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375 lines
14 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>Chapter 4. Concurrency</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="Getting Started with Berkeley DB Transaction Processing" />
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<link rel="up" href="index.html" title="Getting Started with Berkeley DB Transaction Processing" />
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<link rel="prev" href="maxtxns.html" title="Configuring the Transaction Subsystem" />
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<link rel="next" href="blocking_deadlocks.html" title="Locks, Blocks, and Deadlocks" />
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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 11.2.5.3</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">Chapter 4. Concurrency</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="maxtxns.html">Prev</a> </td>
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<th width="60%" align="center"> </th>
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<td width="20%" align="right"> <a accesskey="n" href="blocking_deadlocks.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="chapter" 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"><a id="txnconcurrency"></a>Chapter 4. Concurrency</h2>
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</div>
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</div>
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</div>
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<div class="toc">
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<p>
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<b>Table of Contents</b>
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</p>
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<dl>
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<dt>
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<span class="sect1">
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<a href="txnconcurrency.html#concurrenthandles">Which DB Handles are Free-Threaded</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="blocking_deadlocks.html">Locks, Blocks, and Deadlocks</a>
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</span>
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</dt>
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<dd>
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<dl>
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<dt>
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<span class="sect2">
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<a href="blocking_deadlocks.html#locks">Locks</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="blocking_deadlocks.html#blocks">Blocks</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="blocking_deadlocks.html#deadlocks">Deadlocks</a>
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</span>
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</dt>
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</dl>
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</dd>
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<dt>
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<span class="sect1">
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<a href="lockingsubsystem.html">The Locking Subsystem</a>
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</span>
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</dt>
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<dd>
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<dl>
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<dt>
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<span class="sect2">
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<a href="lockingsubsystem.html#configuringlock">Configuring the Locking Subsystem</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="lockingsubsystem.html#configdeadlkdetect">Configuring Deadlock Detection</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="lockingsubsystem.html#deadlockresolve">Resolving Deadlocks</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="lockingsubsystem.html#setpriority">Setting Transaction Priorities</a>
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</span>
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</dt>
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</dl>
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</dd>
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<dt>
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<span class="sect1">
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<a href="isolation.html">Isolation</a>
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</span>
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</dt>
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<dd>
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<dl>
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<dt>
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<span class="sect2">
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<a href="isolation.html#degreesofisolation">Supported Degrees of Isolation</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="isolation.html#dirtyreads">Reading Uncommitted Data</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="isolation.html#readcommitted">Committed Reads</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="isolation.html#snapshot_isolation">Using Snapshot Isolation</a>
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</span>
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</dt>
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</dl>
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</dd>
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<dt>
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<span class="sect1">
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<a href="txn_ccursor.html">Transactional Cursors and Concurrent Applications</a>
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</span>
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</dt>
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<dd>
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<dl>
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<dt>
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<span class="sect2">
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<a href="txn_ccursor.html#cursordirtyreads">Using Cursors with Uncommitted Data</a>
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</span>
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</dt>
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</dl>
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</dd>
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<dt>
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<span class="sect1">
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<a href="exclusivelock.html">Exclusive Database Handles</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="readmodifywrite.html">Read/Modify/Write</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="txnnowait.html">No Wait on Blocks</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="reversesplit.html">Reverse BTree Splits</a>
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</span>
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</dt>
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</dl>
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</div>
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<p>
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DB offers a great deal of support for multi-threaded
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<span>
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and multi-process
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</span>
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applications even when transactions are not in use. Many of DB's
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handles are
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thread-safe<span>, or
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can be made thread-safe by providing the appropriate flag at handle creation time,</span>
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and DB provides a
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flexible locking subsystem for managing databases in a concurrent
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application. Further, DB provides a robust mechanism for
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detecting and responding to
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<span>deadlocks</span>
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.
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All of these concepts are
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explored in this chapter.
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</p>
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<p>
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Before continuing, it is useful to define a few terms that will appear
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throughout this chapter:
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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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<span class="emphasis"><em>Thread of control</em></span>
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</p>
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<p>
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Refers to a thread that is performing work in your application.
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Typically, in this book that thread will be performing DB
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operations.
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</p>
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<p>
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Note that this term can also be taken to mean a separate process
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that is performing work — DB supports multi-process
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operations on your databases.
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</p>
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<p>
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Also, DB is agnostic with regard to the type or style of
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threads in use in your application. So if you are using
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multiple threads
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(as opposed to multiple processes) to perform concurrent
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database access, you are free to use whatever thread package is
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best for your platform and application. That said, this manual
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will use pthreads for its threading examples because those have
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the best chance of being supported across a large range of
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platforms.
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</p>
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</li>
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<li>
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<p>
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<span class="emphasis"><em>Locking</em></span>
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</p>
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<p>
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When a thread of control obtains
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access to a shared resource, it is said to be
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<span class="emphasis"><em>locking</em></span> that resource. Note that
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DB supports both exclusive and non-exclusive locks. See
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<a class="xref" href="blocking_deadlocks.html#locks" title="Locks">Locks</a> for more information.
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</p>
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</li>
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<li>
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<p>
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<span class="emphasis"><em>Free-threaded</em></span>
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</p>
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<p>
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Data structures and objects are free-threaded if they can be
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shared across threads of control without any explicit locking on
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the part of the application. Some books, libraries, and
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programming languages may use the term
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<span class="emphasis"><em>thread-safe</em></span> for data structures or objects
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that have this characteristic. The two terms mean the
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same thing.
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</p>
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<p>
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For a description of free-threaded DB objects, see
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<a class="xref" href="txnconcurrency.html#concurrenthandles" title="Which DB Handles are Free-Threaded">Which DB Handles are Free-Threaded</a>.
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</p>
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</li>
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<li>
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<p>
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<span class="emphasis"><em>Blocked</em></span>
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</p>
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<p>
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When a thread cannot obtain a lock because some other
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thread already holds a lock on that object, the lock
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attempt is said to be <span class="emphasis"><em>blocked</em></span>. See
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<a class="xref" href="blocking_deadlocks.html#blocks" title="Blocks">Blocks</a> for more information.
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</p>
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</li>
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<li>
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<p>
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<span class="emphasis"><em>Deadlock</em></span>
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</p>
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<p>
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Occurs when two or more threads of control attempt to access conflicting resource in such a way as none
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of the threads can any longer make further progress.
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</p>
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<p>
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For example, if Thread A is blocked waiting for a resource held by Thread
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B, while at the same time Thread B is blocked waiting for a
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resource held by Thread A, then neither thread can make any
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forward progress. In this situation, Thread A and Thread B
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are said to be <span class="emphasis"><em>deadlocked.</em></span>
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</p>
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<p>
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For more information, see <a class="xref" href="blocking_deadlocks.html#deadlocks" title="Deadlocks">Deadlocks</a>.
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</p>
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</li>
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</ul>
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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="concurrenthandles"></a>Which DB Handles are Free-Threaded</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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The following describes to what extent and under what conditions
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individual handles are free-threaded.
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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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<code class="classname">DB_ENV</code>
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</p>
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<p>
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Free-threaded so long as
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<span>
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the <code class="literal">DB_THREAD</code> flag is provided to
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the environment <code class="methodname">open()</code>
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method.
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</span>
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</p>
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</li>
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<li>
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<p>
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<code class="classname">DB</code>
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</p>
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<p>
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Free-threaded so long as
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<span>
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the <code class="literal">DB_THREAD</code> flag is provided to
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the database <code class="methodname">open()</code>
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method, or if the database is opened using a
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free-threaded environment handle.
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</span>
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</p>
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</li>
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<li>
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<p>
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<code class="classname">DBC</code>
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</p>
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<p>
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Cursors are not free-threaded. However, they
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can be used by multiple threads of control so
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long as the application serializes access to the handle.
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</p>
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</li>
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<li>
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<p>
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<code class="classname">DB_TXN</code>
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</p>
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<p>
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Access must be serialized by the application across threads of control.
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</p>
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</li>
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</ul>
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</div>
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</div>
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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="maxtxns.html">Prev</a> </td>
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||
<td width="20%" align="center"> </td>
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<td width="40%" align="right"> <a accesskey="n" href="blocking_deadlocks.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">Configuring the Transaction Subsystem </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"> Locks, Blocks, and Deadlocks</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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