Tvåfaslåsing

Tvåfaslåsing is a Swedish term that translates to "two-phase locking" in English. It is a concurrency control protocol used in database systems to ensure data integrity and consistency during transactions. The protocol operates by dividing the execution of a transaction into two distinct phases: a growing phase and a shrinking phase.

During the growing phase, a transaction can acquire locks on data items. It can request and obtain new locks, but it cannot release any locks it currently holds. Once a transaction enters the shrinking phase, it can release locks, but it cannot acquire any new locks. This strict ordering prevents deadlocks and ensures that all transactions commit or abort in a predictable manner.

The primary goal of two-phase locking is to prevent conflicts between concurrent transactions, such as lost updates, dirty reads, and non-repeatable reads. By adhering to the two-phase locking protocol, database systems can maintain serializability, meaning that the result of concurrently executing transactions is equivalent to some serial execution of those same transactions.

While effective in ensuring consistency, two-phase locking can sometimes lead to performance issues. The strict locking mechanism can reduce the level of concurrency, as transactions may have to wait for locks to be released. Different variations of two-phase locking exist, such as strict two-phase locking and rigorous two-phase locking, which offer different trade-offs between consistency and performance. Strict two-phase locking, for instance, delays the release of all locks until the transaction commits or aborts, further enhancing data integrity.