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<th width="60%" align="center">Chapter 11.
Berkeley DB Transactional Data Store Applications
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<h2 class="title" style="clear: both"><a id="transapp_hotfail"></a>Hot failover</h2>
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<p>
For some applications, it may be useful to periodically snapshot
the database environment for use as a hot failover should the
primary system fail. The following steps can be taken to keep a
backup environment in close synchrony with an active environment.
The active environment is entirely unaffected by these procedures,
and both read and write operations are allowed during all steps
described here.
</p>
<p>
The procedure described here is not compatible with the concurrent
use of the transactional bulk insert optimization (transactions
started with the <a href="../api_reference/C/txnbegin.html#txnbegin_DB_TXN_BULK" class="olink">DB_TXN_BULK</a> flag). After the bulk optimization
is used, the archive must be created again from scratch starting
with step 1.
</p>
<p>
The <a href="../api_reference/C/db_hotbackup.html" class="olink">db_hotbackup</a> utility is the preferred way to automate generating a hot
failover system. The first step is to run <a href="../api_reference/C/db_hotbackup.html" class="olink">db_hotbackup</a> utility without the
<span class="bold"><strong>-u</strong></span> flag. This will create hot
backup copy of the databases in your environment. After that point
periodically running the <a href="../api_reference/C/db_hotbackup.html" class="olink">db_hotbackup</a> utility with the
<span class="bold"><strong>-u</strong></span> flag will copy the new
log files and run recovery on the backup copy to bring it current
with the primary environment.
</p>
<p>
Note that you can also create your own hot backup solution using
the <a href="../api_reference/C/envbackup.html" class="olink">DB_ENV->backup()</a> or <a href="../api_reference/C/envdbbackup.html" class="olink">DB_ENV->dbbackup()</a> methods.
</p>
<p>
To implement your own hot fail over system, the steps below can be
followed. However, care should be taken on non-UNIX based systems
when copying the database files to be sure that they are either
quiescent, or that either the <a href="../api_reference/C/envbackup.html" class="olink">DB_ENV->backup()</a> or <a href="../api_reference/C/db_copy.html" class="olink">db_copy()</a> routine is
used to ensure atomic reads of the database pages.
</p>
<div class="orderedlist">
<ol type="1">
<li>
<p>
Run the <a href="../api_reference/C/db_archive.html" class="olink">db_archive</a> utility with the <span class="bold"><strong>-s</strong></span> option in the active environment
to identify all of the active environment's database files, and
copy them to the backup directory.
</p>
<p>
If the database files are stored in a separate directory from
the other Berkeley DB files, it will be simpler (and much
faster!) to copy the directory itself instead of the individual
files (see <a href="../api_reference/C/envadd_data_dir.html" class="olink">DB_ENV->add_data_dir()</a> for additional information).
</p>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>
If any of the database files did not have an open <a href="../api_reference/C/db.html" class="olink">DB</a>
handle during the lifetime of the current log files,
the <a href="../api_reference/C/db_archive.html" class="olink">db_archive</a> utility will not list them in its output. This
is another reason it may be simpler to use a separate
database file directory and copy the entire directory
instead of archiving only the files listed by the
<a href="../api_reference/C/db_archive.html" class="olink">db_archive</a> utility.
</p>
</div>
</li>
<li>
Remove all existing log files from the backup directory.
</li>
<li>
Run the <a href="../api_reference/C/db_archive.html" class="olink">db_archive</a> utility with the <span class="bold"><strong>-l</strong></span>
option in the active environment to identify all of the active
environment's log files, and copy them to the backup directory.
</li>
<li>
Run the <a href="../api_reference/C/db_recover.html" class="olink">db_recover</a> utility with the <span class="bold"><strong>-c</strong></span>
option in the backup directory to catastrophically recover the
copied environment.
</li>
</ol>
</div>
<p>
Steps 2, 3 and 4 may be repeated as often as you like. If Step 1
(the initial copy of the database files) is repeated, then Steps 2,
3 and 4 <span class="bold"><strong>must</strong></span> be performed at least
once in order to ensure a consistent database environment
snapshot.
</p>
<p>
These procedures must be integrated with your other archival
procedures, of course. If you are periodically removing log files
from your active environment, you must be sure to copy them to the
backup directory before removing them from the active directory.
Not copying a log file to the backup directory and subsequently
running recovery with it present may leave the backup snapshot of
the environment corrupted. A simple way to ensure this never
happens is to archive the log files in Step 2 as you remove them
from the backup directory, and move inactive log files from your
active environment into your backup directory (rather than copying
them), in Step 3. The following steps describe this procedure in
more detail:
</p>
<div class="orderedlist">
<ol type="1">
<li>
Run the <a href="../api_reference/C/db_archive.html" class="olink">db_archive</a> utility with the <span class="bold"><strong>-s</strong></span>
option in the active environment to identify all of the active
environment's database files, and copy them to the backup
directory.
</li>
<li>
Archive all existing log files from the backup directory, moving them
to a backup device such as CD-ROM, alternate disk, or tape.
</li>
<li>
Run the <a href="../api_reference/C/db_archive.html" class="olink">db_archive</a> utility (without any option) in the active environment
to identify all of the log files in the active environment that are
no longer in use, and <span class="bold"><strong>move</strong></span> them to
the backup directory.
</li>
<li>
Run the <a href="../api_reference/C/db_archive.html" class="olink">db_archive</a> utility with the <span class="bold"><strong>-l</strong></span>
option in the active environment to identify all of the remaining
log files in the active environment, and <span class="bold"><strong>copy</strong></span> the log files to the backup
directory.
</li>
<li>
Run the <a href="../api_reference/C/db_recover.html" class="olink">db_recover</a> utility with the <span class="bold"><strong>-c</strong></span>
option in the backup directory to catastrophically recover the
copied environment.
</li>
</ol>
</div>
<p>
As before, steps 2, 3, 4 and 5 may be repeated as often as you
like. If Step 1 (the initial copy of the database files) is
repeated, then Steps 2 through 5
<span class="bold"><strong>must</strong></span> be performed at least once in
order to ensure a consistent database environment snapshot.
</p>
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