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<h2 class="title" style="clear: both"><a id="cam_fail"></a>Handling failure in Data Store and Concurrent Data Store applications</h2>
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<p>
When building Data Store and Concurrent 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 simplest case is handling system failure for any Data
Store or Concurrent Data Store application. In the case of
system failure, it doesn't matter if the application has
opened a database environment or is just using standalone
databases: if the system fails, after the application has
modified a database and has not subsequently flushed the
database to stable storage (by calling either the <a href="../api_reference/C/dbclose.html" class="olink">DB->close()</a>,
<a href="../api_reference/C/dbsync.html" class="olink">DB->sync()</a> or <a href="../api_reference/C/mempsync.html" class="olink">DB_ENV->memp_sync()</a> methods), the database may be left in a
corrupted state. In this case, before accessing the database
again, the database should either be:
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removed and re-created,
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<li>
removed and restored from the last known good
backup, or
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verified using the <a href="../api_reference/C/dbverify.html" class="olink">DB->verify()</a> method or <a href="../api_reference/C/db_verify.html" class="olink">db_verify</a> utility.
If the database does not verify cleanly, the contents may
be salvaged using the <span class="bold"><strong>-R</strong></span>
and <span class="bold"><strong>-r</strong></span> options of the
<a href="../api_reference/C/db_dump.html" class="olink">db_dump</a> utility.
</li>
</ul>
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<p>
Applications where the potential for data loss is
unacceptable should consider the Berkeley DB Transactional
Data Store product, which offers standard transactional
durability guarantees, including recoverability after
failure.
</p>
<p>
Additionally, system failure requires that any persistent
database environment (that is, any database environment not
created using the <a href="../api_reference/C/envopen.html#envopen_DB_PRIVATE" class="olink">DB_PRIVATE</a> flag), be removed. Database
environments may be removed using the <a href="../api_reference/C/envremove.html" class="olink">DB_ENV->remove()</a> method. If
the persistent database environment was backed by the
filesystem (that is, the environment was not created using the
<a href="../api_reference/C/envopen.html#envopen_DB_SYSTEM_MEM" class="olink">DB_SYSTEM_MEM</a> flag), the database environment may also be
safely removed by deleting the environment's files with
standard system utilities.
</p>
<p>
The second case is application failure for a Data Store
application, with or without a database environment, or
application failure for a Concurrent Data Store application
without a database environment: as in the case of system
failure, if any thread of control fails, after the application
has modified a database and has not subsequently flushed the
database to stable storage, the database may be left in a
corrupted state. In this case, the database should be handled
as described previously in the system failure case.
</p>
<p>
The third case is application failure for a Concurrent Data
Store application with a database environment. There are
resources maintained in database environments that may be left
locked if a thread of control exits without first closing all
open Berkeley DB handles. Concurrent Data Store applications
with database environments have an additional option for
handling the unexpected exit of a thread of control, 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> 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. In this case, the application
can continue to use the database environment.
</p>
<p>
Note that by default <a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV->failchk()</a> only notifies the calling thread that
the database environment is unusable. You can optionally cause
<a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV->failchk()</a> to broadcast this 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
<code class="literal">DB_EVENT_FAILCHK_PANIC</code> or <code class="literal">DB_EVENT_MUTEX_DIED</code>
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 Concurrent 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.
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