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The Logging Subsystem
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<h2 class="title"><a id="log"></a>Chapter 17.
The Logging Subsystem
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<p>
<b>Table of Contents</b>
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<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>
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<span class="sect1">
<a href="log_limits.html">Log file limits</a>
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<h2 class="title" style="clear: both"><a id="log_intro"></a>Introduction to the logging subsystem</h2>
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<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>
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<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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