In computer science

Computer science

Computer science or computing science is the study of the theoretical foundations of information and computation and of practical techniques for their implementation and application in computer systems...

, multiple granularity locking (MGL), sometimes called the John Rayner locking method, is a locking method used in database management system

Database management system

A database management system is a software package with computer programs that control the creation, maintenance, and use of a database. It allows organizations to conveniently develop databases for various applications by database administrators and other specialists. A database is an integrated...

s (DBMS) and relational database

Relational database

A relational database is a database that conforms to relational model theory. The software used in a relational database is called a relational database management system . Colloquial use of the term "relational database" may refer to the RDBMS software, or the relational database itself...

s.

In MGL, locks 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 further contain records. This can be thought of as a tree

Tree (data structure)

In computer science, a tree is a widely-used data structure that emulates a hierarchical tree structure with a set of linked nodes.Mathematically, it is an ordered directed tree, more specifically an arborescence: an acyclic connected graph where each node has zero or more children nodes and at...

 of objects, where each node contains its children. A lock

Lock (computer science)

In computer science, a lock is a synchronization mechanism for enforcing limits on access to a resource in an environment where there are many threads of execution. Locks are one way of enforcing concurrency control policies.-Types:...

 locks a node

Node (computer science)

A node is a record consisting of one or more fields that are links to other nodes, and a data field. The link and data fields are often implemented by pointers or references although it is also quite common for the data to be embedded directly in the node. Nodes are used to build linked, often...

 and its descendants.

Multiple granularity locking is usually used with non-strict two-phase locking

Two-phase locking

In databases and transaction processing two-phase locking, is a concurrency control method that guarantees serializability.It is also the name of the resulting set of database transaction schedules...

 to guarantee serializability

Serializability

In concurrency control of databases, transaction processing , and various transactional applications , both centralized and distributed, a transaction schedule is serializable, has the serializability property, if its outcome In concurrency control of databases, transaction processing (transaction...

. MGL uses lock escalation to determine granularity lock on a node and its ancestors.

Lock Modes

In addition to shared (S) locks and exclusive (X) locks from other locking schemes, like strict two-phase locking, MGL also uses intention shared and intention exclusive 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 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).

MGL locking modes are compatible with each other as defined in the following matrix.

Mode	NL	IS	IX	S	SIX	X
NL
IS
IX
S
SIX
X