|
In databases and transaction processing, two-phase locking (2PL) is a concurrency control method that guarantees serializability.〔Philip A. Bernstein, Vassos Hadzilacos, Nathan Goodman (1987): ( ''Concurrency Control and Recovery in Database Systems'' ), Addison Wesley Publishing Company, ISBN 0-201-10715-5〕〔Gerhard Weikum, Gottfried Vossen (2001): ( ''Transactional Information Systems'' ), Elsevier, ISBN 1-55860-508-8〕 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: # Expanding phase: locks are acquired and no locks are released. # Shrinking phase: locks are released and no locks are acquired. 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. ==Data-access locks== 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: *Write-lock (exclusive lock) is associated with a database object by a transaction (Terminology: "the transaction locks the object," or "acquires lock for it") before ''writing'' (inserting/modifying/deleting) this object. *Read-lock (shared lock) is associated with a database object by a transaction before ''reading'' (retrieving the state of) this object. The common interactions between these lock types are defined by blocking behavior as follows: * An existing ''write-lock'' on a database object blocks an intended ''write'' upon the same object (already requested/issued) by another transaction by blocking a respective ''write-lock'' from being acquired by the other transaction. The second write-lock will be acquired and the requested write of the object will take place (materialize) after the existing write-lock is released. * A ''write-lock'' blocks an intended (already requested/issued) ''read'' by another transaction by blocking the respective ''read-lock ''. * A ''read-lock'' blocks an intended ''write'' by another transaction by blocking the respective ''write-lock ''. * A ''read-lock'' does not block an intended ''read'' by another transaction. The respective ''read-lock'' for the intended read is acquired (shared with the previous read) immediately after the intended read is requested, and then the intended read itself takes place. 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: : :X indicates incompatibility, i.e, a case when a lock of the first type (in left column) on an object blocks a lock of the second type (in top row) from being acquired on the same object (by another transaction). An object typically has a queue of waiting requested (by transactions) operations with respective locks. The first blocked lock for operation in the queue is acquired as soon as the existing blocking lock is removed from the object, and then its respective operation is executed. If a lock for operation in the queue is not blocked by any existing lock (existence of multiple compatible locks on a same object is possible concurrently) it is acquired immediately. :Comment: In some publications the table entries are simply marked "compatible" or "incompatible", or respectively "yes" or "no". 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Two-phase locking」の詳細全文を読む スポンサード リンク
|