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92 lines
4.5 KiB
HTML
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<title>Locking with transactions: two-phase locking</title>
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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_notxn.html" title="Locking without transactions" />
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<link rel="next" href="lock_cam_conv.html" title="Berkeley DB Concurrent Data Store locking conventions" />
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<body>
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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">Locking with transactions: two-phase locking</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_notxn.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="lock_cam_conv.html">Next</a></td>
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</tr>
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</table>
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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_twopl"></a>Locking with transactions: two-phase locking</h2>
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</div>
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</div>
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</div>
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<p>Berkeley DB uses a locking protocol called <span class="emphasis"><em>two-phase locking (2PL)</em></span>. This
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is the traditional protocol used in conjunction with lock-based transaction
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systems.</p>
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<p>In a two-phase locking system, transactions are divided into two
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distinct phases. During the first phase, the transaction only acquires
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locks; during the second phase, the transaction only releases locks.
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More formally, once a transaction releases a lock, it may not acquire
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any additional locks. Practically, this translates into a system in
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which locks are acquired as they are needed throughout a transaction
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and retained until the transaction ends, either by committing or
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aborting. In Berkeley DB, locks are released during <a href="../api_reference/C/txnabort.html" class="olink">DB_TXN->abort()</a> or
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<a href="../api_reference/C/txncommit.html" class="olink">DB_TXN->commit()</a>. The only exception to this protocol occurs when we
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use lock-coupling to traverse a data structure. If the locks are held
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only for traversal purposes, it is safe to release locks before
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transactions commit or abort.</p>
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<p>For applications, the implications of 2PL are that long-running
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transactions will hold locks for a long time. When designing
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applications, lock contention should be considered. In order to reduce
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the probability of deadlock and achieve the best level of concurrency
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possible, the following guidelines are helpful.</p>
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<div class="orderedlist">
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<ol type="1">
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<li>When accessing multiple databases, design all transactions so that they
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access the files in the same order.</li>
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<li>If possible, access your most hotly contested resources last (so that
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their locks are held for the shortest time possible).</li>
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<li>If possible, use nested transactions to protect the parts of your
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transaction most likely to deadlock.</li>
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</ol>
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</div>
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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_notxn.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="lock_cam_conv.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">Locking without transactions </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"> Berkeley DB Concurrent Data Store locking conventions</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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