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HTML
109 lines
5 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>Berkeley DB Concurrent Data Store locking conventions</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="lock.html" title="Chapter 16. The Locking Subsystem" />
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<link rel="prev" href="lock_twopl.html" title="Locking with transactions: two-phase locking" />
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<link rel="next" href="lock_am_conv.html" title="Berkeley DB Transactional Data Store locking conventions" />
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</head>
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<body>
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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">Berkeley DB Concurrent Data Store locking conventions</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="lock_twopl.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 16.
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The Locking Subsystem
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</th>
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<td width="20%" align="right"> <a accesskey="n" href="lock_am_conv.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="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_cam_conv"></a>Berkeley DB Concurrent Data Store locking conventions</h2>
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</div>
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</div>
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</div>
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<p>The Berkeley DB Concurrent Data Store product has a simple set of conventions for locking. It
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provides multiple-reader/single-writer semantics, but not per-page
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locking or transaction recoverability. As such, it does its locking
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entirely in the Berkeley DB interface layer.</p>
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<p>The object it locks is the file, identified by its unique file number.
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The locking matrix is not one of the two standard lock modes, instead,
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we use a four-lock set, consisting of the following:</p>
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<div class="variablelist">
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<dl>
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<dt>
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<span class="term">DB_LOCK_NG</span>
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</dt>
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<dd>not granted (always 0)</dd>
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<dt>
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<span class="term">DB_LOCK_READ</span>
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</dt>
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<dd>read (shared)</dd>
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<dt>
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<span class="term">DB_LOCK_WRITE</span>
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</dt>
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<dd>write (exclusive)</dd>
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<dt>
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<span class="term">DB_LOCK_IWRITE</span>
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</dt>
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<dd>intention-to-write (shared with NG and READ, but conflicts with WRITE and IWRITE)</dd>
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</dl>
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</div>
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<p>The IWRITE lock is used for cursors that will be used for updating
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(IWRITE locks are implicitly obtained for write operations through the
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Berkeley DB handles, for example, <a href="../api_reference/C/dbput.html" class="olink">DB->put()</a> or <a href="../api_reference/C/dbdel.html" class="olink">DB->del()</a>). While
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the cursor is reading, the IWRITE lock is held; but as soon as the
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cursor is about to modify the database, the IWRITE is upgraded to a
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WRITE lock. This upgrade blocks until all readers have exited the
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database. Because only one IWRITE lock is allowed at any one time, no
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two cursors can ever try to upgrade to a WRITE lock at the same time,
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and therefore deadlocks are prevented, which is essential because Berkeley DB Concurrent Data Store
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does not include deadlock detection and recovery.</p>
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<p>Applications that need to lock compatibly with Berkeley DB Concurrent Data Store must obey the
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following rules:</p>
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<div class="orderedlist">
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<ol type="1">
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<li>Use only lock modes DB_LOCK_NG, DB_LOCK_READ, DB_LOCK_WRITE,
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DB_LOCK_IWRITE.</li>
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<li>Never attempt to acquire a WRITE lock on an object that is
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already locked with a READ lock.</li>
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</ol>
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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="lock_twopl.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="lock.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="lock_am_conv.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">Locking with transactions: two-phase locking </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"> Berkeley DB Transactional Data Store locking conventions</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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