ACID is an acronym that stands for Atomicity, Consistency, Isolation, and Durability. These are four key properties that ensure the reliability and predictability of transactions in databases.
Atomicity: A transaction either happens completely or not at all. This means that data cannot be partially updated or lost.
Consistency: The data in the database always complies with all the defined rules. For example, the balance in an account cannot be negative.
Isolation: Transactions are executed independently of each other, without affecting each other. This guarantees that data will not be corrupted or lost due to conflicts.
Durability: The changes made by a transaction are stored permanently, even in the event of a system failure.
--------------------------------------------------------------
Achieving ACID depends on the type of database you are using, but generally, there are different methods and mechanisms for each property:
Atomicity:
Journaling: Changes are written to a transaction log before they are applied to the main database. If a transaction needs to be rolled back, the changes can be restored from the log.
Snapshots: A copy of the data is created at the beginning of the transaction. If the transaction needs to be rolled back, the snapshot can be restored.
Consistency:
Constraints: Rules are defined that the data must adhere to (e.g., account balance is always non-negative). The system checks whether a transaction violates any constraints before applying it.
Triggers: Special functions (triggers) are automatically executed after certain actions to maintain consistency.
Isolation:
Locking: A transaction locks access to the data it is using to prevent other transactions from modifying it.
Isolation levels: Databases can offer different transaction isolation levels, which control how much data is visible to other transactions while they are running.
Durability:
Commit protocols: Special protocols ensure that transaction changes are written to disk before it is considered successful.
Data replication: Copies of the data are stored on other servers so that they are available in case the main server fails.
