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119 lines
5.7 KiB
HTML
119 lines
5.7 KiB
HTML
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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>A Note on System Failure</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 Berkeley DB Transaction Processing" />
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<link rel="up" href="introduction.html" title="Chapter 1. Introduction" />
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<link rel="prev" href="introduction.html" title="Chapter 1. Introduction" />
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<link rel="next" href="apireq.html" title="Application Requirements" />
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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">A Note on System Failure</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="introduction.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 1. Introduction</th>
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<td width="20%" align="right"> <a accesskey="n" href="apireq.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="sysfailure"></a>A Note on System Failure</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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From time to time this manual mentions that transactions
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protect your data against 'system or application failure.' This
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is true up to a certain extent. However, not all failures are
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created equal and no data protection mechanism can protect you
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against every conceivable way a computing system can find to
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die.
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</p>
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<p>
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Generally, when this book talks about protection against
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failures, it means that transactions offer protection against
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the likeliest culprits for system and application crashes. So
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long as your data modifications have been committed to disk,
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those modifications should persist even if your application or
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OS subsequently fails. And, even if the application or OS
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fails in the middle of a transaction commit (or abort), the
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data on disk should be either in a consistent state, or there
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should be enough data available to bring your databases into a
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consistent state (via a recovery procedure, for example). You
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may, however, lose whatever data you were committing at the
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time of the failure, but your databases will be otherwise
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unaffected.
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</p>
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<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
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<h3 class="title">Note</h3>
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<p>
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Be aware that many disks have a disk write cache and on
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some systems it is enabled by default. This means that
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a transaction can have committed, and to your
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application the data may appear to reside on disk, but
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the data may in fact reside only in the write cache at
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that time. This means that if the disk write cache is
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enabled and there is no battery backup for it, data can
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be lost after an OS crash even when maximum durability
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mode is in use. For maximum durability, disable the
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disk write cache or use a disk write cache with a
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battery backup.
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</p>
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</div>
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<p>
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Of course, if your <span class="emphasis"><em>disk</em></span> fails, then the transactional benefits described in this book
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are only as good as the backups you have taken.
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<span>
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By spreading your data and log files across separate disks,
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you can minimize the risk of data loss due to a disk
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failure, but even in this case it is possible to conjure a
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scenario where even this protection is insufficient (a fire
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in the machine room, for example) and you must go to your
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backups for protection.
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</span>
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</p>
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<p>
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Finally, by following the programming examples shown in this book, you can write your code so as to protect
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your data in the event that your code crashes. However, no programming API can protect you against logic
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failures in your own code; transactions cannot protect you from simply writing the wrong thing to your
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databases.
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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="introduction.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="introduction.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="apireq.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 1. Introduction </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"> Application Requirements</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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