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<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
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<div>
<h2 class="title" style="clear: both"><a id="elections"></a>Holding Elections</h2>
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<div class="toc">
<dl>
<dt>
<span class="sect2">
<a href="elections.html#influencingelections">Influencing Elections</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="elections.html#winningelections">Winning Elections</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="elections.html#switchingmasters">Switching Masters</a>
</span>
</dt>
</dl>
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<p>
Finding a master environment is one of the fundamental activities that
every replication replica must perform. Upon startup, the
underlying DB replication code will attempt to
locate a master. If a master cannot be found, then the
environment should initiate an election.
</p>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>
In some rare situations, it is desireable for the
application to manually select its master. For these
cases, elections can be turned off.
</p>
<p>
Manually selecting a master is an activity that should
be performed infrequently, if ever. You turn elections
off by using the
<span>
<code class="classname">ReplicationConfig</code>
and
<code class="classname">ReplicationManagerStartPolicy</code>
classes.
</span>
</p>
</div>
<p>
How elections are held depends upon the API that you use to
implement replication. For example, if you are using the
Replication Manager elections are held transparently without any
input from your application's code. In this case,
DB will determine which environment is the master and which
are replicas.
</p>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="influencingelections"></a>Influencing Elections</h3>
</div>
</div>
</div>
<p>
If you want to control the election process, you can
declare a specific environment to be the master. Should
the master become unavailable during run-time for any
reason, an election is held. The environment that receives
the most number of votes, wins the election and becomes the
master. A machine receives a vote because it has the most
up-to-date log records.
</p>
<p>
Because ties are possible when elections are held,
it is possible to influence which environment will win
the election. How you do this depends on which API you
are using. In particular, if you are writing a custom replication
layer, then there are a great many ways to manually influence
elections.
</p>
<p>
One such mechanism is priorities. When votes are cast
during an election, the winner is determined first by
the environment with the most up-to-date log records.
But if this is a tie, the the environment's priority is
considered. So given two environments with log records
that are equally recent, votes are cast for the
environment with the higher priority.
</p>
<p>
Therefore, if you have a machine that you prefer to
become a master in the event of an election, assign it
a high priority. Assuming that the election is held at
a time when the preferred machine has up-to-date log
records, that machine will win the election.
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="winningelections"></a>Winning Elections</h3>
</div>
</div>
</div>
<p>
To win an election:
</p>
<div class="orderedlist">
<ol type="1">
<li>
<p>
There cannot currently be a
master environment.
</p>
</li>
<li>
<p>
The environment must have the most
recent log records. Part of
holding the election is
determining which environments have
the most recent log records.
This process happens
automatically; your code does
not need to involve itself in
this process.
</p>
</li>
<li>
<p>
The environment must receive the most
number of votes from the
replication environments that are
participating in the election.
</p>
</li>
</ol>
</div>
<p>
If you are using the Replication Manager, then in the event of a
tie vote the environment with the highest priority wins
the election. If two or more environments receive the same
number of votes and have the same priority, then
the underlying replication code picks one of the
environments to
be the winner. Which winner will be picked by the
replication code is unpredictable from the
perspective of your application code.
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="switchingmasters"></a>Switching Masters</h3>
</div>
</div>
</div>
<p>
To switch masters:
</p>
<div class="orderedlist">
<ol type="1">
<li>
<p>
Start up the environment that you want
to be master as normal. At this
time it is a replica. Make
sure this environment has a higher
priority than all the other
environments.
</p>
</li>
<li>
<p>
Allow the new environment to run for a
time as a replica. This allows
it to obtain the most recent
copies of the log files.
</p>
</li>
<li>
<p>
Shut down the current master.
This should force an election.
Because the new environment has the
highest priority, it will win
the election, provided it has
had enough time to obtain all
the log records.
</p>
</li>
<li>
<p>
Optionally restart the old
master environment. Because there is
currently a master environment, an
election will not be held and
the old master will now run as
a replica environment.
</p>
</li>
</ol>
</div>
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