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210 lines
11 KiB
HTML
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<title>Chapter 16. The Locking Subsystem</title>
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<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
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<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
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<link rel="start" href="index.html" title="Berkeley DB Programmer's Reference Guide" />
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<link rel="up" href="index.html" title="Berkeley DB Programmer's Reference Guide" />
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<link rel="prev" href="program_faq.html" title="Programmer notes FAQ" />
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<div xmlns="" class="navheader">
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<div class="libver">
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<p>Library Version 11.2.5.3</p>
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</div>
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">Chapter 16.
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The Locking Subsystem
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</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="program_faq.html">Prev</a> </td>
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<th width="60%" align="center"> </th>
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<td width="20%" align="right"> <a accesskey="n" href="lock_config.html">Next</a></td>
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</tr>
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</table>
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<hr />
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</div>
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<div class="chapter" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h2 class="title"><a id="lock"></a>Chapter 16.
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The Locking Subsystem
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</h2>
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</div>
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</div>
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</div>
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<div class="toc">
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<p>
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<b>Table of Contents</b>
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</p>
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<dl>
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<dt>
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<span class="sect1">
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<a href="lock.html#lock_intro">Introduction to the locking subsystem</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="lock_config.html">Configuring locking</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="lock_max.html">Configuring locking: sizing the system</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="lock_stdmode.html">Standard lock modes</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="lock_dead.html">Deadlock detection</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="lock_timeout.html">Deadlock detection using timers</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="lock_deaddbg.html">Deadlock debugging</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="lock_page.html">Locking granularity</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="lock_notxn.html">Locking without transactions</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="lock_twopl.html">Locking with transactions: two-phase locking</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="lock_cam_conv.html">Berkeley DB Concurrent Data Store locking conventions</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="lock_am_conv.html">Berkeley DB Transactional Data Store locking conventions</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="lock_nondb.html">Locking and non-Berkeley DB applications</a>
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</span>
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</dt>
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</dl>
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</div>
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<div class="sect1" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h2 class="title" style="clear: both"><a id="lock_intro"></a>Introduction to the locking subsystem</h2>
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</div>
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</div>
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</div>
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<p>The locking subsystem provides interprocess and intraprocess concurrency
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control mechanisms. Although the lock system is used extensively by
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the Berkeley DB access methods and transaction system, it may also be used as
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a standalone subsystem to provide concurrency control to any set of
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designated resources.</p>
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<p>The Lock subsystem is created, initialized, and opened by calls to
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<a href="../api_reference/C/envopen.html" class="olink">DB_ENV->open()</a> with the <a href="../api_reference/C/envopen.html#envopen_DB_INIT_LOCK" class="olink">DB_INIT_LOCK</a> or <a href="../api_reference/C/envopen.html#envopen_DB_INIT_CDB" class="olink">DB_INIT_CDB</a>
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flags specified.</p>
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<p>The <a href="../api_reference/C/lockvec.html" class="olink">DB_ENV->lock_vec()</a> method is used to acquire and release locks. The
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<a href="../api_reference/C/lockvec.html" class="olink">DB_ENV->lock_vec()</a> method performs any number of lock operations atomically. It
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also provides the capability to release all locks held by a particular
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locker and release all the locks on a particular object. (Performing
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multiple lock operations atomically is useful in performing Btree
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traversals -- you want to acquire a lock on a child page and once
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acquired, immediately release the lock on its parent. This is
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traditionally referred to as <span class="emphasis"><em>lock-coupling</em></span>). Two additional
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methods, <a href="../api_reference/C/lockget.html" class="olink">DB_ENV->lock_get()</a> and <a href="../api_reference/C/lockput.html" class="olink">DB_ENV->lock_put()</a>, are provided. These
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methods are simpler front-ends to the <a href="../api_reference/C/lockvec.html" class="olink">DB_ENV->lock_vec()</a> functionality,
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where <a href="../api_reference/C/lockget.html" class="olink">DB_ENV->lock_get()</a> acquires a lock, and <a href="../api_reference/C/lockput.html" class="olink">DB_ENV->lock_put()</a> releases a
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lock that was acquired using <a href="../api_reference/C/lockget.html" class="olink">DB_ENV->lock_get()</a> or <a href="../api_reference/C/lockvec.html" class="olink">DB_ENV->lock_vec()</a>. All
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locks explicitly requested by an application should be released via
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calls to <a href="../api_reference/C/lockput.html" class="olink">DB_ENV->lock_put()</a> or <a href="../api_reference/C/lockvec.html" class="olink">DB_ENV->lock_vec()</a>. Using <a href="../api_reference/C/lockvec.html" class="olink">DB_ENV->lock_vec()</a>
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instead of separate calls to <a href="../api_reference/C/lockput.html" class="olink">DB_ENV->lock_put()</a> and <a href="../api_reference/C/lockget.html" class="olink">DB_ENV->lock_get()</a> also
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reduces the synchronization overhead between multiple threads or
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processes. The three methods are fully compatible, and may be used
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interchangeably.</p>
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<p>Applications must specify lockers and lock objects appropriately. When
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used with the Berkeley DB access methods, lockers and objects are handled
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completely internally, but an application using the lock manager
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directly must either use the same conventions as the access methods or
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define its own convention to which it adheres. If an application is
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using the access methods with locking at the same time that it is
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calling the lock manager directly, the application must follow a
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convention that is compatible with the access methods' use of the
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locking subsystem. See <a class="xref" href="lock_am_conv.html" title="Berkeley DB Transactional Data Store locking conventions">Berkeley DB Transactional Data Store locking conventions</a> for more information.</p>
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<p>The <a href="../api_reference/C/lockid.html" class="olink">DB_ENV->lock_id()</a> function returns a unique ID that may safely be used
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as the locker parameter to the <a href="../api_reference/C/lockvec.html" class="olink">DB_ENV->lock_vec()</a> method. The access methods
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use <a href="../api_reference/C/lockid.html" class="olink">DB_ENV->lock_id()</a> to generate unique lockers for the cursors
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associated with a database.</p>
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<p>The <a href="../api_reference/C/lockdetect.html" class="olink">DB_ENV->lock_detect()</a> function provides the programmatic interface to
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the Berkeley DB deadlock detector. Whenever two threads of control issue lock
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requests concurrently, the possibility for deadlock arises. A deadlock
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occurs when two or more threads of control are blocked, waiting for
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actions that another one of the blocked threads must take. For example,
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assume that threads A and B have each obtained read locks on object X.
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Now suppose that both threads want to obtain write locks on object X.
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Neither thread can be granted its write lock (because of the other
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thread's read lock). Both threads block and will never unblock because
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the event for which they are waiting can never happen.</p>
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<p>The deadlock detector examines all the locks held in the environment,
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and identifies situations where no thread can make forward progress.
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It then selects one of the participants in the deadlock (according to
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the argument that was specified to <a href="../api_reference/C/envset_lk_detect.html" class="olink">DB_ENV->set_lk_detect()</a>), and
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forces it to return the value <a class="link" href="program_errorret.html#program_errorret.DB_LOCK_DEADLOCK">DB_LOCK_DEADLOCK</a>, which indicates
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that a deadlock occurred. The thread receiving such an error must
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release all of its locks and undo any incomplete modifications to the
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locked resource. Locks are typically released, and modifications
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undone, by closing any cursors involved in the operation and aborting
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any transaction enclosing the operation. The operation may optionally
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be retried.</p>
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<p>The <a href="../api_reference/C/lockstat.html" class="olink">DB_ENV->lock_stat()</a> function returns information about the status of
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the lock subsystem. It is the programmatic interface used by the
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<a href="../api_reference/C/db_stat.html" class="olink">db_stat</a> utility.</p>
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<p>The locking subsystem is closed by the call to <a href="../api_reference/C/envclose.html" class="olink">DB_ENV->close()</a>.</p>
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<p>Finally, the entire locking subsystem may be discarded using the
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<a href="../api_reference/C/envremove.html" class="olink">DB_ENV->remove()</a> method.</p>
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<p>For more information on the locking subsystem methods, see the <a href="../api_reference/C/lock.html#locklist" class="olink">Locking Subsystem and Related Methods</a> section in the <em class="citetitle">Berkeley DB C API Reference Guide.</em> </p>
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</div>
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</div>
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<div class="navfooter">
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<hr />
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<table width="100%" summary="Navigation footer">
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<tr>
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<td width="40%" align="left"><a accesskey="p" href="program_faq.html">Prev</a> </td>
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<td width="20%" align="center"> </td>
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<td width="40%" align="right"> <a accesskey="n" href="lock_config.html">Next</a></td>
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</tr>
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<tr>
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<td width="40%" align="left" valign="top">Programmer notes FAQ </td>
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<td width="20%" align="center">
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<a accesskey="h" href="index.html">Home</a>
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</td>
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<td width="40%" align="right" valign="top"> Configuring locking</td>
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</tr>
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</table>
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</div>
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</body>
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</html>
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