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<title>Berkeley DB Reference Guide: Hot failover</title>
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<td><dl><dt>Berkeley DB Reference Guide:<dd>Berkeley DB Transactional Data Store Applications</dl></td>
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Hot failover
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.
<ol>
<li>Run <a href="../../utility/db_archive.html">db_archive</a> -s in the active environment to
identify all of the active environment's database files, and copy them
to the backup directory.
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_c/env_set_data_dir.html">DB_ENV-&gt;set_data_dir</a>
for additional information). Note: if any of the database
files did not have an open <a href="../../api_c/db_class.html">DB</a> handle during the lifetime of the
current log files, <a href="../../utility/db_archive.html">db_archive</a> 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 <a href="../../utility/db_archive.html">db_archive</a>.
<li>Remove all existing log files from the backup directory.
<li>Run <a href="../../utility/db_archive.html">db_archive</a> -l in the active environment to
identify all of the active environment's log files, and copy them to
the backup directory.
<li>Run <a href="../../utility/db_recover.html">db_recover</a> -c in the backup directory to
catastrophically recover the copied environment.
</ol>
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
must be performed at least once in order to ensure a consistent
database environment snapshot.
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:
<ol>
<li>Run <a href="../../utility/db_archive.html">db_archive</a> -s in the active environment to
identify all of the active environment's database files, and copy them
to the backup directory.
<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>Run <a href="../../utility/db_archive.html">db_archive</a> (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 move them to the backup directory.
<li>Run <a href="../../utility/db_archive.html">db_archive</a> -l in the active environment to
identify all of the remaining log files in the active environment, and
copy the log files to the backup directory.
<li>Run <a href="../../utility/db_recover.html">db_recover</a> -c in the backup directory to
catastrophically recover the copied environment.
</ol>
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 must be performed at least once in order to
ensure a consistent database environment snapshot.
For an example of a hot backup implementation in the Berkeley DB distribution,
see the source code for the <a href="../../utility/db_hotbackup.html">db_hotbackup</a> utility.
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