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<title>Berkeley DB Reference Guide: Berkeley DB and locking</title>
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<td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Locking Subsystem</dl></h3></td>
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<h3 align=center>Berkeley DB and locking</h3>
<p>The locking subsystem provides interprocess and intraprocess concurrency
control mechanisms. Although the lock system is used extensively by
the Berkeley DB access methods and transaction system, it may also be used as
a standalone subsystem to provide concurrency control to any set of
designated resources.</p>
<p>The Lock subsystem is created, initialized, and opened by calls to
<a href="../../api_c/env_open.html">DB_ENV->open</a> with the <a href="../../api_c/env_open.html#DB_INIT_LOCK">DB_INIT_LOCK</a> or <a href="../../api_c/env_open.html#DB_INIT_CDB">DB_INIT_CDB</a>
flags specified.</p>
<p>The <a href="../../api_c/lock_vec.html">DB_ENV->lock_vec</a> method is used to acquire and release locks. The
<a href="../../api_c/lock_vec.html">DB_ENV->lock_vec</a> method performs any number of lock operations atomically. It
also provides the capability to release all locks held by a particular
locker and release all the locks on a particular object. (Performing
multiple lock operations atomically is useful in performing Btree
traversals -- you want to acquire a lock on a child page and once
acquired, immediately release the lock on its parent. This is
traditionally referred to as <i>lock-coupling</i>). Two additional
methods, <a href="../../api_c/lock_get.html">DB_ENV->lock_get</a> and <a href="../../api_c/lock_put.html">DB_ENV->lock_put</a>, are provided. These
methods are simpler front-ends to the <a href="../../api_c/lock_vec.html">DB_ENV->lock_vec</a> functionality,
where <a href="../../api_c/lock_get.html">DB_ENV->lock_get</a> acquires a lock, and <a href="../../api_c/lock_put.html">DB_ENV->lock_put</a> releases a
lock that was acquired using <a href="../../api_c/lock_get.html">DB_ENV->lock_get</a> or <a href="../../api_c/lock_vec.html">DB_ENV->lock_vec</a>. All
locks explicitly requested by an application should be released via
calls to <a href="../../api_c/lock_put.html">DB_ENV->lock_put</a> or <a href="../../api_c/lock_vec.html">DB_ENV->lock_vec</a>. Using <a href="../../api_c/lock_vec.html">DB_ENV->lock_vec</a>
instead of separate calls to <a href="../../api_c/lock_put.html">DB_ENV->lock_put</a> and <a href="../../api_c/lock_get.html">DB_ENV->lock_get</a> also
reduces the synchronization overhead between multiple threads or
processes. The three methods are fully compatible, and may be used
interchangeably.</p>
<p>Applications must specify lockers and lock objects appropriately. When
used with the Berkeley DB access methods, lockers and objects are handled
completely internally, but an application using the lock manager
directly must either use the same conventions as the access methods or
define its own convention to which it adheres. If an application is
using the access methods with locking at the same time that it is
calling the lock manager directly, the application must follow a
convention that is compatible with the access methods' use of the
locking subsystem. See <a href="../../ref/lock/am_conv.html">Access
method locking conventions</a> for more information.</p>
<p>The <a href="../../api_c/lock_id.html">DB_ENV->lock_id</a> function returns a unique ID that may safely be used
as the locker parameter to the <a href="../../api_c/lock_vec.html">DB_ENV->lock_vec</a> method. The access methods
use <a href="../../api_c/lock_id.html">DB_ENV->lock_id</a> to generate unique lockers for the cursors
associated with a database.</p>
<p>The <a href="../../api_c/lock_detect.html">DB_ENV->lock_detect</a> function provides the programmatic interface to
the Berkeley DB deadlock detector. Whenever two threads of control issue lock
requests concurrently, the possibility for deadlock arises. A deadlock
occurs when two or more threads of control are blocked, waiting for
actions that another one of the blocked threads must take. For example,
assume that threads A and B have each obtained read locks on object X.
Now suppose that both threads want to obtain write locks on object X.
Neither thread can be granted its write lock (because of the other
thread's read lock). Both threads block and will never unblock because
the event for which they are waiting can never happen.</p>
<p>The deadlock detector examines all the locks held in the environment,
and identifies situations where no thread can make forward progress.
It then selects one of the participants in the deadlock (according to
the argument that was specified to <a href="../../api_c/env_set_lk_detect.html">DB_ENV->set_lk_detect</a>), and
forces it to return the value <a href="../../ref/program/errorret.html#DB_LOCK_DEADLOCK">DB_LOCK_DEADLOCK</a>, which indicates
that a deadlock occurred. The thread receiving such an error must
release all of its locks and undo any incomplete modifications to the
locked resource. Locks are typically released, and modifications
undone, by closing any cursors involved in the operation and aborting
any transaction enclosing the operation. The operation may optionally
be retried.</p>
<p>The <a href="../../api_c/lock_stat.html">DB_ENV->lock_stat</a> function returns information about the status of
the lock subsystem. It is the programmatic interface used by the
<a href="../../utility/db_stat.html">db_stat</a> utility.</p>
<p>The locking subsystem is closed by the call to <a href="../../api_c/env_close.html">DB_ENV->close</a>.</p>
<p>Finally, the entire locking subsystem may be discarded using the
<a href="../../api_c/env_remove.html">DB_ENV->remove</a> method.</p>
<!--$Id: m4.methods,v 1.5 2004/11/03 15:52:01 bostic Exp $-->
<table border=1 align=center>
<tr><th>Locking Subsystem and Related Methods</th><th>Description</th></tr>
<!--DbDeadlockException-->
<!--DbEnv::lock_detect--><tr><td><a href="../../api_c/lock_detect.html">DB_ENV->lock_detect</a></td><td>Perform deadlock detection</td></tr>
<!--DbEnv::lock_get--><tr><td><a href="../../api_c/lock_get.html">DB_ENV->lock_get</a></td><td>Acquire a lock</td></tr>
<!--DbEnv::lock_id--><tr><td><a href="../../api_c/lock_id.html">DB_ENV->lock_id</a></td><td>Acquire a locker ID</td></tr>
<!--DbEnv::lock_id_free--><tr><td><a href="../../api_c/lock_id_free.html">DB_ENV->lock_id_free</a></td><td>Release a locker ID</td></tr>
<!--DbEnv::lock_put--><tr><td><a href="../../api_c/lock_put.html">DB_ENV->lock_put</a></td><td>Release a lock</td></tr>
<!--DbEnv::lock_stat--><tr><td><a href="../../api_c/lock_stat.html">DB_ENV->lock_stat</a></td><td>Return lock subsystem statistics</td></tr>
<!--DbEnv::lock_vec--><tr><td><a href="../../api_c/lock_vec.html">DB_ENV->lock_vec</a></td><td>Acquire/release locks</td></tr>
<!--DbLockNotGrantedException-->
<!--DbLock-->
<tr><th>Locking Subsystem Configuration</th><th><br></th></tr>
<!--DbEnv::set_lk_conflicts--><tr><td><a href="../../api_c/env_set_lk_conflicts.html">DB_ENV->set_lk_conflicts</a></td><td>Set lock conflicts matrix</td></tr>
<!--DbEnv::set_lk_max_detect--><tr><td><a href="../../api_c/env_set_lk_detect.html">DB_ENV->set_lk_detect</a></td><td>Set automatic deadlock detection</td></tr>
<!--DbEnv::set_lk_max_lockers--><tr><td><a href="../../api_c/env_set_lk_max_lockers.html">DB_ENV->set_lk_max_lockers</a></td><td>Set maximum number of lockers</td></tr>
<!--DbEnv::set_lk_max_locks--><tr><td><a href="../../api_c/env_set_lk_max_locks.html">DB_ENV->set_lk_max_locks</a></td><td>Set maximum number of locks</td></tr>
<!--DbEnv::set_lk_max_objects--><tr><td><a href="../../api_c/env_set_lk_max_objects.html">DB_ENV->set_lk_max_objects</a></td><td>Set maximum number of lock objects</td></tr>
<!--DbEnv::set_timeout--><tr><td><a href="../../api_c/env_set_timeout.html">DB_ENV->set_timeout</a></td><td>Set lock and transaction timeout</td></tr>
</table>
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