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510 lines
19 KiB
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<?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 4. Replica versus Master Processes</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="heartbeats.html" title="Managing Heartbeats" />
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<link rel="next" href="processingloop.html" title="Processing Loop" />
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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 11.2.5.2</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 4. Replica versus Master Processes</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="heartbeats.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="processingloop.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="fwrkmasterreplica"></a>Chapter 4. Replica versus Master Processes</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="fwrkmasterreplica.html#determinestate">Determining State</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="processingloop.html">Processing Loop</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="exampledoloop.html">Example Processing Loop</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="exampledoloop.html#runningit">Running It</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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</dl>
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</div>
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<p>
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Every environment participating in a replicated application
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must know whether it is a <span class="emphasis"><em>master</em></span> or
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<span class="emphasis"><em>replica</em></span>. The reason for this is
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because, simply, the master can modify the database while
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replicas cannot. As a result, not only will you open
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databases differently depended on whether the environment is
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running as a master, but the environment will frequently
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behave quite a bit differently depending on whether it
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thinks it is operating as the read/write interface for
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your database.
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</p>
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<p>
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Moreover, an environment must also be capable of
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gracefully switching between master and replica states.
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This means that the environment must be able to detect when
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it has switched states.
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</p>
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<p>
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Not surprisingly, a large part of your application's code
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will be tied up in knowing which state a given
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environment is in and then in the logic of how to behave depending on
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its state.
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</p>
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<p>
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This chapter shows you how to determine your environment's
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state, and it then shows you some sample code on how
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an application might behave depending on whether it is a
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master or a replica in a replicated application.
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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="determinestate"></a>Determining State</h2>
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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 determine whether your code is running
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as a master or a replica, you must write your
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application as an implementation of
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<code class="classname">com.sleepycat.db.EventHandler</code>.
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This class gives you a series of methods within
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which you can detect and respond to various events
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that occur in your DB code. Some, but not all,
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of these methods have to do with elections:
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</p>
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<p>
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Some of the more commonly handled events are
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described below. For a complete list of events, see
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the
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<span>
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<code class="classname">com.sleepycat.db.EventHandler</code>
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javadoc page.
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</span>
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</p>
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<div class="itemizedlist">
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<ul type="disc">
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<li>
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<p>
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<code class="methodname">EventHandler.handlePanicEvent()</code>
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</p>
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<p>
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An error has occured in the Berkeley DB library requiring your
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application to shut down and then run recovery.
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</p>
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</li>
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<li>
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<p>
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<code class="methodname">EventHandler.handleRepClientEvent()</code>
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</p>
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<p>
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The local environment is now a replica.
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</p>
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</li>
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<li>
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<p>
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<code class="methodname">EventHandler.handleRepConnectBrokenEvent()</code>
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</p>
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<p>
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A previously established connection between
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two sites in the replication group has been
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broken.
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</p>
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</li>
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<li>
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<p>
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<code class="methodname">EventHandler.handleRepConnectEstablishedEvent()</code>
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</p>
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<p>
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A connection has been established between
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two sites in the replication group.
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</p>
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</li>
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<li>
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<p>
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<code class="methodname">EventHandler.handleRepConnectTryFailedEvent()</code>
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</p>
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<p>
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An attempt was made to establish a connection
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to a known remote site, but the connection
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attempt failed.
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</p>
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</li>
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<li>
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<p>
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<code class="methodname">EventHandler.handleRepDupmasterEvent()</code>
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</p>
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<p>
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A duplicate master has been discovered in the
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replication group.
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</p>
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</li>
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<li>
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<p>
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<code class="methodname">EventHandler.handleRepElectedEvent()</code>
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</p>
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<p>
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The local site has just won an election and
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is now the master. Your code should now
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reconfigure itself to operation as a master
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site.
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</p>
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</li>
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<li>
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<p>
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<code class="methodname">EventHandler.handleRepElectionFailedEvent()</code>
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</p>
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<p>
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The local site's attempt to initiate or
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participate in a replication master election
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failed, due to the lack of timely message
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response from a sufficient number of remote
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sites.
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</p>
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</li>
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<li>
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<p>
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<code class="methodname">EventHandler.handleRepJoinFailureEvent()</code>
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</p>
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<p>
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The local site could not synchronize with the
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master because an internal initialization was
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required, but internal initialization has
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been turned off
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</p>
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</li>
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<li>
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<p>
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<code class="methodname">EventHandler.handleRepLocalSiteRemovedEvent()</code>
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</p>
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<p>
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The local site has been removed from the group.
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</p>
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</li>
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<li>
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<p>
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<code class="methodname">EventHandler.handleRepNewMasterEvent()</code>
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</p>
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<p>
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An election was held and a new environment was made a
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master. However, the current environment <span class="emphasis"><em>is
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not</em></span> the master. This event exists so that
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you can cause your code to take some unique action in the
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event that the replication groups switches masters.
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</p>
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</li>
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<li>
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<p>
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<code class="methodname">EventHandler.handleRepMasterEvent()</code>
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</p>
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<p>
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The local environment is now a master.
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</p>
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</li>
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<li>
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<p>
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<code class="methodname">EventHandler.handleRepMasterFailureEvent()</code>
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</p>
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<p>
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The connection to the remote master replication site has failed.
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</p>
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</li>
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<li>
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<p>
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<code class="methodname">EventHandler.handleRepPermFailedEvent()</code>
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</p>
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<p>
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The Replication Manager did not receive enough acknowledgements to
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ensure the transaction's durability within the replicationg group.
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The Replication Manager has therefore flushed the transaction to
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the master's local disk for storage.
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</p>
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<p>
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How the Replication Manager knows whether the acknowledgements it
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has received is determined by the ack policy you have set for your
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applicaton. See <a class="xref" href="fwrkpermmessage.html#fmwrkpermpolicy" title="Identifying Permanent Message Policies">Identifying Permanent Message Policies</a>
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for more information.
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</p>
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</li>
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<li>
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<p>
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<code class="methodname">EventHandler.handleRepSiteAddedEvent()</code>
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</p>
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<p>
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A new site has joined the replication group.
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</p>
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</li>
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<li>
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<p>
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<code class="methodname">EventHandler.handleRepSiteRemovedEvent()</code>
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</p>
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<p>
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An existing site has been removed from the replication group.
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</p>
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</li>
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<li>
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<p>
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<code class="methodname">EventHandler.handleRepStartupDoneEvent()</code>
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</p>
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<p>
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The replica has completed startup
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synchronization and is now processing
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log records received from the master.
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</p>
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</li>
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<li>
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<p>
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<code class="methodname">EventHandler.handleWriteFailedEvent()</code>
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</p>
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<p>
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A Berkeley DB write to stable storage failed.
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</p>
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</li>
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</ul>
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</div>
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<p>
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Note that these events are raised whenever the
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state is established. That is, when the current
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environment becomes a replica, and that includes
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at application startup, the event is raised.
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Also, when an election is held and a replica is elected to be a
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master, then the event occurs.
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</p>
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<p>
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The <code class="classname">EventHandler</code>
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implementation is fairly simple. First you detect
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the event, and then you record the state change
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in some data member maintained in a location that
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is convenient to you.
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</p>
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<p>
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For example:
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</p>
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<pre class="programlisting">
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package db.repquote;
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// We make our main class an EventHandler implementation
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...
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import com.sleepycat.db.EventHandler;
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...
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public class MyReplicationClass implements EventHandler
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{
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...
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// Somewhere we provide a data member that is used to track
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// whether we are a master server. This could be in our main
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// class, or it could be part of a supporting class.
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private boolean isMaster;
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...
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isMaster = false;
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...
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// In the code where we open our environment and start replication,
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// we must identify the class that is the event handler. In this
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// example, we are performing this from within the class that
|
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|
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// implements com.sleepycat.db.EventHandler so we identify
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|
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// "this" class as the event handler
|
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|
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envConfig.setEventHandler(this); </pre>
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|
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<p>
|
|||
|
|
That done, we still need to implement the methods required for handling replication events.
|
|||
|
|
For a simple application like this one, these implementations can be trivial.
|
|||
|
|
</p>
|
|||
|
|
<pre class="programlisting"> public void handleRepClientEvent()
|
|||
|
|
{
|
|||
|
|
dbenv.setIsMaster(false);
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
public void handleRepConnectBrokenEvent()
|
|||
|
|
{
|
|||
|
|
// Ignored for now.
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
public void handleRepConnectEstablishedEvent()
|
|||
|
|
{
|
|||
|
|
// Ignored for now.
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
public void handleRepConnectTryFailedEvent()
|
|||
|
|
{
|
|||
|
|
// Ignored for now.
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
public void handleRepMasterEvent()
|
|||
|
|
{
|
|||
|
|
dbenv.setIsMaster(true);
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
public void handleRepNewMasterEvent(int envId)
|
|||
|
|
{
|
|||
|
|
// Ignored for now
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
public void handleWriteFailedEvent(int errorCode)
|
|||
|
|
{
|
|||
|
|
System.err.println("Write to stable storage failed!" +
|
|||
|
|
"Operating system error code:" + errorCode);
|
|||
|
|
System.err.println("Continuing....");
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
public void handleRepStartupDoneEvent()
|
|||
|
|
{
|
|||
|
|
System.out.println("Replication startup is completed.");
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
public void handleRepPermFailedEvent()
|
|||
|
|
{
|
|||
|
|
System.out.println("This application failed to receive enough" +
|
|||
|
|
"acks for a permanent message. The transaction is flushed" +
|
|||
|
|
"to disk on this master host.");
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
public void handleRepLocalSiteRemovedEvent()
|
|||
|
|
{
|
|||
|
|
// Ignored for now.
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
public void handleRepSiteAddedEvent()
|
|||
|
|
{
|
|||
|
|
// Ignored for now.
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
public void handleRepSiteRemovedEvent()
|
|||
|
|
{
|
|||
|
|
// Ignored for now.
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
public void handleRepElectedEvent()
|
|||
|
|
{
|
|||
|
|
// Safely ignored for Replication Manager applications.
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
public void handleRepElectionFailedEvent()
|
|||
|
|
{
|
|||
|
|
// Safely ignored for Replication Manager applications that do
|
|||
|
|
// not manage their own master selection.
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
public void handleRepJoinFailureEvent()
|
|||
|
|
{
|
|||
|
|
// Safely ignored since this application did not turn off AUTOINIT.
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
public void handleRepMasterFailureEvent()
|
|||
|
|
{
|
|||
|
|
// Safely ignored for Replication Manager applications that do
|
|||
|
|
// not manage their own master selection.
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
public void handleRepDupmasterEvent()
|
|||
|
|
{
|
|||
|
|
// Safely ignored for Replication Manager applications that do
|
|||
|
|
// not manage their own master selection.
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
public void handlePanicEvent()
|
|||
|
|
{
|
|||
|
|
System.err.println("Panic encountered!");
|
|||
|
|
System.err.println("Shutting down.");
|
|||
|
|
System.err.println("You should restart, running recovery.");
|
|||
|
|
try {
|
|||
|
|
terminate();
|
|||
|
|
} catch (DatabaseException dbe) {
|
|||
|
|
System.err.println("Caught an exception during " +
|
|||
|
|
"termination in handlePanicEvent: " + dbe.toString());
|
|||
|
|
}
|
|||
|
|
System.exit(-1);
|
|||
|
|
}</pre>
|
|||
|
|
<p>
|
|||
|
|
Of course, this only gives us the current state of the environment. We
|
|||
|
|
still need the code that determines what to do when the environment
|
|||
|
|
changes state and how to behave depending on the state (described
|
|||
|
|
in the next section).
|
|||
|
|
</p>
|
|||
|
|
</div>
|
|||
|
|
</div>
|
|||
|
|
<div class="navfooter">
|
|||
|
|
<hr />
|
|||
|
|
<table width="100%" summary="Navigation footer">
|
|||
|
|
<tr>
|
|||
|
|
<td width="40%" align="left"><a accesskey="p" href="heartbeats.html">Prev</a> </td>
|
|||
|
|
<td width="20%" align="center"> </td>
|
|||
|
|
<td width="40%" align="right"> <a accesskey="n" href="processingloop.html">Next</a></td>
|
|||
|
|
</tr>
|
|||
|
|
<tr>
|
|||
|
|
<td width="40%" align="left" valign="top">Managing Heartbeats </td>
|
|||
|
|
<td width="20%" align="center">
|
|||
|
|
<a accesskey="h" href="index.html">Home</a>
|
|||
|
|
</td>
|
|||
|
|
<td width="40%" align="right" valign="top"> Processing Loop</td>
|
|||
|
|
</tr>
|
|||
|
|
</table>
|
|||
|
|
</div>
|
|||
|
|
</body>
|
|||
|
|
</html>
|