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109 lines
6.2 KiB
HTML
109 lines
6.2 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>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 16. 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 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">Deadlock detection using 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 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_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 timers</h2>
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</div>
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</div>
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</div>
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<p>Lock and transaction timeouts may be used in place of, or in addition
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to, regular deadlock detection. If lock timeouts are set, lock requests
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will return <a class="link" href="program_errorret.html#program_errorret.DB_LOCK_NOTGRANTED">DB_LOCK_NOTGRANTED</a> from a lock call when it is
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detected that the lock's timeout has expired, that is, the lock request
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has blocked, waiting, longer than the specified timeout. If transaction
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timeouts are 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 has been detected that the transaction has been
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active longer than the specified timeout.</p>
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<p>If lock or transaction timeouts have been set, database operations will
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return <a class="link" href="program_errorret.html#program_errorret.DB_LOCK_DEADLOCK">DB_LOCK_DEADLOCK</a> when the lock timeout has expired or the
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transaction has been active longer than the specified timeout.
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Applications wanting to distinguish between true deadlock and timeout
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can use the <a href="../api_reference/C/envset_flags.html" class="olink">DB_ENV->set_flags()</a> configuration flag, which causes
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database operations to instead return <a class="link" href="program_errorret.html#program_errorret.DB_LOCK_NOTGRANTED">DB_LOCK_NOTGRANTED</a> in the
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case of timeout.</p>
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<p>As lock and transaction timeouts are only checked when lock requests
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first block or when deadlock detection is performed, the accuracy of
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the timeout depends on how often deadlock detection is performed. More
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specifically, transactions will continue to run after their timeout has
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expired if they do not block on a lock request after that time.
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A separate deadlock detection thread (or process) should always
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be used if the application depends on timeouts; otherwise, if
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there are no new blocked lock requests a pending timeout will
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never trigger.</p>
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<p>If the database environment deadlock detector has been configured with
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the <a href="../api_reference/C/lockdetect.html#detect_DB_LOCK_EXPIRE" class="olink">DB_LOCK_EXPIRE</a> option, timeouts are the only mechanism by
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which deadlocks will be broken. If the deadlock detector has been
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configured with a different option, then regular deadlock detection will
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be performed, and in addition, if timeouts have also been specified,
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lock requests and transactions will time out as well.</p>
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<p>Lock and transaction timeouts may be specified on a database environment
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wide basis using the <a href="../api_reference/C/envset_timeout.html" class="olink">DB_ENV->set_timeout()</a> method. Lock timeouts may be
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specified on a per-lock request basis using the <a href="../api_reference/C/lockvec.html" class="olink">DB_ENV->lock_vec()</a> method. Lock
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and transaction timeouts may be specified on a per-transaction basis
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using the <a href="../api_reference/C/txnset_timeout.html" class="olink">DB_TXN->set_timeout()</a> method. Per-lock and per-transaction
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timeouts supersede environment wide timeouts.</p>
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<p>For example, consider that the environment wide transaction timeout has
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been set to 20ms, the environment wide lock timeout has been set to
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10ms, a transaction has been created in this environment and its timeout
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value set to 8ms, and a specific lock request has been made on behalf
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of this 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 block waiting
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for a lock after 20ms. The specific transaction described will be timed
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out if it blocks waiting for a lock after 8ms. By default, any lock
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request in this system will be timed out if it blocks longer than 10ms,
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and the specific lock described will be timed out if it blocks longer
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than 4ms.</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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