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186 lines
7.8 KiB
HTML
186 lines
7.8 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>Non-Durable 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="Getting Started with Berkeley DB Transaction Processing" />
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<link rel="up" href="usingtxns.html" title="Chapter 3. Transaction Basics" />
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<link rel="prev" href="usingtxns.html" title="Chapter 3. Transaction Basics" />
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<link rel="next" href="abortresults.html" title="Aborting a Transaction" />
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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">Non-Durable 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="usingtxns.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 3. Transaction Basics</th>
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<td width="20%" align="right"> <a accesskey="n" href="abortresults.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="nodurabletxn"></a>Non-Durable Transactions</h2>
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</div>
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</div>
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</div>
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<p>
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As previously noted, by default transaction commits are
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durable because they cause the modifications performed
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under the transaction to be synchronously recorded in
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your on-disk log files. However, it is possible to use
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non-durable transactions.
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</p>
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<p>
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You may want non-durable transactions for performance
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reasons. For example, you might be using transactions
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simply for the isolation guarantee.
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<span>
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In this case, you might
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not want a durability guarantee and so you may want to
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prevent the disk I/O that normally accompanies a
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transaction commit.
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</span>
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</p>
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<p>
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There are several ways to remove the durability guarantee
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for your transactions:
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</p>
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<div class="itemizedlist">
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<ul type="disc">
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<li>
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<p>
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Specify
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<span>
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<code class="literal">DB_TXN_NOSYNC</code> using the
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<code class="methodname">DbEnv::set_flags()</code>
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method.
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</span>
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This causes DB to not synchronously force any log
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data to disk upon transaction commit. That is, the modifications are held entirely
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in the in-memory cache and the logging information is not forced to the filesystem for
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long-term storage.
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Note, however, that the logging
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data will eventually make it to the filesystem (assuming no
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application or OS crashes) as a part of DB's
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management of its logging buffers and/or cache.
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</p>
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<p>
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This form of a commit provides a weak durability
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guarantee because data loss can occur due to
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an application
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or OS crash.
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</p>
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<p>
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This behavior is specified on a per-environment
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handle basis. In order for your application to exhibit consistent
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behavior, you need to specify this
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<span>flag</span>
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for all of the environment handles used in your application.
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</p>
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<p>
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You can achieve this behavior on a transaction by transaction basis by
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<span>
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specifying <code class="literal">DB_TXN_NOSYNC</code> to the
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<code class="methodname">DbTxn::commit()</code>
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method.
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</span>
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</p>
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</li>
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<li>
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<p>
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Specify
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<span>
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<code class="literal">DB_TXN_WRITE_NOSYNC</code> using the
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<code class="methodname">DbEnv::set_flags()</code>
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method.
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</span>
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This causes
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<span>
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logging
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</span>
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data to be synchronously
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written to the OS's file system buffers upon
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transaction commit. The data will eventually be
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written to disk, but this occurs when the
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operating system chooses to schedule the
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activity; the transaction commit can complete
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successfully before this disk I/O is performed
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by the OS.
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</p>
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<p>
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This form of commit protects you against application
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crashes, but not against OS
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crashes. This method offers less room for the possibility of data loss than does
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<span><code class="literal">DB_TXN_NOSYNC</code>.</span>
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</p>
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<p>
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This behavior is specified on a per-environment
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handle basis. In order for your application to exhibit consistent
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behavior, you need to specify this
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<span>flag</span>
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for all of the environment handles used in your application.
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</p>
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</li>
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<li>
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<p>
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Maintain your logs entirely in-memory. In this
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case, your logs are never written to disk. The
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result is that you lose all durability guarantees.
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See
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<a class="xref" href="logconfig.html#inmemorylogging" title="Configuring In-Memory Logging">Configuring In-Memory Logging</a>
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for more information.
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</p>
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</li>
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</ul>
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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="usingtxns.html">Prev</a> </td>
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||
<td width="20%" align="center">
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<a accesskey="u" href="usingtxns.html">Up</a>
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||
</td>
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||
<td width="40%" align="right"> <a accesskey="n" href="abortresults.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">Chapter 3. Transaction Basics </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"> Aborting a Transaction</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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