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<b>DbEnv::rep_elect</b>
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<tt>
<b><pre>
#include &lt;db_cxx.h&gt;
<p>
int
DbEnv::rep_elect(u_int32_t nsites, u_int32_t nvotes, u_int32_t flags);
</pre></b>
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<b>Description: DbEnv::rep_elect</b>
<p>The DbEnv::rep_elect method holds an election for the master of a replication
group.</p>
<p>The DbEnv::rep_elect method is not called by most replication
applications.  It should only be called by applications implementing
their own network transport layer, explicitly holding replication group
elections and handling replication messages outside of the replication
manager framework.</p>
<p>If the election is successful, Berkeley DB will notify the application of
the results of the election by means of either the
<a href="../api_cxx/env_event_notify.html#DB_EVENT_REP_ELECTED">DB_EVENT_REP_ELECTED</a> or <a href="../api_cxx/env_event_notify.html#DB_EVENT_REP_NEWMASTER">DB_EVENT_REP_NEWMASTER</a> events
(see <a href="../api_cxx/env_event_notify.html">DbEnv::set_event_notify</a> method for more information).  The
application is responsible for adjusting its relationship to the other
database environments in the replication group, including directing
all database updates to the newly selected master, in accordance with
the results of the election.</p>
<p>The thread of control that calls the DbEnv::rep_elect method must not be the
thread of control that processes incoming messages; processing the
incoming messages is necessary to successfully complete an election.</p>
<b>Parameters</b> <br>
 <b>nsites</b><ul compact><li>The <b>nsites</b> parameter specifies the number of replication sites
expected to participate in the election.  Once the current site has
election information from that many sites, it will short-circuit the
election and immediately cast its vote for a new master. The
<b>nsites</b> parameter must be no less than
<b>nvotes</b>, or 0 if the election should use the value previously set
using the <a href="../api_cxx/rep_nsites.html">DbEnv::rep_set_nsites</a> method.  If an application is using master
leases, then the value <b>must</b> be 0 and the value from
<a href="../api_cxx/rep_nsites.html">DbEnv::rep_set_nsites</a> method must be used.</ul>
 <b>nvotes</b><ul compact><li>The <b>nvotes</b> parameter specifies the minimum number of replication
sites from which the current site must have election information, before
the current site will cast a vote for a new master.  The <b>nvotes</b>
parameter must be no greater than <b>nsites</b>,
or 0 if the election should use the value ((<b>nsites</b> / 2) + 1) as
the <b>nvotes</b> argument.</ul>
 <b>flags</b><ul compact><li>The <b>flags</b> parameter is currently unused, and must be set to 0.</ul>
<br>
<p>Elections are done in two parts: first, replication sites collect
information from the other replication sites they know about, and
second, replication sites cast their votes for a new master.  The second
phase is triggered by one of two things: either the replication site
gets election information from <b>nsites</b> sites, or the election
<b>timeout</b> expires.  Once the second phase is triggered, the
replication site will cast a vote for the new master of its choice if,
and only if, the site has election information from at least
<b>nvotes</b> sites.  If a site receives <b>nvotes</b> votes for it
to become the new master, then it will become the new master.</p>
<p>We recommend <b>nvotes</b> be set to at least:</p>
<blockquote><pre>(sites participating in the election / 2) + 1</pre></blockquote>
<p>to ensure there are never more than two masters active at the same time
even in the case of a network partition.  When a network partitions, the
side of the partition with more than half the environments will elect a
new master and continue, while the environments communicating with fewer
than half of the environments will fail to find a new master, as no site
can get <b>nvotes</b> votes.</p>
<p>We recommend <b>nsites</b> be set to:</p>
<blockquote><pre>number of sites in the replication group - 1</pre></blockquote>
<p>when choosing a new master after a current master fails.  This allows
the group to reach a consensus without having to wait for the timeout
to expire.</p>
<p>When choosing a master from among a group of client
sites all restarting at the same time, it makes more sense to set
<b>nsites</b> to the total number of sites in the group, since there is no
known missing site.  Furthermore, in order to ensure the best choice
from among sites that may take longer to boot than the local site,
setting <b>nvotes</b> also to this same total number of sites will
guarantee that every site in the group is considered.  Alternatively,
using the special timeout for full elections allows full participation
on restart but allows election of a master if one site does not
reboot and rejoin the group in a reasonable amount of time. (See the
<a href="../ref/rep/elect.html">Elections</a>
section in the Berkeley DB Reference Guide for more information.)</p>
<p>Setting <b>nsites</b> to lower values can increase the speed of an
election, but can also result in election failure, and is usually not
recommended.</p>
<a name="2"><!--meow--></a>
<br><b>Errors</b>
<p>The DbEnv::rep_elect method
may fail and throw
<a href="../api_cxx/except_class.html">DbException</a>,
encapsulating one of the following non-zero errors, or return one of
the following non-zero errors:</p>
<br>
<b>DB_REP_UNAVAIL</b><ul compact><li>The replication group was unable to elect a master, or was unable to
complete the election in the election timeout period (see
<a href="../api_cxx/rep_timeout.html">DbEnv::rep_set_timeout</a> method for more information).</ul>
<br>
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<br><b>Class</b>
<a href="../api_cxx/env_class.html">DbEnv</a>
<br><b>See Also</b>
<a href="../api_cxx/rep_list.html">Replication and Related Methods</a>
</tt>
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