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<p>
It is possible to use a replica instead of a master to synchronize another
replica. This serves to take the request load off a master that might otherwise
occur if multiple replicas attempted to synchronize with the master at the same
time.
</p>
<p>
For best results, use this feature combined with the delayed synchronization
feature (see <a href="addfeatures.html#delayedsync">Delayed Synchronization</a>).
</p>
<p>
For example, suppose your replication group consists of four environments. Upon
application startup, all three replicas will immediately attempt to synchronize
with the master. But at the same time, the master itself might be busy with a heavy
database write load.
</p>
<p>
To solve this problem, delay synchronization for two of the three replicas. Allow
the third replica to synchronize as normal with the master. Then, start
synchronization for each of the delayed replicas (since this is a manual process,
you can do them one at a time if that best suits your application).
Assuming you have configured replica to replica synchronization correctly, the
delayed replicas will synchronize using the up-to-date replica, rather than using
the master.
</p>
<p>
When you are using the replication framework, you configure replica to replica synchronization by
declaring one environment to be a peer of another environment. If an
environment is a peer, then
it can be used for synchronization purposes.
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<p>
You can designate one replica to be a peer of
another, and then use this special
status for permanent message acknowledgments,
and for replica-to-replica synchronization.
You might want to do this if you have
machines that you know are on fast, reliable
network connections and so you are willing to
accept the overhead of waiting for
acknowledgments from those specific machines.
</p>
<p>
An environment is currently allowed to have only one
peer.
</p>
<p>
Note that peers are not required to be a
bi-directional. That is, just because machine A
declares machine B to be a peer, that does not mean
machine B must also declare machine A to be a peer.
</p>
<p>
You declare a peer for the current environment
when you add that environment to the list of known
sites. You do this by
<span>specifying the
<tt class="literal">DB_REPMGR_PEER</tt> flag to
<span><tt class="methodname">DB_ENV->repmgr_add_remote_site()</tt>.</span>
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