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<! "@(#)intro.so 10.1 (Sleepycat) 12/18/98">
<!Copyright 1997, 1998 by Sleepycat Software, Inc. All rights reserved.>
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<title>Berkeley DB Reference Guide: Logging Subsystem</title>
<meta name="description" content="Berkeley DB: An embedded database programmatic toolkit.">
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ee,hash,hashing,transaction,transactions,locking,logging,access method,access me
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<h3>Berkeley DB Reference Guide: Logging Subsystem</h3>
<p>
<h1 align=center>Berkeley DB and logging</h1>
<p>
The logging subsystem is the somewhat general-purpose logging facility
used by Berkeley DB. This module is potentially useful outside of the Berkeley DB
package for processes that require write-ahead logging support, but is
less likely to be generally useful than the memory pool or locking
subsystems.
<p>
A log can be shared by any number of processes and threads within
processes. The <a href="../../api_c/DbLog/open.html">log_open</a> interface opens, and optionally creates,
a log. When the log is no longer in use, it should be closed, using the
<a href="../../api_c/DbLog/close.html">log_close</a> interface. Specific log entries are identified by log
sequence numbers. Log sequence numbers are held in an opaque object, a
DB_LSN.
<p>
The <a href="../../api_c/DbLog/put.html">log_put</a> interface is used to append new log records to the
log. Optionally, the <a href="../../api_c/DbLog/put.html#DB_CHECKPOINT">DB_CHECKPOINT</a> flag can be used to cause
any log record to include checkpoint information, such that the log is
consistent to that point and recoverable after a system or application
failure. The <a href="../../api_c/DbLog/get.html">log_get</a> interface is used to retrieve log records
from the log.
<p>
There are additional interfaces for integrating the log subsystem with
a transaction processing system:
<p>
The <a href="../../api_c/DbLog/db_register.html">log_register</a> and <a href="../../api_c/DbLog/db_unregister.html">log_unregister</a> interfaces associate
files with identification numbers. These identification numbers are
logged so that log recovery correctly associates database changes with
the appropriate files.
<p>
The <a href="../../api_c/DbLog/flush.html">log_flush</a> interface flushes the log up to a particular log
sequence number.
<p>
The <a href="../../api_c/DbLog/compare.html">log_compare</a> interface allows applications to compare any
two log sequence numbers.
<p>
The <a href="../../api_c/DbLog/file.html">log_file</a> interface maps a log sequence number to the specific
log file which contains it.
The <a href="../../api_c/DbLog/archive.html">log_archive</a> interface returns various sets of log file names.
These interfaces are used for database administration, e.g., to determine
if log files may be removed from the system.
<p>
The <a href="../../utility/db_stat.html">db_stat</a> utility uses the <a href="../../api_c/DbLog/stat.html">log_stat</a> interface to display
statistics about the log.
<p>
Finally, the log (but not the log files themselves) may be discarded using
the <a href="../../api_c/DbLog/unlink.html">log_unlink</a> interface.
<p>
<a href="../../ref/toc.html"><img src="../../images/toc.gif"></a>
<a href="../../ref/log/limits.html"><img src="../../images/next.gif"></a>
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