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<th width="60%" align="center">Chapter 12.
Berkeley DB Replication
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<h2 class="title" style="clear: both"><a id="rep_filename"></a>Managing Replication Files</h2>
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<p>
Whether you use the Base API or the Replication Manager,
replication creates a set of internal files that are normally stored
on-disk in your environment home directory. These files contain
metadata which is necessary for replication operations, and so you
should never delete these files.
</p>
<p>
You can cause these files to not be stored on disk, but instead to
be held entirely in-memory, by specifying the <a href="../api_reference/C/repconfig.html#config_DB_REP_CONF_INMEM" class="olink">DB_REP_CONF_INMEM</a>
flag to the <a href="../api_reference/C/repconfig.html" class="olink">DB_ENV->rep_set_config()</a> method. Doing this can improve your
application's data throughput by avoiding the disk I/O associated
with these metadata files. However, in the event that your
application is shut down, the contents of these files are lost. This
results in some loss of functionality, including an increased
chance that elections will fail, or that the wrong site will win an
election. See the <a href="../api_reference/C/repconfig.html#config_DB_REP_CONF_INMEM" class="olink">DB_REP_CONF_INMEM</a> flag description for more
information.
</p>
<p>
Note that turning on <a href="../api_reference/C/repconfig.html#config_DB_REP_CONF_INMEM" class="olink">DB_REP_CONF_INMEM</a> means that Replication
Manager cannot store group membership changes persistently. This is
because Replication Manager stores group membership information in
an internal database, which is held in memory when
<a href="../api_reference/C/repconfig.html#config_DB_REP_CONF_INMEM" class="olink">DB_REP_CONF_INMEM</a> is turned on. For this reason, if your
Replication Manager application requires replication metadata to be
stored in memory, then you must manually identify all the sites in
your replication group using the <code class="literal">DB_LEGACY</code> site
configuration attribute. Be aware that this configuration needs to
be made permanent. (Normally, <code class="literal">DB_LEGACY</code> is used
only on a temporary basis for the purpose of upgrading old
Replication Manager applications.)
</p>
<p>
Do the following:
</p>
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<p>
Shut down all the sites in your replication group.
</p>
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<li>
<p>
For every site in your replication group:
</p>
<div class="orderedlist">
<ol type="a">
<li>
<p>
Configure a <a href="../api_reference/C/db_site.html" class="olink">DB_SITE</a> handle for the local site.
Use <a href="../api_reference/C/dbsite_set_config.html" class="olink">DB_SITE->set_config()</a> to indicate that this
is a legacy site by setting the
<code class="literal">DB_LEGACY</code> parameter.
</p>
</li>
<li>
<p>
Configure a <a href="../api_reference/C/db_site.html" class="olink">DB_SITE</a> handle for <span class="emphasis"><em>every
other site</em></span> in the replication
group. Set the <code class="literal">DB_LEGACY</code>
parameter for each of these handles.
</p>
<p>
Please pay careful attention to this step. To
repeat: a <a href="../api_reference/C/db_site.html" class="olink">DB_SITE</a> handle MUST be configured
for EVERY site in the replication group.
</p>
</li>
</ol>
</div>
</li>
<li>
<p>
Restart all the sites in the replication group.
</p>
</li>
</ol>
</div>
<p>
Alternatively, you can store persistent environment metadata files,
including those required by replication, in a location other than
your environment home directory. Doing so can help improve I/O
throughput by placing these files on a spindle that is not being
used for other environment data I/O. You do this using the
<a href="../api_reference/C/envset_metadata_dir.html" class="olink">DB_ENV->set_metadata_dir()</a> method.
</p>
<p>
Note that you must configure the handling of your environment
metadata consistently across your entire replication group. That
is, if you place your replication metadata in-memory on one
site, then it must be placed in-memory on all the sites in the
group. Similarly, if you place your replication metadata files in a
non-standard directory location on one site, then they must be
placed in the exact same directory location on all the sites in
your group.
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