, and Z’s account returns to its previous consistent state.