2011-09-13 17:44:24 +00:00
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<title>Holding Elections</title>
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<link rel="start" href="index.html" title="Getting Started with Replicated Berkeley DB Applications" />
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<link rel="up" href="introduction.html" title="Chapter 1. Introduction" />
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<link rel="prev" href="apioverview.html" title="The Replication APIs" />
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<link rel="next" href="permmessages.html" title="Permanent Message Handling" />
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<body>
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<div xmlns="" class="navheader">
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<div class="libver">
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2012-11-14 21:35:20 +00:00
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<p>Library Version 11.2.5.3</p>
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2011-09-13 17:44:24 +00:00
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</div>
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">Holding Elections</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="apioverview.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 1. Introduction</th>
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<td width="20%" align="right"> <a accesskey="n" href="permmessages.html">Next</a></td>
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</table>
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<hr />
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</div>
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<div class="sect1" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h2 class="title" style="clear: both"><a id="elections"></a>Holding Elections</h2>
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</div>
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</div>
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</div>
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<div class="toc">
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<dl>
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<dt>
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<span class="sect2">
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<a href="elections.html#influencingelections">Influencing Elections</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="elections.html#winningelections">Winning Elections</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="elections.html#switchingmasters">Switching Masters</a>
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</span>
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</dt>
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</dl>
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</div>
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<p>
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Finding a master environment is one of the fundamental activities that
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every replication replica must perform. Upon startup, the
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underlying DB replication code will attempt to
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locate a master. If a master cannot be found, then the
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environment should initiate an election.
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</p>
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<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
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<h3 class="title">Note</h3>
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<p>
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In some rare situations, it is desireable for the
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application to manually select its master. For these
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cases, elections can be turned off.
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</p>
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<p>
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Manually selecting a master is an activity that should
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be performed infrequently, if ever. You turn elections
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off by using the
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<span>
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<code class="classname">ReplicationConfig</code>
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and
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<code class="classname">ReplicationManagerStartPolicy</code>
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classes.
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</span>
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</p>
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</div>
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<p>
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How elections are held depends upon the API that you use to
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implement replication. For example, if you are using the
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Replication Manager elections are held transparently without any
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input from your application's code. In this case,
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DB will determine which environment is the master and which
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are replicas.
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</p>
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<div class="sect2" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h3 class="title"><a id="influencingelections"></a>Influencing Elections</h3>
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</div>
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</div>
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</div>
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<p>
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If you want to control the election process, you can declare
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a specific environment to be the master. Note that for the Replication Manager,
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it is only possible to do this at application startup.
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Should the master become unavailable during run-time for any
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reason, an election is held. The environment that receives
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the most number of votes, wins the election and becomes the
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master. A machine receives a vote because it has the most
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up-to-date log records.
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</p>
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<p>
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Because ties are possible when elections are held,
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it is possible to influence which environment will win
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the election. How you do this depends on which API you
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are using. In particular, if you are writing a custom replication
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layer, then there are a great many ways to manually influence
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elections.
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</p>
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<p>
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One such mechanism is priorities. When votes are cast
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during an election, the winner is determined first by
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the environment with the most up-to-date log records.
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But if this is a tie, the the environment's priority is
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considered. So given two environments with log records
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that are equally recent, votes are cast for the
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environment with the higher priority.
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</p>
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<p>
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Therefore, if you have a machine that you prefer to
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become a master in the event of an election, assign it
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a high priority. Assuming that the election is held at
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a time when the preferred machine has up-to-date log
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records, that machine will win the election.
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</p>
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</div>
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<div class="sect2" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h3 class="title"><a id="winningelections"></a>Winning Elections</h3>
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</div>
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</div>
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</div>
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<p>
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To win an election:
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</p>
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<div class="orderedlist">
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<ol type="1">
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<li>
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<p>
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There cannot currently be a
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master environment.
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</p>
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</li>
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<li>
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<p>
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The environment must have the most
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recent log records. Part of
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holding the election is
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determining which environments have
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the most recent log records.
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This process happens
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automatically; your code does
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not need to involve itself in
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this process.
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</p>
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</li>
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<li>
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<p>
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The environment must receive the most
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number of votes from the
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replication environments that are
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participating in the election.
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</p>
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</li>
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</ol>
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</div>
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<p>
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If you are using the Replication Manager, then in the event of a
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tie vote the environment with the highest priority wins
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the election. If two or more environments receive the same
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number of votes and have the same priority, then
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the underlying replication code picks one of the
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environments to
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be the winner. Which winner will be picked by the
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replication code is unpredictable from the
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perspective of your application code.
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</p>
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</div>
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<div class="sect2" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h3 class="title"><a id="switchingmasters"></a>Switching Masters</h3>
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</div>
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</div>
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</div>
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<p>
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To switch masters:
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</p>
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<div class="orderedlist">
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<ol type="1">
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<li>
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<p>
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Start up the environment that you want
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to be master as normal. At this
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time it is a replica. Make
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sure this environment has a higher
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priority than all the other
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environments.
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</p>
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</li>
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<li>
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<p>
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Allow the new environment to run for a
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time as a replica. This allows
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it to obtain the most recent
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copies of the log files.
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</p>
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</li>
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<li>
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<p>
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Shut down the current master.
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This should force an election.
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Because the new environment has the
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highest priority, it will win
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the election, provided it has
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had enough time to obtain all
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the log records.
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</p>
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</li>
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<li>
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<p>
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Optionally restart the old
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master environment. Because there is
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currently a master environment, an
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election will not be held and
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the old master will now run as
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a replica environment.
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</p>
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</li>
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</ol>
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</div>
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</div>
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</div>
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<div class="navfooter">
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<hr />
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<td width="40%" align="left"><a accesskey="p" href="apioverview.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="introduction.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="permmessages.html">Next</a></td>
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<td width="40%" align="left" valign="top">The Replication APIs </td>
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<td width="20%" align="center">
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<a accesskey="h" href="index.html">Home</a>
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</td>
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<td width="40%" align="right" valign="top"> Permanent Message Handling</td>
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</tr>
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</table>
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</body>
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