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<title>Chapter 16. The Logging Subsystem</title>
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<th colspan="3" align="center">Chapter 16.
The Logging Subsystem
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<div class="chapter" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title"><a id="log"></a>Chapter 16.
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 utility</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>
<div class="informaltable">
<table border="1" width="80%">
<colgroup>
<col />
<col />
</colgroup>
<thead>
<tr>
<th>Logging Subsystem and Related Methods</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td>
<a href="../api_reference/C/lsn.html" class="olink">DB_LSN</a>
</td>
<td>Log Sequence Numbers</td>
</tr>
<tr>
<td>
<a href="../api_reference/C/logcompare.html" class="olink">DB_ENV->log_compare()</a>
</td>
<td>Compare two Log Sequence Numbers</td>
</tr>
<tr>
<td>
<a href="../api_reference/C/logarchive.html" class="olink">DB_ENV->log_archive()</a>
</td>
<td>List log and database files</td>
</tr>
<tr>
<td>
<a href="../api_reference/C/logfile.html" class="olink">DB_ENV->log_file()</a>
</td>
<td>Map Log Sequence Numbers to log files</td>
</tr>
<tr>
<td>
<a href="../api_reference/C/logflush.html" class="olink">DB_ENV->log_flush()</a>
</td>
<td>Flush log records</td>
</tr>
<tr>
<td>
<a href="../api_reference/C/logprintf.html" class="olink">DB_ENV->log_printf()</a>
</td>
<td>Append informational message to the log</td>
</tr>
<tr>
<td>
<a href="../api_reference/C/logput.html" class="olink">DB_ENV->log_put()</a>
</td>
<td>Write a log record</td>
</tr>
<tr>
<td>
<a href="../api_reference/C/logstat.html" class="olink">DB_ENV->log_stat()</a>
</td>
<td>Return log subsystem statistics</td>
</tr>
<tr>
<td>
<span class="emphasis">
<em>Logging Subsystem Cursors</em>
</span>
</td>
<td> </td>
</tr>
<tr>
<td>
<a href="../api_reference/C/logcursor.html" class="olink">DB_ENV->log_cursor()</a>
</td>
<td>Create a log cursor handle</td>
</tr>
<tr>
<td>
<a href="../api_reference/C/logcclose.html" class="olink">DB_LOGC->close()</a>
</td>
<td>Close a log cursor</td>
</tr>
<tr>
<td>
<a href="../api_reference/C/logcget.html" class="olink">DB_LOGC->get()</a>
</td>
<td>Retrieve a log record</td>
</tr>
<tr>
<td>
<span class="emphasis">
<em>Logging Subsystem Configuration</em>
</span>
</td>
<td> </td>
</tr>
<tr>
<td>
<a href="../api_reference/C/envlog_set_config.html" class="olink">DB_ENV->log_set_config()</a>
</td>
<td>Configure the logging subsystem</td>
</tr>
<tr>
<td>
<a href="../api_reference/C/envset_lg_bsize.html" class="olink">DB_ENV->set_lg_bsize()</a>
</td>
<td>Set log buffer size</td>
</tr>
<tr>
<td>
<a href="../api_reference/C/envset_lg_dir.html" class="olink">DB_ENV->set_lg_dir()</a>
</td>
<td>Set the environment logging directory</td>
</tr>
<tr>
<td>
<a href="../api_reference/C/envset_lg_filemode.html" class="olink">DB_ENV->set_lg_filemode()</a>
</td>
<td>Set log file mode</td>
</tr>
<tr>
<td>
<a href="../api_reference/C/envset_lg_max.html" class="olink">DB_ENV->set_lg_max()</a>
</td>
<td>Set log file size</td>
</tr>
<tr>
<td>
<a href="../api_reference/C/envset_lg_regionmax.html" class="olink">DB_ENV->set_lg_regionmax()</a>
</td>
<td>Set logging region size</td>
</tr>
</tbody>
</table>
</div>
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<td width="40%" align="left" valign="top">Locking and non-Berkeley DB applications </td>
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<a accesskey="h" href="index.html">Home</a>
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