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HTML
<?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>Application configuration</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="Berkeley DB Programmer's Reference Guide" />
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<link rel="up" href="apprec.html" title="Chapter 14. Application Specific Logging and Recovery" />
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<link rel="prev" href="apprec_auto.html" title="Automatically generated functions" />
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<link rel="next" href="program.html" title="Chapter 15. Programmer Notes" />
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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 11.2.5.3</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">Application configuration</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="apprec_auto.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 14.
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Application Specific Logging and Recovery
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</th>
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<td width="20%" align="right"> <a accesskey="n" href="program.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="apprec_config"></a>Application configuration</h2>
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</div>
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</div>
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</div>
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<p>The application should include a dispatch function that dispatches to
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appropriate printing and/or recovery functions based on the log record
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type and the operation code. The dispatch function should take the same
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arguments as the recovery function, and should call the appropriate
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recovery and/or printing functions based on the log record type and the
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operation code. For example, the ex_apprec dispatch function is as
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follows:</p>
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<pre class="programlisting">int
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apprec_dispatch(dbenv, dbt, lsn, op)
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DB_ENV *dbenv;
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DBT *dbt;
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DB_LSN *lsn;
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db_recops op;
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{
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u_int32_t rectype;
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/* Pull the record type out of the log record. */
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memcpy(&rectype, dbt->data, sizeof(rectype));
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switch (rectype) {
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case DB_ex_apprec_mkdir:
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return (ex_apprec_mkdir_recover(dbenv, dbt, lsn, op));
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default:
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/*
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* We've hit an unexpected, allegedly user-defined record
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* type.
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*/
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dbenv->errx(dbenv, "Unexpected log record type encountered");
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return (EINVAL);
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}
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}</pre>
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<p>Applications use this dispatch function and the automatically generated
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functions as follows:</p>
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<div class="orderedlist">
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<ol type="1">
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<li>When the application starts, call the <a href="../api_reference/C/envset_app_dispatch.html" class="olink">DB_ENV->set_app_dispatch()</a>
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with your dispatch function.</li>
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<li>Issue a <a href="../api_reference/C/txnbegin.html" class="olink">DB_ENV->txn_begin()</a> call before any operations you want to be
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transaction-protected.</li>
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<li>Before accessing any data, issue the appropriate lock call to lock the
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data (either for reading or writing).</li>
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<li>Before modifying any data that is transaction-protected, issue a call
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to the appropriate log function.</li>
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<li>Call <a href="../api_reference/C/txncommit.html" class="olink">DB_TXN->commit()</a>
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to cancel all of the modifications.</li>
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</ol>
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</div>
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<p>The recovery functions are called in the three following cases:</p>
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<div class="orderedlist">
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<ol type="1">
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<li>During recovery after application or system failure, with op set to
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<a href="../api_reference/C/envset_app_dispatch.html#set_app_dispatch_DB_TXN_FORWARD_ROLL" class="olink">DB_TXN_FORWARD_ROLL</a> or <a href="../api_reference/C/envset_app_dispatch.html#set_app_dispatch_DB_TXN_BACKWARD_ROLL" class="olink">DB_TXN_BACKWARD_ROLL</a>.</li>
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<li>During transaction abort, with op set to <a href="../api_reference/C/envset_app_dispatch.html#set_app_dispatch_DB_TXN_ABORT" class="olink">DB_TXN_ABORT</a>.</li>
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<li>On a replicated client to apply updates from the master, with op set to
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<a href="../api_reference/C/envset_app_dispatch.html#set_app_dispatch_DB_TXN_APPLY" class="olink">DB_TXN_APPLY</a>.</li>
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</ol>
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</div>
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<p>For each log record type you declare, you must write the appropriate
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function to undo and redo the modifications. The shell of these
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functions will be generated for you automatically, but you must fill in
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the details.</p>
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<p>Your code must be able to detect whether the described modifications
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have been applied to the data. The function will be called with the
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"op" parameter set to <a href="../api_reference/C/envset_app_dispatch.html#set_app_dispatch_DB_TXN_ABORT" class="olink">DB_TXN_ABORT</a> when a transaction that wrote
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the log record aborts, with <a href="../api_reference/C/envset_app_dispatch.html#set_app_dispatch_DB_TXN_FORWARD_ROLL" class="olink">DB_TXN_FORWARD_ROLL</a> and
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<a href="../api_reference/C/envset_app_dispatch.html#set_app_dispatch_DB_TXN_BACKWARD_ROLL" class="olink">DB_TXN_BACKWARD_ROLL</a> during recovery, and with <a href="../api_reference/C/envset_app_dispatch.html#set_app_dispatch_DB_TXN_APPLY" class="olink">DB_TXN_APPLY</a>
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on a replicated client.</p>
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<p>The actions for <a href="../api_reference/C/envset_app_dispatch.html#set_app_dispatch_DB_TXN_ABORT" class="olink">DB_TXN_ABORT</a> and <a href="../api_reference/C/envset_app_dispatch.html#set_app_dispatch_DB_TXN_BACKWARD_ROLL" class="olink">DB_TXN_BACKWARD_ROLL</a>
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should generally be the same, and the actions for
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<a href="../api_reference/C/envset_app_dispatch.html#set_app_dispatch_DB_TXN_FORWARD_ROLL" class="olink">DB_TXN_FORWARD_ROLL</a> and <a href="../api_reference/C/envset_app_dispatch.html#set_app_dispatch_DB_TXN_APPLY" class="olink">DB_TXN_APPLY</a> should generally
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be the same. However, if the application is using Berkeley DB replication
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and another thread of control may be performing read operations while
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log records are applied on a replication client, the recovery function
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should perform appropriate locking during <a href="../api_reference/C/envset_app_dispatch.html#set_app_dispatch_DB_TXN_APPLY" class="olink">DB_TXN_APPLY</a>
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operations. In this case, the recovery function may encounter deadlocks
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when issuing locking calls. The application should run with the
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deadlock detector, and the recovery function should simply return
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<a class="link" href="program_errorret.html#program_errorret.DB_LOCK_DEADLOCK">DB_LOCK_DEADLOCK</a> if a deadlock is detected and a locking
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operation fails with that error.</p>
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<p>The <a href="../api_reference/C/envset_app_dispatch.html#set_app_dispatch_DB_TXN_PRINT" class="olink">DB_TXN_PRINT</a> operation should print the log record,
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typically using the auto-generated print function; it is not used in
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the Berkeley DB library, but may be useful for debugging, as in the
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<a href="../api_reference/C/db_printlog.html" class="olink">db_printlog</a> utility. Applications may safely ignore this
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operation code, they may handle printing from the recovery function, or
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they may dispatch directly to the auto-generated print function.</p>
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<p>One common way to determine whether operations need to be undone or
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redone is the use of log sequence numbers (LSNs). For example, each
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access method database page contains the LSN of the most recent log
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record that describes a modification to the page. When the access
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method changes a page, it writes a log record describing the change and
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including the LSN that was on the page before the change. This LSN is
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referred to as the previous LSN. The recovery functions read the page
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described by a log record, and compare the LSN on the page to the LSN
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they were passed.</p>
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<p>If the page LSN is less than the passed LSN and the operation is an
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undo, no action is necessary (because the modifications have not been
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written to the page). If the page LSN is the same as the previous LSN
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and the operation is a redo, the actions described are reapplied to the
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page. If the page LSN is equal to the passed LSN and the operation is
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an undo, the actions are removed from the page; if the page LSN is
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greater than the passed LSN and the operation is a redo, no further
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action is necessary. If the action is a redo and the LSN on the page
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is less than the previous LSN in the log record, it is an error because
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it could happen only if some previous log record was not processed.</p>
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<p>Examples of other recovery functions can be found in the Berkeley DB library
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recovery functions (found in files named XXX_rec.c) and in the
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application-specific recovery example (specifically, ex_apprec_rec.c).</p>
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<p>Finally, applications need to ensure that any data modifications they
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have made, that were part of a committed transaction, must be written
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to stable storage before calling the <a href="../api_reference/C/txncheckpoint.html" class="olink">DB_ENV->txn_checkpoint()</a> method. This is
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to allow the periodic removal of database environment log files.</p>
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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="apprec_auto.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="apprec.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="program.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">Automatically generated functions </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"> Chapter 15.
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Programmer Notes
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</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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