How does a relational database ensure ACID compliance?

A relational database ensures ACID compliance through transaction management, concurrency control, and recovery mechanisms.

ACID stands for Atomicity, Consistency, Isolation, and Durability. These are a set of properties that guarantee that database transactions are processed reliably. To ensure these properties, a relational database uses a combination of techniques.

Atomicity ensures that a transaction is treated as a single, indivisible unit of work. This means that either all the changes made in a transaction are committed to the database, or none of them are. To ensure atomicity, a relational database uses a transaction log. Every change made during a transaction is first recorded in this log. If a transaction is completed successfully, the changes are committed to the database. If a transaction fails at any point, the database can use the log to roll back any changes made during that transaction.

Consistency ensures that a transaction brings the database from one valid state to another. This means that a transaction should not violate any integrity constraints defined on the database. To ensure consistency, a relational database uses a combination of techniques such as constraint checking, trigger execution, and cascading updates or deletes.

Isolation ensures that concurrent transactions do not interfere with each other. This means that the changes made by one transaction should not be visible to other transactions until the first transaction is committed. To ensure isolation, a relational database uses concurrency control techniques such as locking and multi-version concurrency control (MVCC).

Durability ensures that once a transaction is committed, its changes are permanent and survive any subsequent system failures. To ensure durability, a relational database uses recovery mechanisms such as write-ahead logging and checkpointing. Write-ahead logging ensures that all changes made by a transaction are recorded in the transaction log before they are committed to the database. Checkpointing periodically saves the database's current state to disk, which helps in reducing the recovery time in case of a system failure.

In conclusion, a relational database ensures ACID compliance through a combination of transaction management, concurrency control, and recovery mechanisms. These mechanisms work together to ensure that database transactions are processed reliably, even in the face of system failures and concurrent access.