mirror of
https://github.com/yasuhirokimura/db18
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Yasuhiro KIMURA
224 lines
11 KiB
HTML
224 lines
11 KiB
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 15. 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 16. 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 18.1.40</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 15. Application Specific Logging and
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Recovery </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>
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The application should include a dispatch function that
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dispatches to appropriate printing and/or recovery functions
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based on the log record type and the operation code. The
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dispatch function should take the same arguments as the
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recovery function, and should call the appropriate recovery
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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
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is as follows:
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</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>
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Applications use this dispatch function and the
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automatically generated functions 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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When the application starts, call the
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<a href="../api_reference/C/envset_app_dispatch.html" class="olink">DB_ENV->set_app_dispatch()</a> with your dispatch
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function.
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</li>
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<li>
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Issue a <a href="../api_reference/C/txnbegin.html" class="olink">DB_ENV->txn_begin()</a> call before any operations you
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want to be transaction-protected.
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</li>
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<li>
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Before accessing any data, issue the appropriate
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lock call to lock the data (either for reading or
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writing).
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</li>
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<li>
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Before modifying any data that is
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transaction-protected, issue a call to the appropriate log
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function.
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</li>
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<li>
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Call <a href="../api_reference/C/txncommit.html" class="olink">DB_TXN->commit()</a> to cancel all of the
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modifications.
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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 recovery functions are called in the three following
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cases:
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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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During recovery after application or system failure,
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with op set to <a href="../api_reference/C/envset_app_dispatch.html#set_app_dispatch_DB_TXN_FORWARD_ROLL" class="olink">DB_TXN_FORWARD_ROLL</a> or
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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>.
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</li>
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<li>
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During transaction abort, with op set to
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<a href="../api_reference/C/envset_app_dispatch.html#set_app_dispatch_DB_TXN_ABORT" class="olink">DB_TXN_ABORT</a>.
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</li>
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<li>
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On a replicated client to apply updates from the
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master, with op set to <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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</li>
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</ol>
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</div>
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<p>
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For each log record type you declare, you must write the
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appropriate function to undo and redo the modifications. The
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shell of these functions will be generated for you
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automatically, but you must fill in the details.
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</p>
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<p>
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Your code must be able to detect whether the described
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modifications have been applied to the data. The function will
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be called with the "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
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transaction that wrote the log record aborts, with
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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_BACKWARD_ROLL" class="olink">DB_TXN_BACKWARD_ROLL</a> during
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recovery, and with <a href="../api_reference/C/envset_app_dispatch.html#set_app_dispatch_DB_TXN_APPLY" class="olink">DB_TXN_APPLY</a> on a replicated
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client.
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</p>
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<p>
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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 be
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the same. However, if the application is using Berkeley DB
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replication and another thread of control may be performing
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read operations while log records are applied on a replication
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client, the recovery function should perform appropriate
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locking during <a href="../api_reference/C/envset_app_dispatch.html#set_app_dispatch_DB_TXN_APPLY" class="olink">DB_TXN_APPLY</a> operations. In this case, the
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recovery function may encounter deadlocks when issuing locking
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calls. The application should run with the deadlock detector,
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and the recovery function should simply return <a class="link" href="program_errorret.html#program_errorret.DB_LOCK_DEADLOCK">DB_LOCK_DEADLOCK</a>
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if a deadlock is detected and a locking operation fails with that error.
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</p>
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<p>
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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
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used in the Berkeley DB library, but may be useful for
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debugging, as in the <a href="../api_reference/C/db_printlog.html" class="olink">db_printlog</a> utility. Applications may safely
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ignore this operation code, they may handle printing from the
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recovery function, or they may dispatch directly to the
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auto-generated print function.
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</p>
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<p>
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One common way to determine whether operations need to be
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undone or redone is the use of log sequence numbers (LSNs).
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For example, each access method database page contains the LSN
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of the most recent log record that describes a modification to
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the page. When the access method changes a page, it writes a
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log record describing the change and including the LSN that
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was on the page before the change. This LSN is referred to as
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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
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the LSN they were passed.
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</p>
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<p>
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If the page LSN is less than the passed LSN and the
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operation is an undo, no action is necessary (because the
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modifications have not been written to the page). If the page
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LSN is the same as the previous LSN and the operation is a
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redo, the actions described are reapplied to the page. If the
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page LSN is equal to the passed LSN and the operation is an
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undo, the actions are removed from the page; if the page LSN
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is greater than the passed LSN and the operation is a redo, no
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further action is necessary. If the action is a redo and the
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LSN on the page is less than the previous LSN in the log
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record, it is an error because it could happen only if some
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previous log record was not processed.
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</p>
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<p>
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Examples of other recovery functions can be found in the
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Berkeley DB library recovery functions (found in files named
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XXX_rec.c) and in the application-specific recovery example
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(specifically, ex_apprec_rec.c).
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</p>
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<p>
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Finally, applications need to ensure that any data
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modifications they have made, that were part of a committed
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transaction, must be written to stable storage before calling
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the <a href="../api_reference/C/txncheckpoint.html" class="olink">DB_ENV->txn_checkpoint()</a> method. This is to allow the periodic
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removal of database environment log files.
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</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
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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 16. Programmer Notes </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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