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Yasuhiro KIMURA
157 lines
8.4 KiB
HTML
157 lines
8.4 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>Handling failure in Transactional Data Store applications</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="Berkeley DB Programmer's Reference Guide" />
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<link rel="up" href="transapp.html" title="Chapter 11. Berkeley DB Transactional Data Store Applications" />
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<link rel="prev" href="transapp_term.html" title="Terminology" />
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<link rel="next" href="transapp_app.html" title="Architecting Transactional Data Store applications" />
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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 18.1.40</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">Handling failure in Transactional Data Store applications</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="transapp_term.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 11. Berkeley DB Transactional Data Store Applications </th>
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<td width="20%" align="right"> <a accesskey="n" href="transapp_app.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="transapp_fail"></a>Handling failure in Transactional Data Store applications</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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When building Transactional Data Store applications, there
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are design issues to consider whenever a thread of control
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with open Berkeley DB handles fails for any reason (where a
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thread of control may be either a true thread or a process).
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</p>
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<p>
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The first case is handling system failure: if the system
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fails, the database environment and the databases may be left
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in a corrupted state. In this case, recovery must be performed
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on the database environment before any further action is
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taken, in order to:
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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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recover the database environment
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resources,
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</li>
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<li>
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release any locks or mutexes that may have been held
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to avoid starvation as the remaining threads of control
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convoy behind the held locks, and
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</li>
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<li>
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resolve any partially completed operations that may
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have left a database in an inconsistent or corrupted
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state.
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</li>
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</ul>
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</div>
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<p>
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For details on performing recovery, see the <a class="xref" href="transapp_recovery.html" title="Recovery procedures">Recovery procedures</a>.
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</p>
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<p>
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The second case is handling the failure of a thread of
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control. There are resources maintained in database
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environments that may be left locked or corrupted if a thread
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of control exits unexpectedly. These resources include data
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structure mutexes, logical database locks and unresolved
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transactions (that is, transactions which were never aborted
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or committed). While Transactional Data Store applications can
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treat the failure of a thread of control in the same way as
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they do a system failure, they have an alternative choice, the
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<a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV->failchk()</a> method.
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</p>
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<p>
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The <a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV->failchk()</a> method will return
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<a class="link" href="program_errorret.html#program_errorret.DB_RUNRECOVERY">DB_RUNRECOVERY</a>
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if the database environment is unusable as a result of the thread
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of control failure. (If a data structure mutex or a database write
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lock is left held by thread of control failure, the application
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should not continue to use the database environment, as subsequent
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use of the environment is likely to result in threads of control
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convoying behind the held locks.) The <a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV->failchk()</a> call will
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release any database read locks that have been left held by the
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exit of a thread of control, and abort any unresolved transactions.
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In this case, the application can continue to use the database
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environment.
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</p>
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<p>
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Note that you can optionally cause <a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV->failchk()</a> to broadcast a database
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environment failure to other threads of control by using the
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<code class="literal">--enable-failchk_broadcast</code> flag when you compile
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your Berkeley DB library. If this option is turned on, then all
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threads of control using the database environment will return
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<a class="link" href="program_errorret.html#program_errorret.DB_RUNRECOVERY">DB_RUNRECOVERY</a>
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when they attempt to obtain a mutex lock. In this situation, a
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<a href="../api_reference/C/envevent_notify.html#event_notify_DB_EVENT_FAILCHK_PANIC" class="olink">DB_EVENT_FAILCHK_PANIC</a> or
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<a href="../api_reference/C/envevent_notify.html#event_notify_DB_EVENT_MUTEX_DIED" class="olink">DB_EVENT_MUTEX_DIED</a> event will also be raised.
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(You use <a href="../api_reference/C/envevent_notify.html" class="olink">DB_ENV->set_event_notify()</a> to examine events).
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</p>
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<p>
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A Transactional Data Store application recovering from a
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thread of control failure should call <a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV->failchk()</a>, and, if it
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returns success, the application can continue. If <a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV->failchk()</a>
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returns <a class="link" href="program_errorret.html#program_errorret.DB_RUNRECOVERY">DB_RUNRECOVERY</a>,
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the application should proceed as described for the case of system
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failure. In addition, threads notified of failure by <a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV->failchk()</a>
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should also proceed as described for the case of system failure.
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</p>
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<p>
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It greatly simplifies matters that recovery may be
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performed regardless of whether recovery needs to be
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performed; that is, it is not an error to recover a database
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environment for which recovery is not strictly necessary. For
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this reason, applications should not try to determine if the
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database environment was active when the application or system
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failed. Instead, applications should run recovery any time the
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<a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV->failchk()</a> method returns <a class="link" href="program_errorret.html#program_errorret.DB_RUNRECOVERY">
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DB_RUNRECOVERY</a>, or, if the application is not
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calling the <a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV->failchk()</a> method, any time any thread of
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control accessing the database environment fails, as well as
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any time the system reboots.
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</p>
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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="transapp_term.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="transapp.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="transapp_app.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">Terminology </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"> Architecting Transactional Data
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Store applications</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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