Import BSDDB 4.7.25 (as of svn r89086)
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Zachary Ware
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docs/gsg_db_rep/JAVA/processingloop.html
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docs/gsg_db_rep/JAVA/processingloop.html
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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>Processing Loop</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 V1.62.4" />
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<link rel="home" href="index.html" title="Getting Started with Replicated Berkeley DB Applications" />
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<link rel="up" href="fwrkmasterreplica.html" title="Chapter 4. Replica versus Master Processes" />
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<link rel="previous" href="fwrkmasterreplica.html" title="Chapter 4. Replica versus Master Processes" />
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<link rel="next" href="exampledoloop.html" title="Example Processing Loop" />
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</head>
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<body>
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<div class="navheader">
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">Processing Loop</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="fwrkmasterreplica.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 4. Replica versus Master Processes</th>
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<td width="20%" align="right"> <a accesskey="n" href="exampledoloop.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="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="processingloop"></a>Processing Loop</h2>
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</div>
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</div>
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<div></div>
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</div>
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<p>
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Typically the central part of any replication application
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is some sort of a continuous loop that constantly
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checks the state of the environment (whether it is a
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replica or a master), opens and/or closes the
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databases as is necessary, and performs other useful
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work. A loop such as this one must of necessity
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take special care to know whether it is operating
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on a master or a replica environment because all of its
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activities are dependent upon that state.
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</p>
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<p>
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The flow of activities through the loop will
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generally be as follows:
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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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Check whether the environment has
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changed state. If it has, you
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might want to reopen your
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database handles, especially if
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you opened your replica's
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database handles as read-only.
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In this case, you might need to
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reopen them as read-write.
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However, if you always open your
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database handles as read-write,
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then it is not automatically necessary to
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reopen the databases due to a
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state change. Instead, you
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could check for a
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<span>
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<tt class="classname">ReplicationHandleDeadException</tt>
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exception
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</span>
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when you use your
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database handle(s). If you see
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this, then you need to reopen
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your database handle(s).
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</p>
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</li>
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<li>
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<p>
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If the databases are closed,
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create new database handles,
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configure the handle as is
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appropriate, and then open the
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databases. Note that handle
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configuration will be different,
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depending on whether the handle
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is opened as a replica or a
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master. At a minimum, the master
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should be opened with database
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creation privileges, whereas the
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replica does not need to be. You
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must also open the master such
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that its databases are
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read-write. You
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<span class="emphasis"><em>can</em></span> open
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replicas with read-only
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databases, so long as you are
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prepared to closed and the reopen
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the handle in the event the
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client becomes a master.
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</p>
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<p>
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Also, note that if the local
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environment
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is a replica, then it is possible
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that databases do not currently
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exist. In this case, the database
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open attempts will fail. Your
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code will have to take this
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corner case into account
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(described below).
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</p>
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</li>
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<li>
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<p>
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Once the databases are opened,
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check to see if the local
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environment is a
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master. If it is, do whatever it is
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a master should do for your
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application.
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</p>
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<p>
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Remember that the code for your
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master should include some way
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for you to tell the master
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to exit gracefully.
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</p>
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</li>
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<li>
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<p>
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If the local environment is not a
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master, then do whatever it is
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your replica environments should do.
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Again, like the code for your
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master environments, you should provide
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a way for your replicas to exit
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the processing loop gracefully.
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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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The following code fragment illustrates
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these points (note that we fill out this
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fragment with a working example
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next in this chapter):
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</p>
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<pre class="programlisting">// loop to manage replication activities
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public int doloop()
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{
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Database db = null;
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// Infinite loop. We exit depending on how the master and replica code
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// is written.
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for (;;) {
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/* If dbp is not opened, we need to open it. */
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if (db == null) {
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// Create the handle and then configure it. Before you open
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// it, you have to decide what open flags to use:
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DatabaseConfig dbconf = new DatabaseConfig();
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dbconf.setType(DatabaseType.BTREE);
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if (isMaster) {
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dbconf.setAllowCreate(true);
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}
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// Now you can open your database handle, passing to it the
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// optins selected above.
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try {
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db = dbenv.openDatabase
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(null, progname, null, dbconf);
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} catch(java.io.FileNotFoundException e) {
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// Put your error handling code here.
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}
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}
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// Now that the databases have been opened, continue with general
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// processing, depending on whether we are a master or a replica.
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if (isMaster) {
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// Do master stuff here. Don't forget to include a way to
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// gracefully exit the loop.
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} else {
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// Do replica stuff here. As is the case with the master
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// code, be sure to include a way to gracefully exit the
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// loop.
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}
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} </pre>
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</div>
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<div class="navfooter">
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<hr />
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<table width="100%" summary="Navigation footer">
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<tr>
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<td width="40%" align="left"><a accesskey="p" href="fwrkmasterreplica.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="fwrkmasterreplica.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="exampledoloop.html">Next</a></td>
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</tr>
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<tr>
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<td width="40%" align="left" valign="top">Chapter 4. Replica versus Master Processes </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"> Example Processing Loop</td>
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</tr>
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</table>
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</div>
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</body>
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</html>
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