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<th colspan="3" align="center">Recovery procedures</th>
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<h2 class="title" style="clear: both"><a id="transapp_recovery"></a>Recovery procedures</h2>
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<p>The fifth component of the infrastructure, recovery procedures, concerns
the recoverability of the database. After any application or system
failure, there are two possible approaches to database recovery:</p>
<div class="orderedlist">
<ol type="1">
<li>There is no need for recoverability, and all databases can be re-created
from scratch. Although these applications may still need transaction
protection for other reasons, recovery usually consists of removing the
Berkeley DB environment home directory and all files it contains, and then
restarting the application.
Such an application may use the <a href="../api_reference/C/dbset_flags.html#dbset_flags_DB_TXN_NOT_DURABLE" class="olink">DB_TXN_NOT_DURABLE</a> flag to avoid
writing log records.</li>
<li>
<p>
It is necessary to recover information after system or application
failure. In this case, recovery processing must be performed on
any database environments that were active at the time of the
failure. Recovery processing involves running the <a href="../api_reference/C/db_recover.html" class="olink">db_recover</a> utility or
calling the <a href="../api_reference/C/envopen.html" class="olink">DB_ENV->open()</a> method with the <a href="../api_reference/C/envopen.html#envopen_DB_RECOVER" class="olink">DB_RECOVER</a> or
<a href="../api_reference/C/envopen.html#envopen_DB_RECOVER_FATAL" class="olink">DB_RECOVER_FATAL</a> flags.
</p>
<p>
During recovery processing, all database changes made by aborted or
unfinished transactions are undone, and all database changes made
by committed transactions are redone, as necessary. Database
applications must not be restarted until recovery completes. After
recovery finishes, the environment is properly initialized so that
applications may be restarted.
</p>
</li>
</ol>
</div>
<p>If performing recovery, there are two types of recovery processing:
<span class="emphasis"><em>normal</em></span> and <span class="emphasis"><em>catastrophic</em></span>.
Which you choose depends on the source for the database and log files you are
using to recover.</p>
<p>
If up-to-the-minute database and log files are accessible on a stable
filesystem, normal recovery is sufficient. Run the <a href="../api_reference/C/db_recover.html" class="olink">db_recover</a> utility or
call the <a href="../api_reference/C/envopen.html" class="olink">DB_ENV->open()</a> method specifying the <a href="../api_reference/C/envopen.html#envopen_DB_RECOVER" class="olink">DB_RECOVER</a> flag. However, the
normal recovery case <span class="bold"><strong>never</strong></span> includes
recovery using hot backups of the database environment. For example,
you cannot perform a hot backup of databases and log files, restore the
backup and then run normal recovery — you must always run catastrophic
recovery when using hot backups.
</p>
<p>
If the database or log files have been destroyed or corrupted, or
normal recovery fails, catastrophic recovery is required. For example,
catastrophic failure includes the case where the disk drive on which
the database or log files are stored has been physically destroyed, or
when the underlying filesystem is corrupted and the operating system's
normal filesystem checking procedures cannot bring that filesystem to a
consistent state. This is often difficult to detect, and a common sign
of the need for catastrophic recovery is when normal Berkeley DB
recovery procedures fail, or when checksum errors are displayed during
normal database procedures. When catastrophic recovery is necessary,
take the following steps:
</p>
<div class="orderedlist">
<ol type="1">
<li>Restore the most recent snapshots of the database and log files from
the backup media into the directory where recovery will be performed.</li>
<li>
<p>
If any log files were archived since the last snapshot was made,
they should be restored into the directory where recovery will be
performed.
</p>
<p>
If any log files are available from the database environment that
failed (for example, the disk holding the database files crashed,
but the disk holding the log files is fine), those log files should
be copied into the directory where recovery will be performed.
</p>
<p>
Be sure to restore all log files in the order they were written.
The order is important because it's possible the same log file
appears on multiple backups, and you want to run recovery using the
most recent version of each log file.
</p>
</li>
<li>
<p>
Run the <a href="../api_reference/C/db_recover.html" class="olink">db_recover</a> utility, specifying its <span class="bold"><strong>-c</strong></span> option; or call the <a href="../api_reference/C/envopen.html" class="olink">DB_ENV->open()</a> method,
specifying the <a href="../api_reference/C/envopen.html#envopen_DB_RECOVER_FATAL" class="olink">DB_RECOVER_FATAL</a> flag. The catastrophic recovery
process will review the logs and database files to bring the
environment databases to a consistent state as of the time of the
last uncorrupted log file that is found. It is important to
realize that only transactions committed before that date will
appear in the databases.
</p>
<p>
It is possible to re-create the database in a location different
from the original by specifying appropriate pathnames to the
<span class="bold"><strong>-h</strong></span> option of the <a href="../api_reference/C/db_recover.html" class="olink">db_recover</a> utility. In
order for this to work properly, it is important that your
application refer to files by names relative to the database home
directory or the pathname(s) specified in calls to
<a href="../api_reference/C/envset_data_dir.html" class="olink">DB_ENV->set_data_dir()</a>, instead of using full pathnames.
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