559 lines
23 KiB
HTML
559 lines
23 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>The Locking Subsystem</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 V1.62.4" />
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<link rel="home" href="index.html" title="Getting Started with Berkeley DB Transaction Processing" />
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<link rel="up" href="txnconcurrency.html" title="Chapter 4. Concurrency" />
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<link rel="previous" href="blocking_deadlocks.html" title="Locks, Blocks, and Deadlocks" />
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<link rel="next" href="isolation.html" title="Isolation" />
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</head>
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<body>
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<div class="navheader">
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">The Locking Subsystem</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="blocking_deadlocks.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 4. Concurrency</th>
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<td width="20%" align="right"> <a accesskey="n" href="isolation.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="lockingsubsystem"></a>The Locking Subsystem</h2>
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</div>
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</div>
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<div></div>
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</div>
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<p>
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In order to allow concurrent operations, DB provides the locking
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subsystem. This subsystem provides inter- and intra- process
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concurrency mechanisms. It is extensively used by DB concurrent
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applications, but it can also be generally used for non-DB
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resources.
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</p>
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<p>
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This section describes the locking subsystem as it is used to
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protect DB resources. In particular, issues on configuration are
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examined here. For information on using the locking subsystem to
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manage non-DB resources, see the
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<i class="citetitle">Berkeley DB Programmer's Reference Guide</i>.
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</p>
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<div class="sect2" 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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<h3 class="title"><a id="configuringlock"></a>Configuring the Locking Subsystem</h3>
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</div>
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</div>
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<div></div>
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</div>
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<p>
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You initialize the locking subsystem by specifying
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<span>
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<tt class="literal">DB_INIT_LOCK</tt> to the
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<tt class="methodname">DB_ENV->open()</tt>
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method.
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</span>
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</p>
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<p>
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Before opening your environment, you can configure various
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maximum values for your locking subsystem. Note that these
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limits can only be configured before the environment is
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opened. Also, these methods configure the entire environment,
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not just a specific environment handle.
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</p>
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<p>
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Finally, each bullet below identifies the
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<tt class="filename">DB_CONFIG</tt> file parameter that can be used
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to specify the specific locking limit. If used, these
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<tt class="filename">DB_CONFIG</tt> file parameters override any
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value that you might specify using the environment handle.
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</p>
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<p>
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The limits that you can configure are as follows:
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</p>
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<div class="itemizedlist">
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<ul type="disc">
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<li>
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<p>
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The maximum number of lockers
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supported by the environment. This value is used by
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the environment when it is opened to estimate the amount
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of space that it should allocate for various internal
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data structures. By default, 1,000 lockers are
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supported.
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</p>
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<p>
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To configure this value, use the
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<span>
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<tt class="methodname">DB_ENV->set_lk_max_lockers()</tt>
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method.
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</span>
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</p>
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<p>
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As an alternative to this method, you can configure this
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value using the <tt class="filename">DB_CONFIG</tt> file's
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<tt class="literal">set_lk_max_lockers</tt> parameter.
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</p>
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</li>
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<li>
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<p>
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The maximum number of locks supported by the environment.
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By default, 1,000 locks are supported.
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</p>
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<p>
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To configure this value, use the
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<span>
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<tt class="methodname">DB_ENV->set_lk_max_locks()</tt>
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method.
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</span>
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</p>
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<p>
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As an alternative to this method, you can configure this
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value using the <tt class="filename">DB_CONFIG</tt> file's
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<tt class="literal">set_lk_max_locks</tt> parameter.
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</p>
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</li>
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<li>
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<p>
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The maximum number of locked objects supported by the environment.
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By default, 1,000 objects can be locked.
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</p>
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<p>
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To configure this value, use the
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<span>
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<tt class="methodname">DB_ENV->set_lk_max_objects()</tt>
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method.
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</span>
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</p>
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<p>
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As an alternative to this method, you can configure this
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value using the <tt class="filename">DB_CONFIG</tt> file's
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<tt class="literal">set_lk_max_objects</tt> parameter.
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</p>
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</li>
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</ul>
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</div>
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<p>
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For a definition of lockers, locks, and locked objects, see
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<a href="blocking_deadlocks.html#lockresources">Lock Resources</a>.
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</p>
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<p>
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For example, to configure the maximum number of locks that your
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environment can use:
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</p>
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<pre class="programlisting">#include <stdio.h>
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#include <stdlib.h>
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#include "db.h"
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int
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main(void)
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{
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int ret, ret_c;
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u_int32_t env_flags;
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DB_ENV *envp;
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const char *db_home_dir = "/tmp/myEnvironment";
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envp = NULL;
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/* Open the environment */
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ret = db_env_create(&envp, 0);
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if (ret != 0) {
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fprintf(stderr, "Error creating environment handle: %s\n",
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db_strerror(ret));
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return (EXIT_FAILURE);
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}
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env_flags = DB_CREATE | /* If the environment does not
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* exist, create it. */
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DB_INIT_LOCK | /* Initialize locking */
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DB_INIT_LOG | /* Initialize logging */
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DB_INIT_MPOOL | /* Initialize the cache */
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DB_THREAD | /* Free-thread the env handle. */
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DB_INIT_TXN; /* Initialize transactions */
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/* Configure max locks */
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ret = envp->set_lk_max_locks(envp, 5000);
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if (ret != 0) {
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fprintf(stderr, "Error configuring locks: %s\n",
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db_strerror(ret));
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goto err;
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}
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/* Open the environment. */
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ret = envp->open(envp, db_home_dir, env_flags, 0);
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if (ret != 0) {
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fprintf(stderr, "Error opening environment: %s\n",
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db_strerror(ret));
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goto err;
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}
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err:
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/* Close the environment */
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if (envp != NULL) {
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ret_c = envp->close(envp, 0);
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if (ret_c != 0) {
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fprintf(stderr, "environment close failed: %s\n",
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db_strerror(ret_c));
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ret = ret_c;
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}
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}
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return (ret == 0 ? EXIT_SUCCESS : EXIT_FAILURE);
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} </pre>
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</div>
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<div class="sect2" 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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<h3 class="title"><a id="configdeadlkdetect"></a>Configuring Deadlock Detection</h3>
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</div>
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</div>
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<div></div>
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</div>
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<p>
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In order for DB to know that a deadlock has occurred,
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some mechanism must be used to perform deadlock
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detection. There are three ways that deadlock detection can
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occur:
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</p>
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<div class="orderedlist">
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<ol type="1">
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<li>
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<p>
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Allow DB to internally detect deadlocks as they
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occur.
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</p>
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<p>
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To do this, you use
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<span><tt class="methodname">DB_ENV->set_lk_detect()</tt>.</span>
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This method causes DB to walk its internal lock table
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looking for a deadlock whenever a lock request
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is blocked. This method also identifies how DB decides which lock
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requests are rejected when deadlocks are detected. For example,
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DB can decide to reject the lock request for the transaction
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that has the most number of locks, the least number of locks,
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holds the oldest lock, holds the most number of write locks, and
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so forth (see the API reference documentation for a complete
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list of the lock detection policies).
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</p>
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<p>
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You can call this method at any time during your application's
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lifetime, but typically it is used before you open your environment.
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</p>
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<p>
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Note that how you want DB to decide which thread of control should break a deadlock is
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extremely dependent on the nature of your application. It is not unusual for some performance
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testing to be required in order to make this determination. That said, a transaction that is
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holding the maximum number of locks is usually indicative of the transaction that has performed
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the most amount of work. Frequently you will not want a transaction that has performed a lot of
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work to abandon its efforts and start all over again. It is not therefore uncommon for
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application developers to initially select the transaction with the <span class="emphasis"><em>minimum</em></span>
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number of write locks to break the deadlock.
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</p>
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<p>
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Using this mechanism for deadlock detection means
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that your application will never have to wait on a
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lock before discovering that a deadlock has
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occurred. However, walking the lock table every
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time a lock request is blocked can be expensive
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from a performance perspective.
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</p>
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</li>
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<li>
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<p>
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Use a dedicated thread or external process to perform
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deadlock detection. Note that this thread must be
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performing no other database operations beyond deadlock
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detection.
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</p>
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<p>
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To externally perform lock detection, you can use
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either the
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<tt class="methodname">DB_ENV->lock_detect()</tt>
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method, or use the
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<span><b class="command">db_deadlock</b></span> command line
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utility. This method (or command) causes DB to walk the
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lock table looking for deadlocks.
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</p>
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<p>
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Note that like
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<span><tt class="methodname">DB_ENV->set_lk_detect()</tt>,</span>
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you also use this method (or command line utility)
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to identify which lock requests are rejected in the
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event that a deadlock is detected.
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</p>
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<p>
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Applications that perform deadlock detection in
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this way typically run deadlock detection between every few
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seconds and a minute. This means that your
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application may have to wait to be notified of a
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deadlock, but you also save the overhead of walking
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the lock table every time a lock request is blocked.
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</p>
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</li>
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<li>
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<p>
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Lock timeouts.
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</p>
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<p>
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You can configure your locking subsystem such that
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it times out any lock that is not released within a
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specified amount of time. To do this, use the
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<span><tt class="methodname">DB_ENV->set_timeout()</tt></span>
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method.
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Note that lock timeouts are only checked when a
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lock request is blocked or when deadlock
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detection is otherwise performed. Therefore, a lock can have timed out and still be held for
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some length of time until DB has a reason to examine its locking tables.
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</p>
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<p>
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Be aware that extremely long-lived transactions, or
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operations that hold locks for a long time, may be
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inappropriately timed out before the transaction or
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operation has a chance to complete. You should
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therefore use this mechanism only if you know your
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application will hold locks for very short periods
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of time.
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</p>
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</li>
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</ol>
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</div>
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<p>
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For example, to configure your application such that DB
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checks the lock table for deadlocks every time a lock
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request is blocked:
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</p>
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<pre class="programlisting">#include <stdio.h>
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#include <stdlib.h>
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#include "db.h"
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int
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main(void)
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{
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int ret, ret_c;
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u_int32_t db_flags, env_flags;
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DB *dbp;
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DB_ENV *envp;
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DB_TXN *txn;
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const char *db_home_dir = "/tmp/myEnvironment";
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const char *file_name = "mydb.db";
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envp = NULL;
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/* Open the environment */
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ret = db_env_create(&envp, 0);
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if (ret != 0) {
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fprintf(stderr, "Error creating environment handle: %s\n",
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db_strerror(ret));
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return (EXIT_FAILURE);
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}
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env_flags = DB_CREATE | /* If the environment does not
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* exist, create it. */
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DB_INIT_LOCK | /* Initialize locking */
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DB_INIT_LOG | /* Initialize logging */
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DB_INIT_MPOOL | /* Initialize the cache */
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DB_THREAD | /* Free-thread the env handle. */
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DB_INIT_TXN; /* Initialize transactions */
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/*
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* Configure db to perform deadlock detection internally, and to
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* choose the transaction that has performed the least amount of writing
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* to break the deadlock in the event that one is detected.
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*/
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ret = envp->set_lk_detect(envp, DB_LOCK_MINWRITE);
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if (ret != 0) {
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fprintf(stderr, "Error setting lk detect: %s\n",
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db_strerror(ret));
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goto err;
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}
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ret = envp->open(envp, db_home_dir, env_flags, 0);
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if (ret != 0) {
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fprintf(stderr, "Error opening environment: %s\n",
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db_strerror(ret));
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goto err;
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}
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/*
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* From here, you open your databases, proceed with your
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* database operations, and respond to deadlocks as
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* is normal (omitted for brevity).
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*/
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... </pre>
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<p>
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Finally, the following command line call causes
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deadlock detection to be run against the
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environment contained in <tt class="literal">/export/dbenv</tt>. The
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transaction with the youngest lock is chosen to break the
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deadlock:
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</p>
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<pre class="programlisting">> /usr/local/db_install/bin/db_deadlock -h /export/dbenv -a y</pre>
|
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<p>
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For more information, see the
|
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<a href="http://www.oracle.com/technology/documentation/berkeley-db/db/utility/db_deadlock.html" target="_top">
|
||
<tt class="literal">db_deadlock</tt> reference documentation.
|
||
</a>
|
||
</p>
|
||
</div>
|
||
<div class="sect2" lang="en" xml:lang="en">
|
||
<div class="titlepage">
|
||
<div>
|
||
<div>
|
||
<h3 class="title"><a id="deadlockresolve"></a>Resolving Deadlocks</h3>
|
||
</div>
|
||
</div>
|
||
<div></div>
|
||
</div>
|
||
<p>
|
||
When DB determines that a deadlock has occurred, it will
|
||
select a thread of control to resolve the deadlock and then
|
||
<span>
|
||
return <tt class="literal">DB_LOCK_DEADLOCK</tt> to that
|
||
thread.
|
||
</span>
|
||
|
||
|
||
|
||
If a deadlock is detected, the thread must:
|
||
</p>
|
||
<div class="orderedlist">
|
||
<ol type="1">
|
||
<li>
|
||
<p>
|
||
Cease all read and write operations.
|
||
</p>
|
||
</li>
|
||
<li>
|
||
<p>
|
||
Close all open cursors.
|
||
</p>
|
||
</li>
|
||
<li>
|
||
<p>
|
||
Abort the transaction.
|
||
</p>
|
||
</li>
|
||
<li>
|
||
<p>
|
||
Optionally retry the operation. If your application
|
||
retries deadlocked operations, the new attempt must
|
||
be made using a new transaction.
|
||
</p>
|
||
</li>
|
||
</ol>
|
||
</div>
|
||
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
|
||
<h3 class="title">Note</h3>
|
||
<p>
|
||
If a thread has deadlocked, it may not make any
|
||
additional database calls using the handle that has
|
||
deadlocked.
|
||
</p>
|
||
</div>
|
||
<p>
|
||
For example:
|
||
</p>
|
||
<pre class="programlisting">retry:
|
||
ret = envp->txn_begin(envp, NULL, &txn, 0);
|
||
if (ret != 0) {
|
||
envp->err(envp, ret, "txn_begin failed");
|
||
return (EXIT_FAILURE);
|
||
}
|
||
...
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||
/* key and data are Dbts. Their usage is omitted for brevity. */
|
||
...
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||
switch (ret = dbp->put(dbp, txn, &key, &data, 0)) {
|
||
case 0:
|
||
break;
|
||
/* Deadlock handling goes here */
|
||
case DB_LOCK_DEADLOCK:
|
||
/* Abort the transaction */
|
||
(void)txn->abort(txn);
|
||
|
||
/*
|
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* retry_count is a counter used to identify how many times
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* we've retried this operation. To avoid the potential for
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* endless looping, we won't retry more than
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||
* MAX_DEADLOCK_RETRIES times.
|
||
*/
|
||
if (retry_count < MAX_DEADLOCK_RETRIES) {
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||
printf("Got DB_LOCK_DEADLOCK.\n");
|
||
printf("Retrying write operation.\n");
|
||
retry_count++;
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||
goto retry;
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||
}
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printf("Got DB_LOCK_DEADLOCK and out of retries.");
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||
printf("Giving up.\n");
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||
return (EXIT_FAILURE);
|
||
default:
|
||
/* If some random database error occurs, we just give up */
|
||
envp->err(envp, ret, "db put failed");
|
||
ret = txn->abort(txn);
|
||
if (ret != 0) {
|
||
envp->err(envp, ret, "txn abort failed");
|
||
return (EXIT_FAILURE);
|
||
}
|
||
}
|
||
/* If all goes well, commit the transaction */
|
||
ret = txn->commit(txn, 0);
|
||
if (ret != 0) {
|
||
envp->err(envp, ret, "txn commit failed");
|
||
return (EXIT_FAILURE);
|
||
}
|
||
|
||
return (EXIT_SUCCESS); </pre>
|
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