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