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Yasuhiro KIMURA
103 lines
5.2 KiB
HTML
103 lines
5.2 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>Locking and non-Berkeley DB applications</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="lock.html" title="Chapter 17. The Locking Subsystem" />
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<link rel="prev" href="lock_am_conv.html" title="Berkeley DB Transactional Data Store locking conventions" />
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<link rel="next" href="log.html" title="Chapter 18. The Logging Subsystem" />
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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">Locking and non-Berkeley DB
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applications</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="lock_am_conv.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 17. The Locking Subsystem </th>
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<td width="20%" align="right"> <a accesskey="n" href="log.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="lock_nondb"></a>Locking and non-Berkeley DB
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applications</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 Lock subsystem is useful outside the context of Berkeley
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DB. It can be used to manage concurrent access to any
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collection of either ephemeral or persistent objects. That is,
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the lock region can persist across invocations of an
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application, so it can be used to provide long-term locking
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(for example, conference room scheduling).
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</p>
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<p>
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In order to use the locking subsystem in such a general way,
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the applications must adhere to a convention for identifying
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objects and lockers. Consider a conference room scheduling
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problem, in which there are three conference rooms scheduled
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in half-hour intervals. The scheduling application must then
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select a way to identify each conference room/time slot
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combination. In this case, we could describe the objects being
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locked as bytestrings consisting of the conference room name,
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the date when it is needed, and the beginning of the
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appropriate half-hour slot.
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</p>
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<p>
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Lockers are 32-bit numbers, so we might choose to use the
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User ID of the individual running the scheduling program. To
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schedule half-hour slots, all the application needs to do is
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issue a <a href="../api_reference/C/lockget.html" class="olink">DB_ENV->lock_get()</a> call for the appropriate locker/object pair.
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To schedule a longer slot, the application needs to issue a
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<a href="../api_reference/C/lockvec.html" class="olink">DB_ENV->lock_vec()</a> call, with one <a href="../api_reference/C/lockget.html" class="olink">DB_ENV->lock_get()</a> operation per half-hour
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— up to the total length. If the <a href="../api_reference/C/lockvec.html" class="olink">DB_ENV->lock_vec()</a> call fails,
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the application would have to release the parts of the time
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slot that were obtained.
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</p>
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<p>
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To cancel a reservation, the application would make the
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appropriate <a href="../api_reference/C/lockput.html" class="olink">DB_ENV->lock_put()</a> calls. To reschedule a reservation, the
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<a href="../api_reference/C/lockget.html" class="olink">DB_ENV->lock_get()</a> and <a href="../api_reference/C/lockput.html" class="olink">DB_ENV->lock_put()</a> calls could all be made inside of a
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single <a href="../api_reference/C/lockvec.html" class="olink">DB_ENV->lock_vec()</a> call. The output of <a href="../api_reference/C/lockstat.html" class="olink">DB_ENV->lock_stat()</a> could be
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post-processed into a human-readable schedule of conference
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room use.
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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="lock_am_conv.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="lock.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="log.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">Berkeley DB Transactional Data
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Store locking conventions </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 18. The Logging Subsystem </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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