Standard lock modes
Prev  Chapter 15.  The Locking Subsystem  Next

Standard lock modes

The Berkeley DB locking protocol is described by a conflict matrix. A conflict matrix is an NxN array in which N is the number of different lock modes supported, and the (i, j)th entry of the array indicates whether a lock of mode i conflicts with a lock of mode j. In addition, Berkeley DB defines the type db_lockmode_t, which is the type of a lock mode within a conflict matrix.

The following is an example of a conflict matrix. The actual conflict matrix used by Berkeley DB to support the underlying access methods is more complicated, but this matrix shows the lock mode relationships available to applications using the Berkeley DB Locking subsystem interfaces directly.

DB_LOCK_NG
not granted (always 0)
DB_LOCK_READ
read (shared)
DB_LOCK_WRITE
write (exclusive)
DB_LOCK_IWRITE
intention to write (shared)
DB_LOCK_IREAD
intention to read (shared)
DB_LOCK_IWR
intention to read and write (shared)

In a conflict matrix, the rows indicate the lock that is held, and the columns indicate the lock that is requested. A 1 represents a conflict (that is, do not grant the lock if the indicated lock is held), and a 0 indicates that it is OK to grant the lock.

		Notheld	Read    Write	IWrite	IRead	IRW
Notheld		0	0	0	0	0	0
Read*		0	0	1	1	0	1
Write**		0	1	1	1	1	1
Intent Write	0	1	1	0	0	0
Intent Read	0	0	1	0	0	0
Intent RW	0	1	1	0	0	0
*
In this case, suppose that there is a read lock held on an object. A new request for a read lock would be granted, but a request for a write lock would not.
**
In this case, suppose that there is a write lock held on an object. A new request for either a read or write lock would be denied.
Prev  Up  Next
Configuring locking: sizing the system  Home  Deadlock detection