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92 lines
4.9 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 16. 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 17. The Logging Subsystem" />
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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 11.2.5.2</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 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 16.
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The Locking Subsystem
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</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 applications</h2>
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</div>
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</div>
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</div>
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<p>The Lock subsystem is useful outside the context of Berkeley DB. It can be
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used to manage concurrent access to any collection of either ephemeral
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or persistent objects. That is, the lock region can persist across
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invocations of an application, so it can be used to provide long-term
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locking (for example, conference room scheduling).</p>
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<p>In order to use the locking subsystem in such a general way, the
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applications must adhere to a convention for identifying objects and
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lockers. Consider a conference room scheduling problem, in which there
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are three conference rooms scheduled in half-hour intervals. The
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scheduling application must then select a way to identify each
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conference room/time slot combination. In this case, we could describe
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the objects being locked as bytestrings consisting of the conference
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room name, the date when it is needed, and the beginning of the
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appropriate half-hour slot.</p>
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<p>Lockers are 32-bit numbers, so we might choose to use the User ID of
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the individual running the scheduling program. To schedule half-hour
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slots, all the application needs to do is issue a <a href="../api_reference/C/lockget.html" class="olink">DB_ENV->lock_get()</a> call
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for the appropriate locker/object pair. To schedule a longer slot, the
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application needs to issue a <a href="../api_reference/C/lockvec.html" class="olink">DB_ENV->lock_vec()</a> call, with one
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<a href="../api_reference/C/lockget.html" class="olink">DB_ENV->lock_get()</a> operation per half-hour — up to the total length. If
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the <a href="../api_reference/C/lockvec.html" class="olink">DB_ENV->lock_vec()</a> call fails, the application would have to release
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the parts of the time slot that were obtained.</p>
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<p>To cancel a reservation, the application would make the appropriate
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<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
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a 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
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be post-processed into a human-readable schedule of conference room
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use.</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 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 17.
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The Logging Subsystem
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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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