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<title>Chapter 17. The Logging Subsystem</title>
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<th colspan="3" align="center">Chapter 17. The Logging Subsystem </th>
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<div class="chapter" lang="en" xml:lang="en">
<div class="titlepage">
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<div>
<h2 class="title"><a id="log"></a>Chapter 17. The Logging Subsystem </h2>
</div>
</div>
</div>
<div class="toc">
<p>
<b>Table of Contents</b>
</p>
<dl>
<dt>
<span class="sect1">
<a href="log.html#log_intro">Introduction to the logging
subsystem</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="log_config.html">Configuring logging</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="log_limits.html">Log file limits</a>
</span>
</dt>
</dl>
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<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="log_intro"></a>Introduction to the logging
subsystem</h2>
</div>
</div>
</div>
<p>
The Logging subsystem is the logging facility used by
Berkeley DB. It is largely Berkeley DB-specific, although it
is potentially useful outside of the Berkeley DB package for
applications wanting write-ahead logging support. Applications
wanting to use the log for purposes other than logging file
modifications based on a set of open file descriptors will
almost certainly need to make source code modifications to the
Berkeley DB code base.
</p>
<p>
A log can be shared by any number of threads of control. The
<a href="../api_reference/C/envopen.html" class="olink">DB_ENV->open()</a> method is used to open a log. When the log is no
longer in use, it should be closed using the <a href="../api_reference/C/envclose.html" class="olink">DB_ENV->close()</a>
method.
</p>
<p>
Individual log entries are identified by log sequence
numbers. Log sequence numbers are stored in an opaque object,
an <a href="../api_reference/C/lsn.html" class="olink">DB_LSN</a>.
</p>
<p>
The <a href="../api_reference/C/logcursor.html" class="olink">DB_ENV->log_cursor()</a> method is used to allocate a log cursor. Log
cursors have two methods: <a href="../api_reference/C/logcget.html" class="olink">DB_LOGC->get()</a> method to retrieve log
records from the log, and <a href="../api_reference/C/logcclose.html" class="olink">DB_LOGC->close()</a> method to destroy the
cursor.
</p>
<p>
There are additional methods for integrating the log
subsystem with a transaction processing system:
</p>
<div class="variablelist">
<dl>
<dt>
<span class="term">
<a href="../api_reference/C/logflush.html" class="olink">DB_ENV->log_flush()</a>
</span>
</dt>
<dd>
Flushes the log up to a particular log sequence
number.
</dd>
<dt>
<span class="term">
<a href="../api_reference/C/logcompare.html" class="olink">DB_ENV->log_compare()</a>
</span>
</dt>
<dd>
Allows applications to compare any two log
sequence numbers.
</dd>
<dt>
<span class="term">
<a href="../api_reference/C/logfile.html" class="olink">DB_ENV->log_file()</a>
</span>
</dt>
<dd>
Maps a log sequence number to the specific log
file that contains it.
</dd>
<dt>
<span class="term">
<a href="../api_reference/C/logarchive.html" class="olink">DB_ENV->log_archive()</a>
</span>
</dt>
<dd>
Returns various sets of log filenames. These
methods are used for database administration; for
example, to determine if log files may safely be
removed from the system.
</dd>
<dt>
<span class="term">
<a href="../api_reference/C/logstat.html" class="olink">DB_ENV->log_stat()</a>
</span>
</dt>
<dd>
The display <a href="../api_reference/C/db_stat.html" class="olink">db_stat</a> utility used the <a href="../api_reference/C/logstat.html" class="olink">DB_ENV->log_stat()</a> method
to display statistics about the log.
</dd>
<dt>
<span class="term">
<a href="../api_reference/C/envremove.html" class="olink">DB_ENV->remove()</a>
</span>
</dt>
<dd>
The log meta-information (but not the log files
themselves) may be removed using the <a href="../api_reference/C/envremove.html" class="olink">DB_ENV->remove()</a>
method.
</dd>
</dl>
</div>
<p>
For more information on the logging subsystem methods, see
the <a href="../api_reference/C/lsn.html#loglist" class="olink">Logging
Subsystem and Related Methods</a> section in the
<em class="citetitle">Berkeley DB C API Reference Guide.</em>
</p>
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applications </td>
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<td width="40%" align="right" valign="top"> Configuring logging</td>
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