Two-phase locking

In databases and transaction processing, two-phase locking (2PL) is a concurrency control method that guarantees serializability. It is also the name of the resulting set of database transaction schedules (histories). The protocol utilizes locks, applied by a transaction to data, which may block (interpreted as signals to stop) other transactions from accessing the same data during the transaction's life.

By the 2PL protocol, locks are applied and removed in two phases:

Two types of locks are utilized by the basic protocol: Shared and Exclusive locks. Refinements of the basic protocol may utilize more lock types. Using locks that block processes, 2PL may be subject to deadlocks that result from the mutual blocking of two or more transactions.

A lock is a system object associated with a shared resource such as a data item of an elementary type, a row in a database, or a page of memory. In a database, a lock on a database object (a data-access lock) may need to be acquired by a transaction before accessing the object. Correct use of locks prevents undesired, incorrect or inconsistent operations on shared resources by other concurrent transactions. When a database object with an existing lock acquired by one transaction needs to be accessed by another transaction, the existing lock for the object and the type of the intended access are checked by the system. If the existing lock type does not allow this specific attempted concurrent access type, the transaction attempting access is blocked (according to a predefined agreement/scheme). In practice, a lock on an object does not directly block a transaction's operation upon the object, but rather blocks that transaction from acquiring another lock on the same object, needed to be held/owned by the transaction before performing this operation. Thus, with a locking mechanism, needed operation blocking is controlled by a proper lock blocking scheme, which indicates which lock type blocks which lock type.

Two major types of locks are utilized:

The common interactions between these lock types are defined by blocking behavior as follows:

Several variations and refinements of these major lock types exist, with respective variations of blocking behavior. If a first lock blocks another lock, the two locks are called incompatible; otherwise the locks are compatible. Often, lock types blocking interactions are presented in the technical literature by a Lock compatibility table. The following is an example with the common, major lock types:

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