ZweiPhasenSperrlogik

ZweiPhasenSperrlogik, also known as two-phase locking (2PL), is a concurrency control protocol used in database management systems to ensure serializability of transactions. Serializability guarantees that the outcome of concurrently executing transactions is the same as if they were executed one after another in some sequential order. This prevents data inconsistencies that can arise from overlapping operations.

The protocol divides the execution of each transaction into two distinct phases: a growing phase and a shrinking phase. In the growing phase, a transaction can acquire locks on data items, but it cannot release any locks it holds. Once a transaction releases its first lock, it enters the shrinking phase, during which it can release locks but cannot acquire any new ones.

There are several variations of 2PL. Strict two-phase locking is a common variant where all exclusive locks are held until the transaction commits or aborts. This ensures that a transaction can only read data that has been written by transactions that committed before it. Basic two-phase locking simply follows the growing and shrinking phase rules without the strictness of holding exclusive locks.

The primary benefit of 2PL is its ability to guarantee serializability, thus maintaining data integrity. However, it can also lead to problems such as deadlocks, where two or more transactions are blocked indefinitely, waiting for each other to release locks. Various deadlock detection and prevention mechanisms are employed alongside 2PL to mitigate this issue.