computer science Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to practical disciplines (includi ...

, multiple granularity locking (MGL) is a locking method used in database management systems (DBMS) and relational databases. In multiple granularity locking,

lock Lock(s) may refer to: Common meanings *Lock and key, a mechanical device used to secure items of importance *Lock (water navigation), a device for boats to transit between different levels of water, as in a canal Arts and entertainment * ''Lock ...

s are set on objects that contain other objects. MGL exploits the hierarchical nature of the ''contains'' relationship. For example, a database may have files, which contain pages, which contain records. This can be thought of as a

tree In botany, a tree is a perennial plant with an elongated stem, or trunk, usually supporting branches and leaves. In some usages, the definition of a tree may be narrower, including only woody plants with secondary growth, plants that are ...

of objects, where each node contains its children. A lock on this structure (such as a shared or exclusive lock) locks the targeted

node In general, a node is a localized swelling (a "knot") or a point of intersection (a vertex). Node may refer to: In mathematics * Vertex (graph theory), a vertex in a mathematical graph *Vertex (geometry), a point where two or more curves, lines ...

as well as all of its descendants. Multiple granularity locking is usually used with non-strict

two-phase locking 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'' ...

to guarantee

serializability In concurrency control of databases, Philip A. Bernstein, Vassos Hadzilacos, Nathan Goodman (1987)''Concurrency Control and Recovery in Database Systems''(free PDF download), Addison Wesley Publishing Company, Gerhard Weikum, Gottfried Vossen (20 ...

Lock modes

In addition to shared (S) locks and exclusive (X) locks from other locking schemes, like strict two-phase locking, MGL also uses intentional "locks", which do not directly lock a node, but instead denote the existence, or intent to add, a lock of the specified type lower in the node hierarchy. Intentional locks include "intention shared" (IS), "intention exclusive" (IX), and the combined "shared and intention exclusive" (SIX) locks. IS locks conflict with X locks, while IX locks conflict with S and X locks. The null lock (NL) is compatible with everything. To lock a node in S (or X), MGL has the transaction lock on all of its ancestors with IS (or IX), so if a transaction locks a node in S (or X), no other transaction can access its ancestors in X (or S and X). This protocol is shown in the following table: Determining what level of granularity to use for locking is done by locking the finest level possible (i.e., at the lowest leaf), and then escalating these locks to higher levels in the file hierarchy to cover more records or file elements as needed in a process known as "lock escalation". MGL locking modes are compatible with each other as defined in the following matrix. Following the locking protocol and the compatibility matrix, if one transaction holds a node in S mode, no other transactions can have locked any ancestor in X mode.

References

{{DEFAULTSORT:Multiple Granularity Locking Concurrency control

Lock modes

See also

References