mirror of
https://github.com/berkeleydb/libdb.git
synced 2024-11-16 17:16:25 +00:00
92 lines
4.5 KiB
HTML
92 lines
4.5 KiB
HTML
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
|
||
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
|
||
<html xmlns="http://www.w3.org/1999/xhtml">
|
||
<head>
|
||
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
|
||
<title>Locking with transactions: two-phase locking</title>
|
||
<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
|
||
<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
|
||
<link rel="start" href="index.html" title="Berkeley DB Programmer's Reference Guide" />
|
||
<link rel="up" href="lock.html" title="Chapter 16. The Locking Subsystem" />
|
||
<link rel="prev" href="lock_notxn.html" title="Locking without transactions" />
|
||
<link rel="next" href="lock_cam_conv.html" title="Berkeley DB Concurrent Data Store locking conventions" />
|
||
</head>
|
||
<body>
|
||
<div xmlns="" class="navheader">
|
||
<div class="libver">
|
||
<p>Library Version 11.2.5.2</p>
|
||
</div>
|
||
<table width="100%" summary="Navigation header">
|
||
<tr>
|
||
<th colspan="3" align="center">Locking with transactions: two-phase locking</th>
|
||
</tr>
|
||
<tr>
|
||
<td width="20%" align="left"><a accesskey="p" href="lock_notxn.html">Prev</a> </td>
|
||
<th width="60%" align="center">Chapter 16.
|
||
The Locking Subsystem
|
||
</th>
|
||
<td width="20%" align="right"> <a accesskey="n" href="lock_cam_conv.html">Next</a></td>
|
||
</tr>
|
||
</table>
|
||
<hr />
|
||
</div>
|
||
<div class="sect1" lang="en" xml:lang="en">
|
||
<div class="titlepage">
|
||
<div>
|
||
<div>
|
||
<h2 class="title" style="clear: both"><a id="lock_twopl"></a>Locking with transactions: two-phase locking</h2>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
<p>Berkeley DB uses a locking protocol called <span class="emphasis"><em>two-phase locking (2PL)</em></span>. This
|
||
is the traditional protocol used in conjunction with lock-based transaction
|
||
systems.</p>
|
||
<p>In a two-phase locking system, transactions are divided into two
|
||
distinct phases. During the first phase, the transaction only acquires
|
||
locks; during the second phase, the transaction only releases locks.
|
||
More formally, once a transaction releases a lock, it may not acquire
|
||
any additional locks. Practically, this translates into a system in
|
||
which locks are acquired as they are needed throughout a transaction
|
||
and retained until the transaction ends, either by committing or
|
||
aborting. In Berkeley DB, locks are released during <a href="../api_reference/C/txnabort.html" class="olink">DB_TXN->abort()</a> or
|
||
<a href="../api_reference/C/txncommit.html" class="olink">DB_TXN->commit()</a>. The only exception to this protocol occurs when we
|
||
use lock-coupling to traverse a data structure. If the locks are held
|
||
only for traversal purposes, it is safe to release locks before
|
||
transactions commit or abort.</p>
|
||
<p>For applications, the implications of 2PL are that long-running
|
||
transactions will hold locks for a long time. When designing
|
||
applications, lock contention should be considered. In order to reduce
|
||
the probability of deadlock and achieve the best level of concurrency
|
||
possible, the following guidelines are helpful.</p>
|
||
<div class="orderedlist">
|
||
<ol type="1">
|
||
<li>When accessing multiple databases, design all transactions so that they
|
||
access the files in the same order.</li>
|
||
<li>If possible, access your most hotly contested resources last (so that
|
||
their locks are held for the shortest time possible).</li>
|
||
<li>If possible, use nested transactions to protect the parts of your
|
||
transaction most likely to deadlock.</li>
|
||
</ol>
|
||
</div>
|
||
</div>
|
||
<div class="navfooter">
|
||
<hr />
|
||
<table width="100%" summary="Navigation footer">
|
||
<tr>
|
||
<td width="40%" align="left"><a accesskey="p" href="lock_notxn.html">Prev</a> </td>
|
||
<td width="20%" align="center">
|
||
<a accesskey="u" href="lock.html">Up</a>
|
||
</td>
|
||
<td width="40%" align="right"> <a accesskey="n" href="lock_cam_conv.html">Next</a></td>
|
||
</tr>
|
||
<tr>
|
||
<td width="40%" align="left" valign="top">Locking without transactions </td>
|
||
<td width="20%" align="center">
|
||
<a accesskey="h" href="index.html">Home</a>
|
||
</td>
|
||
<td width="40%" align="right" valign="top"> Berkeley DB Concurrent Data Store locking conventions</td>
|
||
</tr>
|
||
</table>
|
||
</div>
|
||
</body>
|
||
</html>
|