mirror of
https://github.com/yasuhirokimura/db18
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Yasuhiro KIMURA
130 lines
6.6 KiB
HTML
130 lines
6.6 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>Deadlock detection using timers</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_dead.html" title="Deadlock detection" />
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<link rel="next" href="lock_deaddbg.html" title="Deadlock debugging" />
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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">Deadlock detection using
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timers</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_dead.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="lock_deaddbg.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_timeout"></a>Deadlock detection using
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timers</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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Lock and transaction timeouts may be used in place of, or in
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addition to, regular deadlock detection. If lock timeouts are
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set, lock requests will return <a class="link" href="program_errorret.html#program_errorret.DB_LOCK_NOTGRANTED">DB_LOCK_NOTGRANTED</a>
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from a lock call when it is
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detected that the lock's timeout has expired, that is, the
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lock request has blocked, waiting, longer than the specified
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timeout. If transaction timeouts are set, lock requests will
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return <a class="link" href="program_errorret.html#program_errorret.DB_LOCK_NOTGRANTED">DB_LOCK_NOTGRANTED</a>
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from a lock call when it has been detected that the transaction has been
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active longer than the specified timeout.
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</p>
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<p>
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If lock or transaction timeouts have been set, database
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operations will return <a class="link" href="program_errorret.html#program_errorret.DB_LOCK_DEADLOCK">DB_LOCK_DEADLOCK</a>
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when the lock timeout has expired
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or the transaction has been active longer than the specified
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timeout. Applications wanting to distinguish between true
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deadlock and timeout can use the <a href="../api_reference/C/envset_flags.html" class="olink">DB_ENV->set_flags()</a> configuration
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flag, which causes database operations to instead return <a class="link" href="program_errorret.html#program_errorret.DB_LOCK_NOTGRANTED">DB_LOCK_NOTGRANTED</a>
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in the case of timeout.
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</p>
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<p>
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As lock and transaction timeouts are only checked when lock
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requests first block or when deadlock detection is performed,
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the accuracy of the timeout depends on how often deadlock
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detection is performed. More specifically, transactions will
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continue to run after their timeout has expired if they do not
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block on a lock request after that time. A separate deadlock
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detection thread (or process) should always be used if the
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application depends on timeouts; otherwise, if there are no
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new blocked lock requests a pending timeout will never
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trigger.
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</p>
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<p>
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If the database environment deadlock detector has been
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configured with the <a href="../api_reference/C/lockdetect.html#detect_DB_LOCK_EXPIRE" class="olink">DB_LOCK_EXPIRE</a> option, timeouts are the
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only mechanism by which deadlocks will be broken. If the
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deadlock detector has been configured with a different option,
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then regular deadlock detection will be performed, and in
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addition, if timeouts have also been specified, lock requests
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and transactions will time out as well.
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</p>
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<p>
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Lock and transaction timeouts may be specified on a database
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environment wide basis using the <a href="../api_reference/C/envset_timeout.html" class="olink">DB_ENV->set_timeout()</a> method. Lock
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timeouts may be specified on a per-lock request basis using
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the <a href="../api_reference/C/lockvec.html" class="olink">DB_ENV->lock_vec()</a> method. Lock and transaction timeouts may be
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specified on a per-transaction basis using the
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<a href="../api_reference/C/txnset_timeout.html" class="olink">DB_TXN->set_timeout()</a> method. Per-lock and per-transaction timeouts
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supersede environment wide timeouts.
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</p>
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<p>
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For example, consider that the environment wide transaction
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timeout has been set to 20ms, the environment wide lock
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timeout has been set to 10ms, a transaction has been created
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in this environment and its timeout value set to 8ms, and a
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specific lock request has been made on behalf of this
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transaction where the lock timeout was set to 4ms. By default,
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transactions in this environment will be timed out if they
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block waiting for a lock after 20ms. The specific transaction
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described will be timed out if it blocks waiting for a lock
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after 8ms. By default, any lock request in this system will be
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timed out if it blocks longer than 10ms, and the specific lock
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described will be timed out if it blocks longer than
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4ms.
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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_dead.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_deaddbg.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">Deadlock detection </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"> Deadlock debugging</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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