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87 lines
4.5 KiB
HTML
87 lines
4.5 KiB
HTML
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<?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>Locking without transactions</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_page.html" title="Locking granularity" />
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<link rel="next" href="lock_twopl.html" title="Locking with transactions: two-phase locking" />
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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 11.2.5.2</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">Locking without transactions</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_page.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_twopl.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_notxn"></a>Locking without transactions</h2>
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</div>
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</div>
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</div>
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<p>If an application runs with locking specified, but not transactions (for
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example, <a href="../api_reference/C/envopen.html" class="olink">DB_ENV->open()</a> is callsed with <a href="../api_reference/C/envopen.html#envopen_DB_INIT_LOCK" class="olink">DB_INIT_LOCK</a> or
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<a href="../api_reference/C/envopen.html#envopen_DB_INIT_CDB" class="olink">DB_INIT_CDB</a> specified, but not <a href="../api_reference/C/envopen.html#envopen_DB_INIT_TXN" class="olink">DB_INIT_TXN</a>), locks are
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normally acquired during each Berkeley DB operation and released before the
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operation returns to the caller. The only exception is in the case of
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cursor operations. Cursors identify a particular position in a file.
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For this reason, cursors must retain read locks across cursor calls to
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make sure that the position is uniquely identifiable during a subsequent
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cursor call, and so that an operation using <a href="../api_reference/C/dbcget.html#dbcget_DB_CURRENT" class="olink">DB_CURRENT</a> will
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always refer to the same record as a previous cursor call. These cursor
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locks cannot be released until the cursor is either repositioned and a
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new cursor lock established (for example, using the <a href="../api_reference/C/dbcget.html#dbcget_DB_NEXT" class="olink">DB_NEXT</a>
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or <a href="../api_reference/C/dbcget.html#dbcget_DB_SET" class="olink">DB_SET</a> flags), or the cursor is closed. As a result,
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application writers are encouraged to close cursors as soon as
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possible.</p>
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<p>It is important to realize that concurrent applications that use locking
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must ensure that two concurrent threads do not block each other.
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However, because Btree and Hash access method page splits can occur at
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any time, there is virtually no way to guarantee that an application
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that writes the database cannot deadlock. Applications running without
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the protection of transactions may deadlock, and can leave the database
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in an inconsistent state when they do so. Applications that need
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concurrent access, but not transactions, are more safely implemented
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using the Berkeley DB Concurrent Data Store Product.</p>
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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_page.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_twopl.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 granularity </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"> Locking with transactions: two-phase 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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