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420 lines
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420 lines
18 KiB
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<title>Chapter 1. Introduction</title>
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<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
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<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
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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="index.html" title="Getting Started with Replicated Berkeley DB Applications" />
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<link rel="prev" href="moreinfo.html" title="For More Information" />
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<link rel="next" href="repadvantage.html" title="Replication Benefits" />
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</head>
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<body>
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<div xmlns="" class="navheader">
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<div class="libver">
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<p>Library Version 12.1.6.2</p>
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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">Chapter 1. Introduction</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="moreinfo.html">Prev</a> </td>
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<th width="60%" align="center"> </th>
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<td width="20%" align="right"> <a accesskey="n" href="repadvantage.html">Next</a></td>
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</tr>
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</table>
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<hr />
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</div>
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<div class="chapter" 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"><a id="introduction"></a>Chapter 1. Introduction</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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<p>
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<b>Table of Contents</b>
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</p>
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<dl>
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<dt>
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<span class="sect1">
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<a href="introduction.html#overview">Overview</a>
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</span>
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</dt>
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<dd>
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<dl>
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<dt>
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<span class="sect2">
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<a href="introduction.html#repenvirons">Replication Environments</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="introduction.html#repdbs">Replication Databases</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="introduction.html#commlayer">Communications Layer</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="introduction.html#masterselect">Selecting a Master</a>
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</span>
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</dt>
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</dl>
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</dd>
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<dt>
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<span class="sect1">
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<a href="repadvantage.html">Replication Benefits</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="apioverview.html">Replication Mechanisms</a>
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</span>
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</dt>
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<dd>
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<dl>
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<dt>
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<span class="sect2">
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<a href="apioverview.html#repframeworkoverview">Replication Manager Overview</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="apioverview.html#writefor_overview">Write Forwarding Overview</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="apioverview.html#repapioverview">Replication Base API Overview</a>
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</span>
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</dt>
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</dl>
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</dd>
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<dt>
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<span class="sect1">
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<a href="elections.html">Holding Elections</a>
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</span>
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</dt>
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<dd>
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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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</dd>
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<dt>
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<span class="sect1">
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<a href="permmessages.html">Permanent Message Handling</a>
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</span>
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</dt>
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<dd>
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<dl>
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<dt>
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<span class="sect2">
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<a href="permmessages.html#permmessagenot">When Not to Manage
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Permanent Messages</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="permmessages.html#permmanage">Managing Permanent Messages</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="permmessages.html#permimplement">Implementing Permanent
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Message Handling</a>
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</span>
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</dt>
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</dl>
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</dd>
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<dt>
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<span class="sect1">
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<a href="eventhandling.html">Event Handling</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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This book provides a thorough introduction and discussion on
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replication as used with Berkeley DB (DB). It begins by offering a
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general overview to replication and the benefits it provides. It also
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describes the APIs that you use to implement replication, and it
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describes architecturally the things that you need to do to your
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application code in order to use the replication APIs. Finally, it
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discusses the differences in backup and restore strategies that you
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might pursue when using replication, especially where it comes to log
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file removal.
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</p>
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<p>
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You should understand the concepts from the
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<span>
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<em class="citetitle">Berkeley DB Getting Started with Transaction Processing</em>
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</span>
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guide before reading this book.
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</p>
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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="overview"></a>Overview</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="introduction.html#repenvirons">Replication Environments</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="introduction.html#repdbs">Replication Databases</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="introduction.html#commlayer">Communications Layer</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="introduction.html#masterselect">Selecting a Master</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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The DB replication APIs allow you to distribute your database
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write operations (performed on a read-write master) to one or
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more read-only <span class="emphasis"><em>replicas</em></span>.
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For this reason, DB's replication implementation is said to be a
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<span class="emphasis"><em>single master, multiple replica</em></span> replication strategy.
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</p>
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<p>
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Note that your database write operations can occur only on the
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master; any attempt to write to a replica results in an error
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being
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<span>raised by</span>
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the DB API used to perform the write.
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</p>
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<p>
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A single replication master and all of its replicas are referred
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to as a <span class="emphasis"><em>replication group</em></span>. While all
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members of the replication group can reside on the same
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machine, usually each replication participant is placed on a
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separate physical machine somewhere on the network.
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</p>
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<p>
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Note that all replication applications must first be
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transactional applications. The data that the master transmits
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to its replicas are log records that are generated as records are
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updated. Upon transactional commit, the master transmits a
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transaction record which tells the replicas to commit the
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records they previously received from the master. In order for
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all of this to work, your replicated application must also be a
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transactional application. For this reason, it is
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recommended that you write and debug your DB application as
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a stand-alone transactional application before introducing the
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replication layer to your code.
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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="repenvirons"></a>Replication Environments</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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The most important requirement for a replication
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participant is that it must use a unique Berkeley DB database
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environment independent of all other replication
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participants. So while multiple replication participants
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can reside on the same physical machine, no two such participants
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can share the same environment home directory.
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</p>
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<p>
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For this reason, technically replication occurs between
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unique <span class="emphasis"><em>database environments</em></span>. So in the strictest sense,
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a replication group consists of a <span class="emphasis"><em>master
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environment</em></span> and
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one or more <span class="emphasis"><em>replica environments</em></span>. However, the reality
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is that for production code, each such environment will
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usually be located on its own unique machine. Consequently,
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this manual sometimes talks about <span class="emphasis"><em>replication sites</em></span>, meaning the
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unique combination of environment home directory, host and port that a specific
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replication application is using.
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</p>
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<p>
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There is no DB-specified limit to the number of
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environments which can participate in a replication group.
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The only limitation here is one of resources —
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network bandwidth, for example.
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</p>
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<p>
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(Note, however, that the Replication Manager does place a limit on the
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number of environments you can use. See
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<a class="xref" href="apioverview.html#repframeworkoverview" title="Replication Manager Overview">Replication Manager Overview</a>
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for details.)
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</p>
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<p>
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Also, DB's replication implementation requires all
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participating environments to be assigned IDs that are
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locally unique to the given environment. Depending on the
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replication APIs that you choose to use, you may or may not
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need to manage this particular detail.
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</p>
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<p>
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For detailed information on database environments, see
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the <em class="citetitle">Berkeley DB Getting Started with Transaction Processing</em>
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guide. For more information on environment IDs, see
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the <em class="citetitle">Berkeley DB Programmer's Reference Guide</em>.
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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="repdbs"></a>Replication Databases</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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DB's databases are managed and used in exactly the same way
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as if you were writing a non-replicated application, with
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a couple of caveats. First, the databases maintained in a replicated environment
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must reside either in the <code class="literal">ENV_HOME</code>
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directory, or in the directory identified by the
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<code class="methodname">DbEnv::add_data_dir()</code>
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method. Unlike non-replication applications, you cannot place your
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databases in a subdirectory below these locations. You should
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also not use full path names for your databases or
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environments as these are likely to break when they are replicated
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to other machines.
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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="commlayer"></a>Communications Layer</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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In order to transmit database writes to the replication
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replicas, DB requires a communications layer.
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DB is agnostic as to what this layer should
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look like. The only requirement is that it
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be capable of passing two opaque data objects and an
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environment ID from the master to its replicas without
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corruption.
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</p>
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<p>
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Because replicas are usually placed on different machines on
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the network, the communications layer is usually some kind
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of a network-aware implementation. Beyond that, its
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implementation details are largely up to you. It could use
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TCP/IP sockets, for example, or it could use
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raw sockets if they perform better for your particular
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application.
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</p>
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<p>
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Note that you may not have to write your own communications
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layer. DB provides a Replication Manager that
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includes a fully-functional TCP/IP-based communications layer,
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<span>which uses IPv6 whenever possible but also supports IPv4.</span>
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See <a class="xref" href="apioverview.html" title="Replication Mechanisms">Replication Mechanisms</a>
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for more information.
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</p>
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<p>
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See the <em class="citetitle">Berkeley DB Programmer's Reference Guide</em>
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for a description of how to
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write your own custom replication communications layer.
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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="masterselect"></a>Selecting a Master</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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Every replication group is allowed one and only one
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master environment. Usually masters are selected by
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holding an <span class="emphasis"><em>election</em></span>, although it
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is possible to turn elections off and manually select
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masters (this is not recommended for most replicated
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applications).
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</p>
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<p>
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When elections are being used, they are performed by the
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underlying Berkeley DB replication code so you have to
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do very little to implement them.
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</p>
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<p>
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When holding an election, replicas "vote" on who should
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be the master. Among replicas participating in the
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election, the one with the most up-to-date set of log
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records will win the election. Note that it's possible
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for there to be a tie. When this occurs, priorities are
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used to select the master. See
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<a class="xref" href="elections.html" title="Holding Elections">Holding Elections</a>
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for details.
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</p>
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<p>
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For more information on holding and managing elections,
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see <a class="xref" href="elections.html" title="Holding Elections">Holding Elections</a>.
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</p>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
<div class="navfooter">
|
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<hr />
|
|
<table width="100%" summary="Navigation footer">
|
|
<tr>
|
|
<td width="40%" align="left"><a accesskey="p" href="moreinfo.html">Prev</a> </td>
|
|
<td width="20%" align="center"> </td>
|
|
<td width="40%" align="right"> <a accesskey="n" href="repadvantage.html">Next</a></td>
|
|
</tr>
|
|
<tr>
|
|
<td width="40%" align="left" valign="top">For More Information </td>
|
|
<td width="20%" align="center">
|
|
<a accesskey="h" href="index.html">Home</a>
|
|
</td>
|
|
<td width="40%" align="right" valign="top"> Replication Benefits</td>
|
|
</tr>
|
|
</table>
|
|
</div>
|
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</body>
|
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</html>
|
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